YEAR(S) - '90-'91

PROBLEM - Some owners of '90 - '91 ZR-1's may experience stalls, rough running and high idles.

CORRECTION - Replace Prom kit, part number:
1990: #16163905
1991: #16193416

SERVICE BULLETIN/RECALL: 476503

These are the latest versions of the LT5 calibrations:

'90 AYBK Corrects high idle speed when the engine is very hot.
'91 BFXB Corrects long cranking time during a hot restart.
'92 BPPA GM never published a description of why this chip was released.
'93 -'95 BMCB Corrects a sag in acceleration at 1800 rpm. Noticeable in 4th gear or higher. Also corrects the intermittent ASR error at start up.

The four letter code is printed on the silver label on top of the chip.

YEAR(S) - All

PROBLEM - When the LT5 is cold, some owners may experience a rapping noise on start up until oil pressure comes up. Once oil pressure is up the noise goes away. Most likely this is the cam chain tensioner (CCT).

No damage is done during this time and the noise is more a nuisance then anything else. CORRECTION - Some owners have repaired or cut down this noise in several ways. Usually the correction is not made with just one change but by making several changes. One is with heavier oil like 10W40 or 20W50. But understand that GM recommends 10W30. Second is with using a different oil filter other than GM's AC PF 970C. There are several on the market that seem to have a better check valve that holds the oil longer in the filter. The new Fram Oil filter (PH-3985) and the NAPA Gold 1324 are two that are the most popular. Opinions seem to like the NAPA Gold 1324. Another but a more costly fix is to replace the right side cam chain tensioner with a newer one, kit part number 12550580. The kit has a housing with a machined in cavity that provides an effective oil reservoir for the CCT.

SERVICE BULLETIN/RECALL - No

YEAR(S) - All

PROBLEM - Some owners when they have changed their thermostat to an aftermarket one may find themselves with a leaking/blown radiator. The problem is when you bring the rpm's above 5500 the water pump will pump at 85 gallons per minute. To much pressure for the plastic tank radiator. The stock thermostats take 5 lbs of pressure to collapse the round metal plate to access the bypass holes. Some aftermarket thermostats take 9 lbs of pressure to collapse and have no bypass holes. End result, the water can't flow though the block to relief pressure from the radiator....

CORRECTION - Stick with the stock thermostat or make sure the aftermarket thermostat meets GM specs.  

SERVICE BULLETIN/RECALL - No

YEAR(S) - '93-'95

PROBLEM - Near the end of 1993 GM found themselves with a weight problem on the 1993 ZR-1. To meet the same EPA class GM had to come up with a way to cut some weight off the '93 ZR-1. They did this by installing a rubber extension on the fuel filler pipe to limit the fuel capacity to 18 gallons. The fuel level float arm was also adjusted to reflect the 18 gallon capacity. All 1994 and 1995 ZR-1's were suppose to once again have 20 gallon fuel tanks. But some owners may find that their reserve light comes on before other '94 and '95 owners but take the same amount of fuel. The problem seems to be that the fuel level float arm that was adjusted for the '93 may not of been re-adjusted for the '94's and '95's

CORRECTION - The tank sender and fuel pumps can be removed and recalibrated (90 Ohms produces a full gauge reading and 0 Ohms reads empty). A new gasket will be needed to complete the repair.  

SERVICE BULLETIN/RECALL - No

YEAR(S) - '90-'91

PROBLEM - Low Tire pressure sending units are known to go bad. Specially in earlier ZR-1's. If you see your LTPWS light come on then most likely you have a bad Low Tire pressure sensor in one of your wheels

CORRECTION - Follow the GM service manual to find and replace the bad sensor.

SERVICE BULLETIN/RECALL - 90-340-3E and 90-389-3E

YEAR(S) - All

PROBLEM - If you have intermittent starting problems it maybe the starter motor (see starter motor problem) but it also maybe the battery. A bad battery may lead you to believe you have a bad starter.

CORRECTION - Try a new battery before taking half your engine apart to replace the starter.

SERVICE BULLETIN/RECALL - No

YEAR(S) - All

PROBLEM - ZR-1's seem to have poor radio reception.

CORRECTION - GM has come out with a radio grounding kit that will improve radio reception. Kit Part number is: 10246147.

SERVICE BULLETIN/RECALL - 339008

YEAR(S) - All

PROBLEM - The LT5 engine can stain easily. Do not use any chemicals or get any anti freeze on the painted surfaces of the LT5. These stains are almost impossible to remove.

CORRECTION - You can try something mild like Simple Green to try and clean the spot. If you have to repaint the LT5 there are two paint numbers, G9889J from Dupont called Cronar and DAU35598 from PPG. But be warned that the color may not match exactly.

SERVICE BULLETIN/RECALL - 92-283-0A

YEAR(S) - '94-'95

PROBLEM - Some ZR-1 owners may experience glass scratching on the side window glass. This can be caused by several different problems. Cleanliness, number of window cycles, etc.

CORRECTION - A service bulletin is out for ZR-1's UNDER warranty that if the window glass gets replaced to also replace the outer seal strip.

SERVICE BULLETIN/RECALL - 53-10-17

YEAR(S) - All

PROBLEM - ZR-1 may start to run rough or won't start. If your ZR-1 is stored outside or has been in the rain you may of gotten water in the fuel tank. This can happen though the fuel filler cap if your drain hose in the fuel filler area is clog.

