Replacing the Front Spoiler: 1984 to 1996

Time: 2 Hours

Tools: Sockets and screwdrivers

Talent: (3)

Applicable Years: 1984 to 1996

Tools: ¼-inch drive socket set and metric combination wrenches

Tab: $100

Tip: You don’t have to replace the entire spoiler. Remember it comes in three pieces. The way these spoilers get abused you might want to consider doing one section each year. That way you wouldn’t be spending a lot money and your car would always look decent.

Performance Gain: Improved radiator flow may help the car run a little cooler on the highway. They don’t have much effect on local driving.

Complementary Work: If you have an overheating problem, it’s probably because of a lot of dirt is trapped between the radiator and the air conditioning condensor. Broken spoilers usually won’t cause your Corvette to overheat.

The front spoiler on the C4 Corvette will be the bane of your existence. If you keep your 1984 to 1996 Corvette long enough you’ll go through several of these spoilers. No matter how careful you might be this spoiler is going to scrape the pavement at some time.

There’s one basic fact that gets overlooked by most people: on the early cars there were two different spoilers available. The base suspension cars got a shorter spoiler than the cars equipped with the Z51 suspension. The theory here was that the Corvettes with the Z51 suspensions wouldn’t bounce as much going over bumps so the spoiler could be closer to the pavement. Hey, a 1984 Corvette with the Z51 option would barely move at all on its suspension.

The reality of the situation was the Z51 spoilers just got torn up quicker. If I only drove my Corvette on the street I would opt for the non-Z51 spoiler. If I were concerned about originality with my Z51-optioned car I would go for the original spoiler. The third choice, and more people are doing this every year, is to keep a street spoiler and put the really nice correct one on the day before you go to a show.

This is really a two person job. I have difficulty thinking about how I would do it by myself. One person has to use a ¼” ratchet with a long extension and maybe even a universal on the end. One person can use this socket from the top of the car while the second person is under the car keeping the nuts from turning. The center section is really easy. The side extensions test your collection of ¼” extensions and swivel sockets.

With the old spoiler in the trash you get to reverse the process. After a break one person gets back on the ground and pushes the bolts up through the holes. The second person reaches around all of the stuff in the front corner and places the nut on the bolt. You continue this process for about an hour and you’ll be done.

The other thing to remember is to leave everything loose until you get it lined up properly. Carefully align the top edge of the spoiler with the body panel and then tighten each bolt as you go around. All that’s left is to apply a coat of your favorite vinyl treatment to your shiny new spoiler.

10-110-1.tif: By the time the average C4 gets over a decade old the spoiler is usually a mess and is held together with an assortment of nuts and bolts, not to mention cable ties. Take a good look at the situation before you place your order. You may want to order the mounting kit that’s available as well as the spoiler.

IMG007IMG007: This was one of my more foolish moves. This aftermarket spoiler has a place for attaching a brake cooling duct. We won’t even discuss the fact the opening is in the wrong place. What really amazed me was that there was no provision for attaching the cooling hose anyplace near the front brakes. Brake ducts are a great idea, but this particular one it the wrong answer. Save your money and stay with the original spoiler.

IMG008IMG008: Everyone has tried to sell fiberglass spoilers for the front of the C4 Corvette. Even Chevrolet offered one in 1988. They’ve just never caught on with the Corvette masses. The first reason is that they’re generally pretty ugly and detract from the original C4 design. Secondly if you hit the pavement with this spoiler it’s going to cost you a lot of money. Any of us who drive a C4 Corvette know that no matter how careful we might be we’re going to rub the spoiler on the pavement.

This is an adaptation from Richard Newton’s most recent book 101 Projects for Your Corvette 1984 – 1996. He has also written two other best selling Corvette One deals with the 1968 to 1982 Corvettes, How to Restore and Modify Your Corvette 1968-82 , while another deals with the Sting Rays from 1963 to 1967, Corvette Restoration Guide 1963-1967 All of these books are available from Corvette Central.

