C4 Fuel Injection Overview

Speed density was the first fuel control system used on 1982-1984 Corvettes. It was also used on the 1990-1993 cars. Speed density uses a manifold absolute pressure sensor (MAP sensor) to calculate the necessary fuel load. The 1990-1991 cars have a MAP sensor bolted directly to the intake plenum. The 1992-1993 cars have the MAP sensor bolted to the intake manifold. The throttle position sensor, coolant temperature sensor, and intake air temperature sensor are also used to calculate fuel.

Mass-air flow fuel control was introduced in 1985 and used until 1989. It was reintroduced in 1994 along with sequential fuel injection. The 1985-1989 mass-air system had a few changes along the way.

1985 cars have a slower baud rate computer and use a mass-air sensor module to control burn-off and to power the mass-air sensor.

1985-1988 cars use a stand-alone cold-start system that requires intake manifold porting to allow cold start fuel flow. This porting hurts power output.

1989 cars had no cold-start system, along with intake manifold porting changes. The 1989 cold start event is handled by the PROM (chip). The fuel injector pulsewidth was increased. 1986-1989 computers were replaced with a faster baud rate unit.

1994 cars have an On Board Diagnostic (OBD) system (first generation). The OBD system was introduced to bring diagnostic uniformity to all makes and manufacturers. The 1994-1996 OBD computer system utilizes mass-air and MAP sensors to control fuel delivery. The 1994-1996 cars utilize sequential fuel injection, which means fuel is injected only when needed. These cars have better fuel control and respond better to high performance modifications.

The 1995-1996 cars have OBDII fuel control systems that follow more stringent fuel emission control standards. Catalytic converters have oxygen sensors checking their performance as well as controlling fuel in closed loop.

Troubleshooting Basic Engine Control Module (ECM) Failures

The most important thing to remember when troubleshooting a computer-controlled fuel injection system is to check the basics first. Many times the first thought is to condemn the computer; it rarely fails. Check all connections, especially the computer grounds when intermittent conditions occur. A good battery and battery cables are very important. If you have constant problems with dead batteries you can damage your Engine Control Module (ECM) from a voltage spike. It is very rare that a sensor fails and the Service Engine Soon (SES) light is not lit to warn of a problem. Therefore, you most likely have a basic fuel or ignition problem if the Service Engine Soon light is not on. There is an exception to the rule with an ECM: the 1986-1989 ECM can cause a hard start condition that acts like a flooded engine when starting. The 1990-1991 ECM can cause a stall intermittently and I have found a few 1990-1991 ECM’s that cause an elevated engine temperature. This elevated temperature occurs when the ECM goes into a limp home mode. 1982-1991 ECM’s have failures that will exhibit strange sensor values when using a scanner.

All of the ECM’s I mentioned did not turn on the Service Engine Soon light or set any codes. I have seen a few ECM’s that will show coolant temperature readings 50° higher than actual temperature. Anytime you see multiple codes and strange readings, most likely the ECM has failed. The later 1992-1993 ECM and the 1994-2001 PCM (Powertrain Control Module) has a better success rate. It is rare that an ECM will turn on the Service Engine Soon light when a short-cycle intermittent failure occurs. I have also found that usually you can rap on a failed ECM with your fist and make the engine stall or start. Anytime an ECM or PCM has failed all the control solenoids must be checked for shorts to ground. I check all control solenoids at the ECM or PCM connector for shorts to ground; this will test for a possible shorted wire also. The control solenoids must have at least 20 ohms of resistance when testing. A factory service manual has the necessary information to locate the control solenoid wire position for testing. The testing should be done at the ECM connector. This way the wiring itself can be tested for possible shorting to ground.

The most important tool to use when trouble-shooting is a good memory or notes. If necessary, write down as many pieces of information as possible. This will help compile a troubleshooting workbook. A service manual is also a very important diagnosis tool. During road testing I take notes detailing when a problem occurs: hot engine, cold engine, or possibly after a hot restart, etc. In the shop I try to get as much information as possible about any previous repairs and parts that have been replaced. A simple audio test of the engine’s performance can be performed. Listen to the engine as it cranks over. An engine that has comparable compression on all cylinders has an even rhythm as it cranks over. If you hear what sounds like a rhythmic increase or uneven pulses during cranking, most likely a cylinder(s) have low compression, which may be caused by a burned valve or possibly a hole in a piston. If a cranking problem is encountered a compression test should be performed.

Crankcase pressure test

If compression seems to be okay during cranking, the next test I would perform is a crankcase pressure test. A crankcase pressure test consists of removing the PCV valve and breather tube on an engine that is at operating temperature. Watch for oil vapors coming out of the PCV valve port. If you have oil vapors coming out of the PCV valve port in pulses like a steam engine, you have broken piston rings or possibly a broken piston. A light amount of smoke (blow by) without rhythmic pulses is normal on high mileage engines. If I find either excessive blow by or uneven cranking I do a compression test first, then a cylinder leakage test before proceeding.

