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.

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C2, C3 & C4 A/C Retrofit

Basic System Operation

The compressor pressurizes the refrigerant then it enters the A/C condenser as a gas. The high temperature, high pressure gas is cooled as it goes through the condenser. This changes the gas to a liquid. Then the liquid goes to the evaporator through an expansion valve or metered orifice. This quickly depressurizes the liquid. This vaporizing liquid lowers its temperature rapidly. The vaporized refrigerant returns to the compressor (the compressor cannot handle any liquid).

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C2 & C3 Troubleshooting

By Chris Petris

The most important tool to use when troubleshooting is a good memory or notes. If necessary, write down as much information as possible. This will help compile a troubleshooting notebook. A factory service manual is an invaluable tool when diagnosing problems. A road test is very important and notes should be taken then also. When road testing I take notes about when a problem occurs: hot engine, cold engine, or possibly after a hot restart. In the shop I try to get as much information as possible about any previous repairs and parts that were replaced. A simple audio test of the engine’s performance is to listen to the engine cranking over and note any irregularities. A V-8 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) has low compression, which may be caused by a burned valve or possibly a hole in a piston.

I would perform 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. The most important thing is to have good engine compression with minimal blow-by before proceeding.

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C4 Interior Overview

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The C-4 interior is comprised mostly of plastic. The 1984-1989 dash carrier is made entirely of plastic. All of the plastic dash fascia can be damaged quite easily just by over-tightening the screws. I use a screwdriver or a clutch-operated power screwdriver. When using a clutch-operated screwdriver, be sure to use the lowest setting to avoid cracking the dash fascia. Then use a Phillip’s head screwdriver to check for tightness. A cordless drill with a Phillip’s head bit is a good idea for removal only; definitely use a screwdriver when tightening. Early cordless drills did not have any torque setting options. They require extreme care if used for any dash screw tightening.

The light switch area of the dash carrier typically cracks when this occurs a rattle or creaking noise will be heard that is difficult to locate. I repair these by fabricating a reinforcement plate using a piece of .020 gauge aluminum. The light switch stud nut and the dash carrier mounting 13mm stud nut must be removed to fabricate and install the reinforcement plate. The light switch shaft will have an outer nut holding the light switch to the dash, which will be referred to as the light switch stud nut. This can be removed with a special socket or a pair of snap ring pliers. A pair of needle nose pliers will work in a pinch. A paper file folder can be used to make a pattern- and save a lot of aluminum. A file folder is thicker than paper but cuts easily with scissors this will allow you to cut out the holes for the light switch stud nut and the dash carrier mounting stud. Make sure the reinforcement plate pattern covers the entire cracked area. Now transfer the pattern over to the aluminum plate. Then drill a 7/16” hole in the lower corner of the reinforcement plate for the light switch shaft. Next drill a 3/8” hole in the upper corner of the reinforcement plate for the dash mounting stud. I use a good quality epoxy to bond the reinforcement plate to the dash carrier. The reinforcement plate and dash carrier must be cleaned carefully and free of all oil. Isopropyl alcohol or automotive brake cleaner spray will remove any oily residue. After applying epoxy to the reinforcement plate I install it over the light switch shaft and dash carrier mounting stud. Tighten the dash carrier mounting nut. Install the light switch stud nut into the light switch. Then I like to put two 1/8” rivets in the reinforcement plate to keep it tight until the epoxy sets up avoid riveting in the cracked area. I have only seen one, possibly two cars that did not have a badly broken dash carrier, so look for this during a dash restoration.

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