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Clutch Hydraulics on the 1984 to 1996 Corvette

Time: 3 Hours

Tools: 3/8” drive Metric sockets, box end and flare nut wrenches, GM hydraulic clutch fluid and vinyl hose.

Talent: (3)

Applicable Years: 1984 to 1996

Tab: $150

Tip: Replace both the master and slave units at the same time

When the 1984 Corvette was introduced it used a totally different system for working the clutch. Previously the Corvette had used a series of rods and pivots to connect the clutch to the clutch pedal.  Suddenly the Corvette had a hydraulic system. Nothing really new here since this hydraulic clutch actuation system had been used on other cars for a long time.

The Corvette engineering team felt that the engagement of the clutch was much smoother with a hydraulic system. This system uses two major components. The clutch pedal is attached to the clutch master cylinder and the fluid is transmitted to the slave cylinder

The C4 clutch hydraulic system is both self-bleeding and non-adjustable. Cycling the clutch pedal multiple times while on a level grade before driving will displace any trapped air pockets formed within either cylinder. If the threaded fittings of the hydraulic line between the clutch master and slave cylinder are tight there are only three places where air can enter into the slave cylinder assembly: The slave cylinder seal, the master cylinder seal or the fluid reservoir inlet.

Both the master and slave cylinder assemblies are horizontally mounted. The master cylinder assembly faces forward and the slave cylinder assembly faces rearward. When the vehicle is parked facing uphill, the clutch hydraulic master cylinder is more likely to develop an air pocket. When the vehicle is parked facing downhill,  the clutch actuator cylinder is more likely to develop an air pocket.

A Simple Test

There’s a fairly simple test to check out your clutch hydraulic system. The first step is to make sure that you’ve bled the hydraulic system and use GM hydraulic clutch fluid (P/M 12345347). When you’re convinced that you’ve done a good job and all the air is out of the system you need to do the following things to check the master and slave cylinder for leak down, or internal leaks.

  • Simulate being parked on an incline by placing jack stands under the front of the car for 12-24 hours.  Raise the front at least 1 foot higher than the rear.
  • Lower the car being careful not to depress the clutch
  • With the clutch pedal to the floor start the vehicle.
  • Try to put it into reverse when it first gets started.
  • If you have difficulty getting into reverse there’s a very good chance that the clutch master cylinder is not at 100%.

The following day you need to follow the same procedure but with the rear of your Corvette raised. This is the leak down test for the slave cylinder.

What happens is the hydraulic cylinders develop a leak and no pressure is available to actuate the clutch. The first thing you need to realize is that you can’t rebuild these units.  Even though there are rebuild kits available for both cylinders it simply isn’t worth the effort.

When a master or slave cylinder goes bad it’s because of wear in the bore. The anodizing wears out and leaves a rough spot in the bore. That spot accelerates the wear on the rubber O-ring. You can hone the cylinder out,  but when you do that you’ll enlarge the size of the bore.  The O-ring seals only come in standard sizes. This means that after rebuilding the unit will fail again in a very short time because the seal will not be able to seal the enlarged bore.

The other thing to remember is that you’ve got two units and both are used every time you push down on the clutch pedal. If one of the units fails the other one is going fail also. The best idea is to replace both units at the same time. This means you only have to do the job once. This is an especially good idea since bleeding the system can be aggravating.

Don’t hesitate to replace the slave cylinder if it doesn’t  bleed properly. Some people have found that they need to replace the clutch master hydraulic cylinder once for every 2 slave cylinder replacements. The hydraulic clutch system should always operate in the 97+% efficiency range to provide for normal transmission life expectancy, which is normally 75,000 – 150,000 miles.

The first thing you want to do is get the car as high up on four jack stands as you can. Is this beginning to sound familiar? You’re going to go under the car to replace the master and slave units. You’ll be under it again to bleed the system.