When compression testing be sure to hold the throttle wide open and remove all spark plugs before testing. Remove the distributor power + lead from the distributor cap on 1982-1991 cars to disable the ignition. Disconnect the coil connector on 1992-1996 cars to disable the ignition.

Lower than usual compression readings that are even will not typically make an engine run rough. Cylinders that vary more than 20 % when performing compression testing can make an engine run rough. If cylinder compression is low put 1 tablespoon of light oil into the cylinder in question then retest compression. If the compression rises in the cylinder with the addition of oil, worn piston rings are likely. Cylinder pressure testing is accomplished with a supply of compressed air. First the valve cover should be removed and the valve adjustment backed off on the cylinder being tested. Now compressed air is introduced into the cylinder with an air-hold. Listen for air leaking at the throttle body for an intake valve leak and at the tailpipe for a leaking exhaust valve. If you hear air leaking in the crankcase you have broken piston rings or a hole in a piston.

Engine Vacuum Testing

Check engine vacuum. A good vacuum gauge that has not been banged around in your toolbox can be very helpful in diagnosing engine performance. Vacuum gauges are sensitive and will not be accurate if treated roughly. Expensive vacuum gauges will not tolerate abuse either. Please refer to Figure 1.1 provided for vacuum gauge diagnosis. A vacuum gauge can also help locate an unexplained power loss. The vacuum gauge will check for a clogged exhaust system. A clogged exhaust system can be difficult to diagnose because it usually takes quite a while for it to cause a significant power loss that will accumulate over time. The most reliable test of exhaust system backpressure is to remove the A.I.R. check valve on vehicles with an A.I.R. pump system. These are the valves on the top of the exhaust manifold. A 1-1/16th” wrench is required for removal. Be sure to use a 7/8th” wrench to secure the manifold tubes while removing them. Sometimes a torch is required to heat the A.I.R. check valve during removal. An adapter can be put together to connect the vacuum gauge to the A.I.R. tube. Three pounds of exhaust system pressure is acceptable. Any pressure reading in excess of three pounds indicates a clogged exhaust system.

PCV System Testing

PCV (Positive Crankcase Ventilation) valves are very important for proper engine performance. The PCV valve allows a pressure drop (vacuum) in the engine’s crankcase; this helps piston rings seal. The engine oil stays cleaner because moisture and oil vapors are burned off. The PCV valve also acts as check valve under hard acceleration when engine vacuum is zero. This allows the crankcase ventilation tube that is connected to the air cleaner to draw a vacuum in the crankcase when the throttle is wide open. PCV valves are often neglected, but they play an important role in engine drivability and engine life. An incorrect PCV valve can cause hesitation, poor idling, and stalling. Also keep in mind that the ECM fuel map is setup for the PCV valve’s calculated vacuum leak. The crankcase should never be totally closed; it will cause extreme oil usage and oil leaks! Use an equivalent O.E. PCV system and crankcase breather system to relieve pressure in the crankcase. Never disable the system. All the PCV components should be in good shape: rubber grommets soft and pliable, all hoses clean and sealing properly, and the crankcase hose properly sealed and attached to the throttle body. Do not use heater hose for the short PCV tube connector hose on 1985-1988 cars; it will become soft causing a vacuum leak. Do not use fuel hose for the PCV valve to intake manifold fitting on 1989-1996 cars; it can collapse due to high vacuum conditions. The O.E. PCV hose part number 14088538 is available from GM. This 14088538 PCV hose came on 1989-1991 cars and will take the place of the O.E. aluminum tube and connector. The 1985-1988 crankcase breather tube that is connected to the throttle body below the throttle position sensor and then to the valve cover is also replaceable with part number 10055872. The 10055872 was used on 1989-1991 cars and allowed greater flow than the earlier design.

Vacuum Sensor and Coolant Sensor Troubleshooting

Speed density fuel control systems are sensitive to vacuum leaks. If a vacuum leak occurs the idle speed will be increased because the MAP sensor recognizes the vacuum loss and adds fuel as necessary. Therefore, on speed density cars, changing leaking deteriorated vacuum hoses can make a significant change in performance. Many times I have been able to drop the idle speed significantly with a thorough check and repair of all minor and major vacuum leaks on 1982-1984 cars. 1982-1984 cars have a vacuum hose that connects the MAP sensor to the throttle body. The 1982-1984 MAP sensor is located on the firewall near the brake booster. If this hose is damaged or disconnected you may have a difficult time starting and experience extremely rich conditions. In most cases the engine will not run at all. If the hose is off the MAP sensor and there is no vacuum signal, the ECM goes full rich because it thinks the throttle is wide open. I always inspect this hose and replace it if any deterioration is evident. If you have an unexplained extremely rich condition, the sensor hose and connections should be checked on any speed density fuel system.