On the later cars you’ll have to move the computer unit out of the way in order to access the clutch master cylinder. The good part is that everything will come apart easily. Start at the top and use your  flare nut wrench to break the hydraulic lines loose. Don’t remove it yet, just make sure they can be loosened  It’s easier to do this before you start to unbolt the actual cylinders.

Now comes the hard part. You have to release the clutch hydraulic master cylinder linkage. This has to be done from inside the car, from under the dash. If you have the car up on jack stands and the hush panel ( this is the panel that is above the pedals and acts as a sound deadener) removed, the clip can be accessed from directly below. You can see this clip by looking through the hood release handle with a flash light.

It’s not that bad of a job as long as you have a long regular screwdriver and extra long needle nose pliers. The trick is to have the vehicle up on jack stands. That way you can sit on the floor and have your eye level be at the lower door jam level.

Bleeding the Clutch Hydraulic System

One method for bleeding this system is to let fluid simply run through the system until clean  fluid comes out. Close the bleeder screw , climb into  the car and press down on the clutch pedal about fifty times. Then go back under the car and allow a little more fluid out one last time.

The best bleeding process is still the conventional method where one person actuates the clutch pedal while another opens and closes the bleeder after the pedal is depressed to the floor and before the pedal is let back up. .

After the bleeding process is complete, fill the reservoir to 1/16″ below the “Low” mark so that when the “clean and dry” moisture barrier is re-inserted and lid screwed on, the fluid level remains between the LOW and HI mark. This provides the best level of atmospheric pressure isolation within the hydraulic system.

9-19-1.tif: The most difficult part of working on the clutch system is getting under the dash to release the small clevis pin that holds the clutch master cylinder actuating rod to the clutch pedal. This job goes a lot quicker if you have the car up on four jackstands. You don’t have to twist around quite so much to access the pin. Since the car will have to be on jackstands anyway for the bleeding process, start this project right off by getting it up in the air.

This is an adaptation from Richard Newton’s most recent book 101 Projects for Your Corvette 1984 – 1996. He has also written two other best selling Corvette One deals with the 1968 to 1982 Corvettes, How to Restore and Modify Your Corvette 1968-82 , while another deals with the Sting Rays from 1963 to 1967, Corvette Restoration Guide 1963-1967 All  of these books are available from Corvette Central.

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Automatic Transmission Service for the 1984 to 1996 Corvette

Time: 1 hours

Tools: 3/8” drive metric sockets

Talent: (4)

Applicable Years: 1984 to 1996

Tab: $50.00

Tip: Make sure you ask a lot questions before you let a shop tear into your transmission. The expense is great and a lot of things can happen to your car while it’s in the shop. Make sure you find a top quality shop. The quality of the work is going to be a lot more important that final price.

Complementary Work: If you’re considering the installation of a shift kit in your automatic keep in mind that you’ll also be changing the fluid and filter at that time.

If you own a 1984 to 1996 Corvette you probably have an automatic transmission. Most Corvette owners prefer the automatic transmissions to the manual transmission. This means that most of you reading this article have an automatic transmission in your Corvette.

The good part is that very little ever goes wrong with these automatics. These are very solid transmissions and seldom need repair. The most common reason for repair is simple abuse and the most damaging abuse comes from neglect.

Since most of you purchased your C4 Corvette on the used car market there’s a need to check out the transmission. You can make two very simple checks. First, you should check the fluid level. Make sure you do this when the car is completely warmed up. Also, make sure you always check the car with the engine running.

Checking the Fluid Level

This one is supposed to be so easy I shouldn’t even have to write about it. On the other hand, after twenty years in the service business and a few years with a major transmission chain, I have to point out that very few people know how to properly check the level of the automatic transmission fluid. Low fluid levels are a major reason that automatic transmissions have to be repaired.