Another common problem on 1982-1984 Corvettes is an open or shorted coolant sensor. The early coolant sensors were prone to bad corrosion. The coolant sensor connector was redesigned in 1985 and is much more reliable. The coolant sensor wires have a tendency to lose their insulation. I have seen a few situations where the insulation was worn off the coolant sensor wires and work was done on the engine that moved the wires causing them to short to each other or ground. The owner then started the engine and it ran poorly. Shorted coolant sensor wires would make the ECM think the engine temperature is 300° plus, making it run lean. It would also be very difficult to start. If the coolant sensor wires are broken allowing an open circuit, the ECM thinks the engine temperature is minus 40° or less. The engine would also run very rich with a lot of black smoke out of the tailpipe.

Throttle Bodies

A common problem on 1982-1989 cars is worn throttle bodies. The throttle shaft bore wears and the shaft is loose, causing sticking throttle plates and stalling. A typical complaint is that the throttle seems to return to idle when the engine is cold, and as the engine warms up the throttle sticks. Another problem I have seen is when the engine is off, the throttle returns to the idle stop, but when the engine is running it will not return. Another complaint is erratic idle speeds. The throttle body itself should be cleaned every 30,000 miles to maintain peak performance. A dirty throttle body can cause hesitation and stalling when the A/C is on. An extremely carboned-up throttle body can create a hard hot start condition.

Typical Code Problems

1985 problems were EGR codes (32).

1986-1989 code problems are EGR (32), mass airflow rate high (34) (possible mass air sensor failure or relay failure), mass air burn-off failure (36) (typically a relay failure, Service Engine Soon light will come on at start up after a drive cycle).

1990-1991 code problems were EGR failure (32) due to exhaust system modifications that relieved exhaust backpressure.

1992-1996 code problems are rare.

Distributor failures are common on 1992-1994 LT-1 cars. A revised distributor was introduced on the 1995-1996 LT-1 engines. The revised distributor uses engine vacuum to eliminate moisture and heat.

Camshaft and Valve Spring Troubleshooting

If you have an unexplained miss at idle and under load, there are two possible problems that can be difficult to find in some situations. A worn camshaft lobe is possible on 1982-1986 cars. 1987-1996 cars have a roller lifter camshaft that rarely has any wear concerns. As the camshaft lobe wears, power is reduced and a miss will be evident. A slightly worn camshaft lobe may emit a minor tapping noise from a lifter. Severely worn exhaust camshaft lobes will cause a rhythmic popping noise in the throttle body as the engine is accelerated. The exhaust valve does not open enough to allow the exhaust gases to escape so they are released into the intake manifold. If an intake camshaft lobe is worn, it will not allow fuel and air to be drawn into the cylinder, which will make the engine have a steady miss. A severely worn camshaft lobe usually gives a warning of impending disaster. If you have lifter that starts to tap and you adjust it and it requires adjustment again and again, this usually indicates that the camshaft lobe is wearing away. A compression test will show a lower than normal reading if the intake camshaft lobe is worn badly.

A broken valve spring can cause a steady miss also. Vacuum gauge readings will be erratic and there will also be a noticeable valve train noise when a valve spring is broken badly. If the valve spring is broken near the top coil of the spring, it will only make a minor effect on the vacuum gauge reading. The engine will have a steady miss that improves as the engine is accelerated. Also, during a compression test it may not affect the readings at all and make no excessive valve train noise if the valve spring is broken at the top coil. Therefore, if you have checked everything: ignition, fuel system, vacuum leaks, and still have a steady miss, take a close look for broken valve springs or worn camshaft lobes on early cars.

EGR System Troubleshooting

The EGR (Exhaust Gas Recirculation) valve can be a source of lean misfire at float or light engine loads (sometimes referred to as a surge). Typically this misfire (surge) will be felt at 45 MPH cruise speeds. The EGR valve has a vacuum diaphragm that operates a pintle. The pintle rests against a seat closing off the exhaust flow. The 1982-1993 Corvette’s Engine Control Module controls the EGR valve. The 1994-2001 Corvette Powertrain Control Module (PCM) controls the EGR valve. The EGR system should allow flow at cruise speeds or light acceleration, but not at idle or hard acceleration. When exhaust gas is reintroduced into the combustion chamber, it leans and cools the fuel mixture allowing an increase in fuel mileage and helps prevent detonation. Spark knock can occur when the EGR system is inoperative.

The 1982-1996 EGR valve control system utilizes a 12-volt solenoid to regulate vacuum that opens the EGR valve. The 1982-1984 EGR solenoid is normally open which was sometimes a problem because it allowed vacuum flow when the solenoid failed. The 1982-1984 control systems had design limitations that made most of them hard to live with due to lean misfire conditions. You can disconnect the EGR valve’s vacuum supply and plug the hose, then road test the car to see if the misfire has been eliminated. If the misfire has been eliminated you can purchase an aftermarket EGR valve, which has changeable orifices to cut down on the amount of EGR flow. One word of caution: the 1982-1984 misfire (lean condition) I am referring to is very slight. If the misfire is severe most likely there is a fuel delivery or supply problem. Do not leave the EGR valve disconnected- detonation can occur that will damage pistons!

The 1985-1996 cars use a normally closed solenoid that will not allow vacuum to operate the EGR valve if the solenoid fails. This helps prevent stalling. If an EGR valve sticks open the engine will idle very roughly or not at all. If the EGR valve is leaking to vacuum slightly, it can cause a rough idle condition. A sticking EGR pintle is common on 1982-1991 cars. If you tap on a sticking EGR valve lightly with a heavy screwdriver it sometimes will reseat the EGR valve pintle. If tapping on the EGR valve smoothes out the idle an EGR valve replacement may be necessary. Remove the EGR valve and check the operation of the EGR pintle. If the EGR pintle sticks when testing, spray the pintle and seat with throttle body cleaner while holding the valve open. This may break loose carbon that is preventing the pintle from seating.

EGR flow rate causing a misfire (lean condition) is typically not a problem on the 1985-1996 cars. The 1985-1989 cars typically have EGR code 32 problems. If you have a Service Engine Soon light that comes on after cruising at highway speeds and code 32, most likely the EGR valve is bad. If the vacuum supply or vacuum solenoid has failed, it will also display code 32 at highway speeds. If the code 32 occurs when you start your car the EGR temperature switch has failed (part number 14087415). Low backpressure exhaust systems can render the EGR system inoperative. There is a check valve inside the EGR valve that closes from exhaust system backpressure to modify the EGR flow rate.

When EGR systems are disabled or inoperative, the exhaust passages have a tendency to build up carbon, blocking EGR flow. Blocked passages can cause spark knock problems. Excessive carbon in the EGR passages can prevent the EGR valve from sealing properly. If you change any 1985-1991 L-98 intake manifold base or plenum be sure that it has the same passages as the original pieces- some of these pieces can block EGR flow.

A.I.R. System Troubleshooting

The Air Injection Reaction (A.I.R.) system consists of a pump, control valves, and solenoids. The A.I.R. system has a diverter valve control system that controls flow to the exhaust manifolds or catalytic converter. The A.I.R. system diverts air to the exhaust manifolds for the first two minutes of operation to heat the exhaust and speed up the oxygen sensor on time. After the engine warms up the A.I.R. flow is diverted to the catalytic converter or atmosphere. The A.I.R. system should divert to atmosphere when the throttle is closed on deceleration. If the A.I.R. system is not controlling the airflow properly a lean condition (misfire) may occur or catalytic converter failure is possible.

This lean misfire can occur at the same speed (45 MPH) as the EGR misfire. If misfire is a problem remove the hoses at the check valves to see if air is flowing at all times to the exhaust manifolds. Then drive the car with the hoses disconnected to see if the misfire has been eliminated. If the lean misfire has been eliminated the diverter valves or control system has a problem that will require a service manual for diagnosis.

After-fire (popping in the exhaust system on deceleration) can be caused by an improperly operating A.I.R. control system. The after-fire is heard on deceleration if the A.I.R. pump is incorrectly pumping air into the exhaust system. After-fire can be also be caused by cold air being introduced into the exhaust system. This condition can cause drivability problems because the fuel system will be lean, especially if the exhaust leak is before the oxygen sensor. This popping in the exhaust is often mistaken for backfiring.

Ignition System Troubleshooting

A steady miss may indicate an ignition problem or a fouled spark plug. If the miss occurs primarily when the engine is cold you may have an open circuit in a plug wire. If you have a miss when accelerating it indicates that there is a secondary circuit (plug wire, distributor cap, or rotor) that is shorting to ground because of an open circuit. A plug wire that is laying on an exhaust manifold or A.I.R. tube may not look burned, but once it gets hot it will allow an easier path to ground. Many cars that come into my shop have incorrectly routed plug wires.

Over the years I have used many brands of plug wires. I always use Moroso for performance applications and A/C Delco for everyday use. These particular brands of custom fit plug wires fit well and are very reliable. Remember though, there is no plug wire that will sit directly on a hot exhaust manifold or header without misfiring.

Corvettes equipped with headers can be very difficult when it comes time to install plug wires and spark plugs. If a plug wire boot is resting directly on the header pipe it will misfire. I also do not use plug wires larger than 8 millimeter in diameter. The larger diameter plug wires are difficult to install and route properly. Correct plug wire routing can eliminate misfire and missing under load especially on hot engines. When a plug wire is shorting to a header-pipe or exhaust manifold it will be random and usually will be more noticeable at light throttle applications. When the plug wire or boot is burned badly it will show up under hard acceleration also.