One last point is that you should never check the fluid immediately after coming off the interstate. High speed driving will give you a false reading on the dipstick, or as GM calls it, the fluid level indicator blade. Wait at least three minutes with the car in Park before you check the fluid level.

You not only need to see that the transmission is properly filled but you also want to get an indication about the condition of the fluid. After you remove the dipstick place it on a white paper towel. The fluid should soak into the towel fairly easily. If the fluid doesn’t readily soak into the paper towel and/or is black in color it’s probably compacted with fiber material which is the first sign of clutch wear.

If you can actually see clutch material on the paper towel then you can be almost certain that the clutches need to be replaced. Just for comparison drop some brand new Dexron III ATF fluid next to the spot being checked. This will give you a good side by side comparison of how drastically your fluid may have changed since it was last serviced.


The only thing you need to do is change the fluid and filter on a regular basis.  I’m a fanatic about regular fluid and filter changes for an automatic transmission. I recommend that you change it on an annual basis even though that may be a little excessive.

I will offer one suggestion though: If you have over 75,000 miles on your car, and you’re pretty certain it still contains the original factory installed fluid, don’t change anything. There’s a good chance that the dirt and crud inside your transmission is what’s holding it together. At this point any possible damage has been done and changing the fluid and filter might only make things worse. Just start putting money aside for a rebuilt transmission.

Corvettes haven’t had a factory-equipped drain plug since the old Powerglide days. Changing the automatic transmission fluid in a Corvette requires dropping, or removing, the oil pan. Some shops use transmission service equipment that can replace the fluid without dropping the transmission pan. This equipment either taps into the ATF oil cooler lines or connects to the dipstick filler tube. The problem with this approach is that the service doesn’t include a filter change. The only way to change the filter is to drop the oil pan. You should be more concerned about changing the filter than you are about changing the fluid.

If you decide to change your own transmission filter make sure you get the car as high as possible. The higher the car is off the ground the easier this job will be. Next I would put some large plastic sheeting on the garage floor. Make sure that you spread it the width of the entire car and possibly three feet in front of and behind the transmission. Just assume you’re going to make a mess. It wouldn’t hurt to have a bag of kitty litter handy as well.

Pan gasket and filter sets are available at your local discount parts house. They may be either boxed or in flat shrink-wrap packages. Flat shrink wraps are typically used with cork pan gaskets to protect the gasket from bending that could damage it. Rubber (neoprene) pan gaskets, on the other hand, are flexible and can be folded to fit in a box. Different technicians have different preferences as to which is the best gasket material to use.

One thing you don’t need is silicone gasket material that comes in a tube. Silicone gasket material can cause you more problems than you can imagine. When you use silicone gasket material it’s squeezed between the transmission body and the pan. You’ll notice how it squeezes out of the sealing faces. Just keep in mind that just as much has been squeezed into the oil pan area. If this material gets loose inside the pan there’s a very good chance it will find it’s way into internals of the transmission and clog a very important passage.

In addition to the pan and gasket set you’ll also need four or more quarts of Dexron III ATF to refill the transmission. The transmission’s fluid capacity is important because automatics must be run very close to the full level, and never over that level. Overfilling your automatic transmission can aerate the fluid causing shifting problems. Underfilling can cause slow engagement and slipping. The amount of ATF required is usually listed in the vehicle owners manual. Also, make sure you read the dipstick properly.

Don’t forget proper lighting. It’s dark under your Corvette and you want to make sure that you see everything that’s going on. This is one case where those inexpensive halogen lights carried by Home Depot come in very handy. They focus the light directly into the correct area.

When you have the oil pan down check for metal shavings and other debris which are indicators of impending transmission problems. Some shavings in the oil pan are normal – don’t be alarmed. You only need to worry if you find lots of fiber material and big chunks of metal. If you do find a lot of material don’t be alarmed – you didn’t hurt anything. Just continue on the task and remember to start setting some money aside for a rebuilt transmission.


A lot of additives are sold on the market for transmissions. None of them are good for your transmission. The only time I would use and additive is out of desperation. A case in point is where the seals have started to leak and you want to postpone any real repair. The additives designed to stop leaks contain chemicals that swell the seals and gaskets to reduce fluid leakage. This might hold you over until you can reduce the balance on your Visa card so your transmission can be properly repaired. Just don’t think of it as a real repair.

Getting Your Transmission Back

One thing you need to be concerned about with a Corvette is that your car comes back  from the repair shop with it’s own transmission. The large transmission shops always keep a variety of units on the shelf. That’s how they can offer same day service on a transmission repair. Since the transmission used in the Corvette is popular across the GM product line they may have a couple on the shelf.

You need to carefully explain to the shop that you want your transmission rebuilt, not simply a rebuilt transmission. Then just to be on the safe side crawl under the car with a light and locate the VIN number on your transmission. This VIN number can be in more than a half dozen locations so check carefully.  Then when you get your car back home check this number again.

This might not seem important to you right now but trust me at some point it will be a big deal. Just ask the people who own 1968 and 1972 Corvettes how much they would be willing to pay to get the original transmission back in their Corvettes. Having the original driveline in your Corvette will become a big deal at some point. Don’t neglect it now.

8-18-1.tif: It’s important to keep track of the fluid level in your transmission. It may sound a little silly if you know what you’re doing, but on the other hand fluid neglect is the single largest cause of transmission failure. Make sure you read the dipstick correctly and that you also check the condition of the fluid at the same time. You really won’t look all that silly with your nose on the dipstick to check for a burning smell. You’ll feel a lot sillier if you have to get a home equity loan to pay for a new transmission in your Corvette.

8-28-2.tif: Check the VIN Number before you take the car to a transmission shop. The Vehicle Identification Number could be in any one of six different locations. Just keep looking until you find the numbers. While you’re under the car you can also copy down the Identification Code which has a lot of interesting information once you decode the letters and numbers.

IMG005IMG0005: The automatic transmission in the C4 Corvette began life as the 700 R4 and ended up being called the 4L60-E. Basically it was the same transmission except that a lot of the hydraulic functions were handled by electronics in the later years. It was still a 4-speed transmission and it still had fluid and a filter that needed changing. All of the changes were part of the quest for improved fuel mileage and smoother shifting.

IMG006IMG006: Transmission builders will argue the merits of different gaskets for the next several decades. Actually, if you look around a transmission shop you’ll most likely find a variety of different materials. The real secret to keeping your transmission oil pan from leaking is to make sure that both surfaces are very clean and you use a torque wrench to tighten the oil pan bolts evenly, using a variation of the star pattern you use when tightening lug nuts.

8-58-5.tif: The interesting thing is that for all the parts that are involved in an automatic most technicians would much rather rebuild an automatic transmission than a manual transmission. When your transmission is rebuilt everything comes out of the case and then parts are checked, replaced and everything goes back in as a stack


Make sure you specify that you want a total rebuild if you have your transmission rebuilt. Some shops only repair the first broken item they find and stop. Everything is put back together and they hope it holds long enough for the check to clear. Stay away from that sort of shop. Ask questions and shop around.

This is an adaptation from Richard Newton’s most recent book 101 Projects for Your Corvette 1984 – 1996. He has also written two other best selling Corvette One deals with the 1968 to 1982 Corvettes, How to Restore and Modify Your Corvette 1968-82 , while another deals with the Sting Rays from 1963 to 1967, Corvette Restoration Guide 1963-1967 All  of these books are available from Corvette Central.

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Servicing the Overdrive on the 4+3 Transmission

Time: 1 hour

Tools: Drain pain, sockets and 3/8” torque wrench

Talent: (3)

Applicable Years: 1984 to 1988

Tab: $45.00

Tip: Treat this job the same way you would treat changing the fluid and filter with an automatic transmission. You’ll probably end up making just as big a mess on your garage floor.