When installing the plug wires on the driver’s side, the number five and seven plug wires should not be routed parallel with each other. High-Energy Ignition (HEI) system cars with an open circuit plug wire can also burn a hole in a rotor or distributor cap.

It is possible to install a new H.E.I. distributor cap and create an engine miss. Sometimes the ignition coil hold-down screws are not the correct length and will break through the distributor cap causing a short to ground. If you have an unexplained miss look carefully at the inside of the distributor cap for burn spots at each of the four ignition coil hold-down screw locations.

There may be problems with the plug wires if they have 75,000 miles or more on them. When removing the plug wire boot for spark plug replacement they can be damaged easily due to fatigue. I found that twisting the plug wire boot before pulling on the plug wire and boot will allow easy removal and prevent terminal damage. Usually plug wires that have been tugged hard will cause an open circuit. If you look at the terminal inside the plug wire boot on plug wires that have had rough handling there will be black carbon sooty deposits on the terminal. This is an indication that the plug wire has an open circuit that will cause a problem very soon. If you use silicone dielectric grease when installing the plug wire boot you will promote better contact between the terminal and spark plug. It also makes spark plug wire removal easier in the future. Do not put more silicone dielectric grease than the size of a wooden match head at each plug wire connection. One other word of caution: if you do not feel or hear the plug wire terminal in the boot as it engages the spark plug or distributor cap terminal, you may have a plug wire create a higher resistance or possibly have it pop off the terminal.

It is a good idea to install a better distributor cap and rotor- they will stand a short to ground better and help prevent possible ignition module damage. I prefer Accel or Standard Ignition’s performance line distributor cap and rotor. Both of these manufacturers use brass contacts in their caps and rotors with heavy plastic components. When replacing the distributor cap look carefully at the coil where the metal frame goes around the plastic housing of the coil; if there are burned spots the coil should be replaced.

Spark plugs are good for 50,000 miles typically in crossfire cars. Spark plugs last 75,000 miles or longer in 1985-1996 cars unless there is a fuel control problem. Fouled spark plugs can make an engine hard or impossible to start. If you do not drive your car regularly fouled spark plugs are possible. Figure 1.2 (Spark Plug Analysis) illustrates many spark plug conditions. Use it with care and consider any other problems that may be occurring before making a spark plug condition diagnosis.

With a high performance ignition coil spark plug gaps can be increased; this will possibly smooth a rough idle and will increase fuel economy. Do not increase your plug gap beyond .050” unless you have an ignition coil that has more than 50,000-volt output. Remember that with the increased secondary voltage distributor caps, rotors and plug wires must be in top condition. When setting the spark plug gap on platinum tipped spark plugs do not pry between the center electrode and ground piece. The platinum tip can be broken and destroyed. Be careful when installing spark plugs if you hear even a minor pop noise the porcelain can be cracked that can cause an intermittent miss that is difficult to find.


Fuel injectors deliver exactly the right amount of fuel for one cylinder. 1982 and later Corvettes deliver their fuel through fuel injectors, one to a cylinder except LT-5 (ZR1) engines that have two per cylinder. The term fuel injector is misleading: just as shock absorbers don’t absorb shocks (the springs do), fuel injectors don’t inject fuel (the fuel pump does). Fuel injectors simply block the flow of fuel. Only when the computer activates them by completing their electrical circuit do they open and let fuel spray. Fuel injectors are fast-acting, precise electromechanical nozzles, metering the pressurized fuel into a finely atomized spray for the next power stroke. 1982-1991 Corvettes have batch-fired fuel injection. This means that as long as the ignition distributor is turning, fuel is injected no matter where the piston is (on a combustion stroke or exhaust stroke).

There have been many types of fuel injectors. The majority now work when an electromagnetic coil lifts a ferrous pintle or needle off its seat so fuel can squeeze through to the nozzle’s aperture and into the intake air channel. In 1989, GM introduced their own fuel injector. These are Multec fuel injectors. The Multec injector uses a ball to close off fuel flow and the injectors are raised up out of the intake runner slightly. Raising the injector kept the tip cleaner and the ball provided multiple seats to close off fuel flow. Because gasoline has such low viscosity, even minuscule pintle or ball movement is enough to release the amount of fuel the engine needs. The Engine Control Module (ECM) or Powertrain Control Module (PCM) operates the fuel injectors, varying their delivery volume by the time it actuates their coils, the on-time or pulsewidth is measured in milliseconds. The computer calculates this time based on information from its sensors. These include the throttle position sensor, the engine coolant temperature sensor, the intake manifold pressure sensor, the mass airflow sensor, the crankshaft position sensor on 1996 Corvettes, and various other inputs depending on the year of your car.

Most fuel injection systems determine fuel pressure at the injectors by a combination of the fuel pump and the fuel pressure regulator. A fuel pressure regulator is used to release fuel back to the tank. The regulator’s return flow corresponds inversely to pressure in the intake manifold. As manifold pressure increases (increased load or low vacuum), the regulator returns less fuel back to the tank. As manifold pressure decreases (reduced load or high vacuum), the regulator returns more. The engine rarely uses all the fuel pumped.

At low coolant temperature, wide-open throttle, high load and high rpm, the engine burns almost all the gasoline the pump delivers. At high coolant temperature, idle and no load, the fuel pressure regulator returns most of the fuel back to the tank. This way the difference between the pressure of the fuel in the rail and the pressure of the air in the intake manifold remains constant. With a constant pressure difference, the only variable determining the quantity of fuel injected is pulsewidth. This is the time the computer turns the injector on allowing fuel to be injected.

As fuel re-enters the tank, it produces substantial vapor. With tighter emissions standards, 1999 and later Corvette fuel injection systems have no external pressure regulator or return line and run at a constant fuel pressure. It turned out easier to control fuel mixture at a constant fuel pressure (regardless of manifold vacuum/pressure) rather than to expand and complicate the charcoal canister vapor purge system. Fuel injectors on these ‘returnless’ fuel injection systems work the same as on the previous systems, but with shorter pulsewidths at low manifold pressure. When there is a greater difference between the fuel and the intake air pressure- at idle and deceleration- injector pulsewidth is extremely brief, or even non-existent.


Injector problems fall into two groups: mechanical and electrical. Bad fuel and water in the fuel have damaged many injector nozzles and/or pintle tips. You can identify mechanical problems by a visual inspection of the fuel spray pattern. Listening for an injector’s click (or its absence) can point to the first injector to observe. If grit or contaminated fuel gets past the main filter they’ll quickly plug the small mesh filter in the top of the injector. Deposits formed from fuel droplets that evaporate after and engine shuts off can form gums and tars that clog an injector or skew its spray pattern, preventing the fuel from vaporizing completely. Sometimes such deposits prevent the pintle from seating completely, so the injector dribbles fuel. While fuel cushions the injector pintles from closing against the seat, it’s still a mechanical part and will eventually show wear and begin to leak. Even the slowest unmetered fuel leak can often transmit more fuel than necessary at unloaded warm idle.

Rust and sediment in the fuel tank of 1982-1996 Corvettes damages many fuel injectors. The typical complaint would be difficult hot restarts. If the 1985-1996 Corvette fuel filler boot drain is plugged, water will accumulate and slowly be drawn into the tank. The 1982-1996 fuel tank has a plastic bladder, but the steel sending unit is prone to corrosion. This corrosion falls into the fuel tank then the minute particles are forced through the fuel system to the fuel injector. The fuel filter will not stop these minute particles or water.

Most failed electrical circuits first become a short circuit and then quickly burn open, but with fuel injectors, you often notice drivability problems before the injector shorts completely. An injector that loses electrical resistance can have an interesting and puzzling effect. The usual way a coil changes resistance is by melting the thin insulation between adjacent windings of the wire. The failure does not occur all of a sudden. If the coil has 100 loops of wire, a short across the insulation between two loops can reduce the coil’s resistance by no more than one percent, even if all the current went through the partial short circuit. Such a short may not noticeably affect the injector. It could be very hard to detect with even the best ohmmeter since new injector’s resistance vary more than that and temperature changes account for even more. As the resistance goes down, the current goes up, and as current goes up so does the heat. The heat increase will melt the next most vulnerable insulation barrier until it also forms a short.

Before long, that partially shorted injector pulls noticeably more current than the others. It is rare for an injector to burn all the way shorted or open. The ECM or PCM will shut the injector off to protect the driver transistors when the current exceeds or falls below a certain threshold. Fuel injectors on 1985-1993 cars are grouped in two banks. The injector with lower resistance draws more current because electricity always follows the path of least resistance. Increased current to the partially shorted injector may ‘starve’ the paired injectors of current so it can’t lift its pintle and its cylinder gets little or no fuel. We then have the odd circumstance that the good injector does not spray long enough for combustion but the defective one keeps working- not for long, but long enough to leave a difficult troubleshooting situation.

Fuel injector cleaning with harsh chemicals is detrimental to the 1989-1996 Multec fuel injectors because the windings are cooled with fuel. Because the Multec fuel injectors sit higher in the intake manifold the tips stay clean enough that pouring in an injector cleaner in the fuel tank should keep them in good shape. The best possible situation is to keep good quality clean fuel in your fuel tank at all times. Standard Motor Products and Tomco have a very reasonably priced replacement injector available. I order fuel injectors for the 1989 model year for all 1985-1988 cars because they will be Multec style and have the same resistance. These fuel injectors can be purchased for as little as $35.00 a piece! The A/C Delco fuel injectors cost $100.00 minimum per injector.