Complementary Work: As long as you have a mess on your garage floor you might as well change the gear oil inside the 4-speed part of the transmission.

The overdrive unit was simply hung off the back of the 4-speed transmission. In a great many ways the overdrive was very similar to the old two-speed Powerglide. This particular overdrive unit was originally designed for the Jeep CJ-7 and CJ-5. Chevrolet was working on such a tight deadline for the fourth generation Corvette that there was no time to develop a new unit specifically for the Corvette.

Delco Electronics solved the computer problems and made sure that the manual section and the overdrive were truly integrated units. All of this was really done to meet the EPA fuel mileage regulations, but at the same time the overdrive fourth gear provided a high speed cruising capability that easily exceeded all the earlier Corvettes. Thanks to the EPA the Corvette was a 150 mile per hour car.

The electronics necessary to create this dual transmission weren’t all that complicated but it did make for some interesting driving. There are really three unique, and different, algorithms programmed into the overdrive ECM, one each for second, third and fourth gears. The overdrive ECM logic is incorporated in into the main ECM, or the chip, as it’s commonly known.

This program monitors miles per hour, engine temperature, and throttle position in an effort to properly engage the overdrive. The program will normally not engage under 184°, although my ’85 always liked 176°. I guess all computers are not created equal. The real trick, however, is to get rid of all the complicated electronics and turn this transmission into an 8-speed.

A Quick Easy Modification

I discovered how to do this by accident. Well, not quite by accident. Actually Chris Petris, who used to build transmissions for the Corvette Challenge racing series, helped me with this. There’s a switch on the side of the transmission that tells the computer which gear is engaged. This 2nd gear switch is one of the first things to go bad on the 4+3 . The switch is engaged every single time you shift the transmission, whether the overdrive is engaged or not. It’s no wonder that this is usually the first item to wear out.

When my 2nd gear switch went bad the car simply wouldn’t engage overdrive properly. I could hit the switch on the console to turn on the overdrive and it might not actually engage until I drove some twenty miles down the road. You can imagine how aggravating this was. When Chris and I couldn’t locate a replacement switch right away we simply grounded the switch by running a very short shunt from one terminal to the other.  This effectively made the OD a completely mechanical unit, operated only by the interior switch, which in my case is on the console.

Eliminating this switch made all the difference in the world. This was like giving me a new Corvette. While Chevrolet designed all the electronics to pass the EPA fuel mileage cycle, I simply wanted a useful transmission. The 4+3 actually works better if you take some of the sophistication out of the system.

What we did was fool the computer into thinking that I’m always in 2nd gear or higher. This switch is on the left side of the transmission, and can be seen easily if you have the car on a lift. This switch moved around a little during the years, but it was always the switch towards the rear.

Remove the wiring harness from the switch and hook it out of the way with a cable tie. You won’t be using this harness. Now make a little jumper wire that fits into the two terminals. You can remove the switch from the transmission and make the jumper wire on your workbench if you prefer. If it takes more than fifteen minutes you’re goofing off.

The Filter

Since the overdrive unit in the 4+3 transmission is really a version of the old Powerglide automatic transmission is shouldn’t be a surprise that it has a transmission filter. You should treat this situation just as you would any automatic transmission. You’ll even create the same type of mess on your garage floor.

The technique is to simply drop the oil pan on the bottom of the overdrive unit and hope that most of the fluid goes into the large drain pan you’ve place under the pan. Be careful not to lose any of the bolts.

The best way to clean the pan in your home garage is to use spray cans of brake clean and a roll of paper towels. Pay particular attention to the sealing edges of the pan. At one time GM used RTV sealant on this surface. That wasn’t such a great idea and now you can get the filter kits with gaskets. If you don’t find a gasket in your car you can assume that the fluid hasn’t been changed in a few years.