Do not forget to check the fuel filter; many times it is forgotten

when diagnosing a fuel delivery problem.

The fuel pressure should be checked before condemning any fuel system components. 1982-1984 Corvette fuel pressure should be 9-12 PSI. 1985-1996 Corvette fuel pressure should be 32-38 PSI with the vacuum hose connected, and 42-48 PSI with the vacuum hose disconnected. When electric fuel pumps have accumulated 100,000 miles or more they are on borrowed time.

Fuel Injection System Maintenance

  • Fuel filters should be replaced every 30,000 miles.
  • Throttle bodies should be cleaned every 30,000 miles.
  • Oxygen sensors should be replaced every 75,000 miles.
  • When installing spark plugs in aluminum cylinder heads, Anti-seize or Never-seize should be put on the threads. A small dab of Anti-seize or Never-seize is all that is required; excessive amounts will cause misfiring.

Story and photos courtesy Chris Petris

46 thoughts to “C4 Fuel Injection Overview”

  1. I have a 1994 c4 lt1 6 speed with 76k miles. It runs great except for one strange issue: a power loss that is only noticeable during WOT acceleration runs. No tire slippage or anything like that. It accelerates hard through the first 1/2 of the rpm range and then there is a sudden noticeable drop in power through the rest of the range. It still pulls hard all the way up and runs smooth, but the drop in power is noticable and comes on like a light switch ….and very reapeatable. Distributor, wires and plugs have been replaced with no effect. I thought about a knock sensor pulling spark out, but no sign of knock or ping at all. Wondering if there is rings of something in the air/fuel system that souns characteristically plausible.

    1. My first inclination would be a fuel issue, possibly a weak fuel pump. The best thing to do would be the check fuel pressure, and make sure it isn’t dropping. If you don’t have a GM Shop Service manual, it is a good idea to get one. There are some troubleshooting and testing procedures for these types of issues in there. These are high pressure fuel systems, so an issue such as this could arise if the pump isn’t keeping up with engine demand.

  2. Hello I have a 94 corvette with lt1 automatic, it starts normal and smooth, runs for 2-3 seconds and dies clean, its losing injector pulse only, will run as long as you spray intake, sys light flashes. I already replaced CCM and key, checked serial data wire to PCM for breaks. Also ohmed injectors, all are 12.5. checked all engine and pcm grounds. I’m looking toward replacing PCM next. Any ideas?

  3. I have an 85 Corvette and I had all the injectors replaced, and as before if it sits overnight or longer it won’t start unless you give it fuel. When it’s warm it starts fine. I have a small cam and lifters, and a first stage chip. Any ideas.

  4. My 1996 corvette service engine light is on, diagnosed as a vacuum part and the computer also replaced. Drove 10 miles and ses light is back on, any ideas?

  5. My 1990 started bogging down and choking out when I turned onto a road and had to floor it to get with traffic. I have replaced filter and pump with no fix. When it does this, it starts sucking a lot of air but it seems to be lean coming out of exhaust. I’m thinking tps or fuel regulator, but fuel getting in return line or vent tube is a maybe. I think it has a roll over fuel cut off so could have something wrong there.

  6. I have a 1987 c4 corvette with the tpis intake (mini ram) but the idle is very up and down and will die if you goose the engine. Which keeps me from being able to drive it anywhere. It has no map sensor because it’s an 87. Anyone have any clue what it could be or where to start?

  7. 1989 tuneport corvette raw fuel leaking out of front of throttle body, could this be plugged return line? also fuel cap has sucking sound after trying to start car

  8. Nice report. I went WOT earlier today and now my ’86 c4 is running rough. It was running great before. The engine now feels like it is running “uneven” and making the car shake slightly. I will go through this article and hopefully be able to pinpoint what happened.

    1. Did you ever find out what the problem was? My 91 did the same. Basically went from cruise to WOT. Within 1-2 secs, the car shook, then acted as though the fuel pressure dropped to near zero. I ended up replacing the ECM only, and so no problems since. Apparently one (WOT vs fauly ECM) had nothing to do with the other?

  9. Thanks for advice. I have a 94 Lt1 73000. Car idles a little rough. Will not downshift in 3rd at wot but will at lower speeds. Car has stalled after short drive when shut off then restarted. Also had a backfire once under good at Wot. New plugs, wires, clean maf throttle body iac , new coil, ICM was tested.

  10. Any hints on an ’88 that has had a Pro Charger put in 10 yrs ago? Issue is intermittent 036. I’ve replaced both relays 4 times, MAF sensor 3 times. I never get into the throttle enough to kick in the Pro Charger, but the old owner did. He claims he never had any issues. After clearing ECM, cold start is fine, no check engine light, engine runs great. At around 180 degrees (190 thermostat), the check engine comes on. Engine appears to run fine at all speeds, but idle is a bit rough and smell of un-burnt fuel very noticeable. Replaced all temp sensors and knock sensor, among other non-related parts. Mechanic said a vacuum leak can cause a MAF error. If that is the case, which vacuum hose is connected to a sensor that opens when engine is hot?

    1. The last thing you want is a boosted application with any vacuum leaks. Depending on the leak it could interfere with how the car idles. I would go through all your vacuum lines and double check especially since the vehicle has a pro charger.

    2. Maybe look at the temp, the computer goes by the temp and it should be 235 not 190 isn’t it? Also some performance chips will state the needed temp thermostat, i dunno just tryin to help.

    3. Check wiring harness 6 inches from relays and see if frayed. otherwise, order a 1987 Service manual and fault tree out the diagnostics. Don’t know why you keep replacing MAF 3 times and relays 4 times. Makes no sense.

  11. I have a 1984 corvette and after 50 miles or so it starts to run rough (lean). If I full throttle it after it runs rough it sets a code 44 (lean). If I turn the key off and back on while running down the road, it runs better and I can pick up a little speed again. If I turn the key off and then on after a few seconds and do this 3 or 4 times in a 5 minute span, I have full power back for 10 miles or so. If I pull over and shut the engine down for 4 to 8 minutes, I have full power back for 20 miles or so. Parts replaced: fuel filter, O2 sensor, fuel pump, emc temp sensor and spark plugs. What should I check now?

    1. I also replaced the ignition module, oil sending unit (3 wire unit), fuel pump relay, egr valve, cap and rotor and found a lot of vacuum leaks (was surprised it ran as good as it did). Changed out the computer and a new prom.

      1. After replacing all that you have and still no sign of what the issue is, I’d recommend you have the car looked at by a certified Corvette technician.

  12. My 1986 corvette rear end has just started to make a noise under the driver side behind wheel below the antenna hole – noise sound like a small appliance similar to a dishwasher. Engine has been off for while (put away for winter) and I have trickle charger hooked up. This noise just started, so I have now unplugged charger. What do you believe is causing this and how shall I fix it?

  13. I have a 1990 base model that runs fine till it gets warm, then runs rough. It has new plugs, wires, cap, coil, rotor and module and set at factory timing. It has bosch injectors with about 15k miles on them. All cylinders have 180+ lbs of compression. It has a full dual exhaust system with only only pre-cats on it. The A.I.R. system is non-functional and the EGR is blocked off. Fuel pressure is 43 psi at idle and no vacuum applied. There are no codes present. Any suggestions?

    1. I would suggest testing your injectors. Shorted injectors are fine when cold, but short when warmed up. Since they’re wired in series, one shorted injector shorts the others. Check impedance when cold and again when warm. It’s possible you were sold a bad injector.

  14. What is the serial number break in 1993 C4 when they switched injectors from 22 to 24 lb? When I first got my car, they had put the wrong ones in and there was a problem associated with pulse rate/impudence values as well. My VIN was newer and had early VIN injectors, the reason it was running poorly. Thanks.

  15. Thank you so much, this answered so many questions. Now back to an educated troubleshooting of a very strange issue: ’82 vet will start fine then while driving will dump fuel and flood out. This can happen at any speed and even at an idle. Issue came on without warning while on a trip.

    1. I would suggest taking a look at the oxygen sensors. Something is making the computer think it’s running lean and dumping more fuel than needed, causing it to flood.

  16. My 1992 corvette with the LT1 engine runs kinda rough when cold but runs much better when the engine is warmed up. I just changed spark plug wires and it has 63,000 miles.

  17. In the throttle body section you mention the condition “throttle returns to the stop when car is off but when running it sticks”. Is this condition due to a worn throttle body shaft? I own a 1988 corvette 80,000 miles original throttle body. I have cleaned the throttle body, IAC any and all help would be appreciated

    1. The throttle body should be cleaned every 30,000 miles to maintain peak performance. If the throttle shaft bore wears and the shaft becomes loose, it may cause sticking throttle plates and stalling.

  18. This report is very helpful. I do most of the tuning and repair on my 1991 C-4 9 (45,000 mi) for fun and enjoyment. Now that I have retired I intend to put many more miles on the C-4. Thanks for the many tips.

      1. I have a 1988 corvette and had replaced a lot of parts on it to make it a super daily driver, I have replaced the ignition coil with oem part, distributor cap/rotor, spark plugs, fuel pressure regulator, egr valve, idle air control valve, air injection pump and the drive belt tensioner as well, it starts right away-no more hard starting as it did before when I first got it- also it has raw fuel odor by the right side of the engine bay. The issue I have now is that when I put the transmission in D, it idles @ around 8 to 7 to 6 thousand rpm and it seems to bog down as if the engine will stall, then it will settle again to 7 or 6 thousand rpm and will do the same cycle again-at stop lights I am afraid that it will stall on me. I drove it last night and it ran great except for this issue. HELP!

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