Make sure that you also clean the transmission surface as well. These overdrive units are notorious leakers. Using a gasket on the surface will solve most of the sealing problems. Once everything is clean you can install a couple of bolts to hold the oil pan back in place and carefully tighten the bolts using a cross pattern to get it nice and even.

I would use Dexron III in the unit and probably opt for a synthetic fluid. Mobil 1 is the obvious choice but it seems everyone has a different preference here. Just ask some questions and see what other owners are using.

These units have proven pretty reliable once they got past the 1984 model year. There were some internal changes made and the clutch material is different on the later cars. The best thing you can do for this overdrive is change the fluid and filter once a year, or at least every 15,000 miles.

7-17-1.tif: GM placed a magnet in the oil pan to capture any metallic particles. Don’t be shocked if your magnet is covered with steel shavings. That simply means the magnet is doing what is designed to do. You can remove the magnet for cleaning and then place it back in the correct location.

7-27-2.tif: The overdrive gave the C4 an incredible top speed for the mid-‘80s. You’ll notice that the difference between fourth gear and fourth gear overdrive is almost forty miles an hour at 4,000 rpm. Since most of us don’t drive much over 80 mph on the highway it simply means we get pretty incredible gas mileage. I’m still wondering where you can actually use all that gear ratio. Even at Sebring, which is a very fast course, I only get into 3rd overdrive. Maybe I need to drive the banking at Daytona. Yep – that ought to get me into 4th overdrive.

IMG004IMG004: This little jumper switch did wonders for my driving enjoyment. Placing the jumper wire in this switch turned my overdrive into a manual overdrive. All of the stuff that was designed to meet fuel mileage requirements is now gone. I even have overdrive in 1st gear now – not that I really need it. The switch is easy to modify, but if you have any questions about your abilities just call the Corvette Clinic in Sanford Florida.

7-47-4.tif: This is an interesting combination of parts. We have the traditional small block Chevrolet engine bolted to the very traditional T-10 transmission. Then that’s hooked to yet another transmission which looks strangely similar to the old Powerglide from the fifties. Then that second transmission (or overdrive) is bolted to a huge length of aluminum U-channel (the Driveline Support), which in turn is bolted to a differential carrier in the rear of the car. What’s amazing is that it works so well.

This is an adaptation from Richard Newton’s most recent book 101 Projects for Your Corvette 1984 – 1996. He has also written two other best selling Corvette One deals with the 1968 to 1982 Corvettes, How to Restore and Modify Your Corvette 1968-82 , while another deals with the Sting Rays from 1963 to 1967, Corvette Restoration Guide 1963-1967 All  of these books are available from Corvette Central.

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The LT1 Opti-Spark Distributor

Time: 2 hours

Tools: A Visa card with a large credit line.

Talent: (4)

Applicable Years: 1992 to 1996

Tab: $500 to $1,000

Tip: Don’t ever let any water get in the area of the Opti-Spark if you detail your engine compartment.

Complementary Work: This would be a good time to install new spark plug wires. Spark plug wires on the LT1 are a major effort. It usually takes around four hours. I would install the new Opti-Spark and when you’re sure the engine runs properly get started on the spark plugs and the spark plug wires.

This new Opti-Spark distributor was supposed to be the greatest thing since GM did away with ignition points. Instead there was an internal mistake and the Opti-Spark caused Corvette owners a tremendous amount of aggravation.

One of the goals was to control spark scatter during transient maneuvers, such as acceleration. GM also wanted to eliminate the large timing errors that happen during starting. This was a traditional problem with magnetic reluctance timing sensors.

The Opti-Spark is a distributor with a two track optical position sensor, a keyed drive shaft, an ECM, a single ignition coil and driver and conventional secondary wires and park plugs. Contrary to what many people believe, most spark errors actually occur at low speeds, especially starting, not at high rpm.  Spark errors also occur during transient movements such as acceleration, deceleration and transmission shifts.

A key element in this Opti-Spark system was the increased overall diameter of the distributor which allowed for greater separation between the secondary terminals within the cap. This reduced cap and rotor wear by reducing the ozone formed when the spark is jumping large rotor-to-cap gaps.

The Problem

For all of its innovations, one major problem occurred in the transition to actual production. The casing of the Opti-Spark, which was mounted to the front of the engine, just behind the water pump, was designed with a small hole at the base. This would allow any condensation formed within the distributor to drain out. The design engineering team was very precise about the size of this hole.

After the design was completed the unit was passed on to what is called validation engineering. This group changed the size of the hole without communicating this change to anyone else. The validation team felt the hole was simply too large and would allow water to enter into the distributor. The problem was this new drain hole was simply too small to allow condensation to flow out of the distributor.

Then to make matters just a little worse the validation team changed the composition of the internal components, making them less corrosion resistant. That was all that was needed to make this one of the most failure prone components in Corvette history. Eventually a recall campaign led to all (or most) of the units being replaced.

Today there are several units on the market that incorporate some design changes that make the unit very reliable. This is important since there are no alternatives on the market for the Opti-Spark. It’s not as if you can simply install a different type of distributor.

Petris Enterprises is the only firm that sells the Opti-Spark with all the upgrades. They actually use a flow through ventilation system that circulates air through the distributor cap. I’ve never heard of anyone having a problem with their units.

  • Petris Enterprises’ Optispark unit is equipped with a Mitsubishi-Grade optical sensor module. Our optical sensor is built using components that are equal to or higher quality than the original equipment optical sensor found in factory-supplied distributors.
  • Our Optispark distributor cap has a ventilation port cast into it with high quality brass terminals and high voltage distribution straps.
  • The brass tipped rotor of each unit is secured with Loctite, assembled, and then tested for leakage. The final touch is the installation of a vacuum ventilation hose assembly with the correct check and restrictor valves to prevent distributor cap damage.
  • The Petris Enterprises Optispark kit includes all the pieces and attachments necessary to make the conversion, including manifold fittings, the vacuum hose assembly, and installation instructions. You can rest assured that you will not need to make trips back and forth to the auto parts store to buy another little piece to finish the project.

Just remember to cover the hole when you detail your engine. You have to be very careful when you do any engine detailing with the LT1. A lot of folks have gotten a little carried away cleaning their engine compartments and killed the Opti-Spark in the process. That’s a very expensive mistake.

Before you do any detailing you should take a towel or shop rag and cover the hole in the Opti-Spark casing. I use a shop rag and then duct tape it so no moisture can possibly destroy the unit. You simply can’t take chances.

IMG001IMG001: These dreaded holes create the infamous Opti-Spark problem. Once you kill the unit be prepared to spend some very serious money. I would search out one of the upgraded units – don’t even consider another original equipment part.

IMG002IMG002: This is the Opti-Spark distributor cap. You can easily see how far the terminals are from each other. This greatly improved the low speed performance of the new LT1 engine. At least until the whole distributor failed.

IMG003IMG003: This assembly hides directly behind your water pump and almost directly under the thermostat housing. It’s a shame that the various groups within GM didn’t have a chance to talk to each other and resolve the whole Opti-Spark issue. The LT1 is a great engine – with one huge problem — a problem that could cost you over a thousand dollars.

66.tif: This cutaway LT1 picture from GM shows exactly what is going on with the infamous distributor. While you look at the distributor also look at the spark plug wire routing for the LT1. If you have to replace your spark plug wires you can expect to pay for about four hours of labor. That sort of bill makes me happy that I have an L98 engine.

This is an adaptation from Richard Newton’s most recent book 101 Projects for Your Corvette 1984 – 1996. He has also written two other best selling Corvette One deals with the 1968 to 1982 Corvettes, How to Restore and Modify Your Corvette 1968-82 , while another deals with the Sting Rays from 1963 to 1967, Corvette Restoration Guide 1963-1967 All  of these books are available from Corvette Central.

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