CORRECTION - Make sure the drain tube is free and unclogged. Take you ZR-1 into the dealer for verification/repair. Also in some cases it has been discovered that the tie wrap keeping the hose from moving around behind the license plate is to tight, crimping the hose and keeping water from draining.

SERVICE BULLETIN/RECALL - No

YEAR(S) - All

PROBLEM - Oil lines running from the oil cooler maybe leaking where they connect into the block just under the oil filter.

CORRECTION - You can either replace the lines with an updated version from GM or do what some members have done. Remove the lines, and clean the ends, both male and female, thoroughly. Take some RTV and put it on the female end (some owners also put some on the male end too), let it sit and get tacky, then reattach and let it sit for several hours to set up.

Thanks to Hib Halverson and Jim Milstead for this info.

SERVICE BULLETIN/RECALL - No

YEAR(S) - All

PROBLEM - When you go and fill up your fuel tank even though the gas gage reads empty, your tank takes less than 20 gallons.

CORRECTION - There is what is called a "bladder" inside the fuel tank. This bladder is not pliable but a rigid black plastic. What can and does happen is this plastic liner pulls away from the fuel tank walls causing bulges or bubbles. These bulges can shrink the size of the liner and therefore reduce the capacity of the fuel tank. The only correction is to replace the fuel tank. Thanks to the ZR-1 Registry, Dennis Gratton and Jim Ingle for this information.

SERVICE BULLETIN/RECALL - No

YEAR(S) - All

PROBLEM - Common problem is for the inflatable seat bladder for your back stops working. The pump is ok but the actuated bellows housing membrane has rupture.

CORRECTION - Instead of replacing the whole pump from GM you can order just the membrane from the manufacture Jasco Products in Sun Valley, CA. Their phone number is 818-504-2516. Ask for Ramona. Also you will find an article on replacing this piece in the May '95 issue of Corvette Fever.

Another fix from member Mike Zeeff:

If your one of the 99% of ZR-1 owners whose seat bladders don't work, then try this procedure. I know you can order the diaphragms from Jasco, but here is a better fix for 99 cents. Some of the info came from the vette net by Jim Mason, but I found an easier way to make the fix. Both of my diaphragms were blown out on my 90 ZR-1. Total time to fix both sides was 45 minutes total.

1. Purchase a pair of latex rubber dishwashing gloves from your local 5&10. Make sure they are the heavy duty kind, and not the ones the thickness of a condom. Also make sure they have a good portion of flat texture on the cuff. You'll need enough to cut out two 1" round diaphragms.

2. Remove the lower seat cushion. There is a wire retainer at the front bottom of the seat. Carefully flip the seat bottom up and out of the way to expose the bladder pump. It is the one with the tubings attached to it.

3. On one one end of the pump you will notice two small phillips head screws, holding on a retaining plate. Remove both screws and the retaining plate. The bottom one is a little hard, but it can be done with a small jewelers screwdriver. You do not have to remove the pump itself.

4. Once you remove the retainer, you can wiggle out the diaphragm assembly. It has three tubes attached to it on one end, and the plunger assembly on the other. If your diaphragm is busted, then the plunger will be separated.

5.The diaphragm assembly, is held together by four small bolts and nuts. Remove all four, and break apart the diagram assembly. You can now take it to your workbench.

6. By this time you will clearly see how the diaphragm works. Pull the old diaphragm out, and use it as a template for your new one. Once you cut out a new one, you can mount it on the plunger. Since there is a screw that retains the diagram to the plunger, it is a good idea to use a small dab of silicon sealer to the screw hole.

7. Re-assemble the entire diaphragm assembly and pump. Be careful to get the plunger back on the motor rod.

8. Test the unit before you put on the retaining plate. You will see the pump motor spinning the plunger. Make sure it is inflating the bladders.

9. Enjoy your sport seats.

SERVICE BULLETIN/RECALL - No

YEAR(S) - 1990-1991

PROBLEM - The following information comes from page 47 of the 9/10 issue of the Registry Newsletter from Gordon Killebrew:

First off, you may or may not notice your radio controls acting odd as well...

"...the cause was a so called "logic loss." The memory programmer in the AC programmer lost track of where it was. Just like a home computer when the power is removed from the system, then re-powered up, the computer did a re-learn and again performed as it should. A bulletin was released to all dealers telling them if this occurred to removed the radio and courtesy fuse for 15 sec. with the ignition off. This did a reset of the AC logic. A defective radio data signal could cause a AC logic loss, or vice versa.

CORRECTION - In 1991, the AC data lines were removed from pin J ALDL data line to the radio. The #1996 circuit then became number 461A and 461B going from the AC control head to the AC programmer. The pin J ALDL #1996 wire became the #720 circuit going directly to cavity C6 in the radio control head's white connector. This resolved the AC radio interface problem, the root cause of the logic loss that was coming from the new AC air mix motor. All 1990 Corvettes have a BULER flat pack air mix motor (part number 16145772). It was causing the logic loss in the AC system. The motor did not have built-in stops and if the tip would break or wear, the motor would go beyond its travel range and was extensively damaged; thus causing the AC system to flash lights and fail. The fiber tip, GM part # 52452753, did in fact crack and wear on many cars. As the cars get older this may happen more often.

In the mid-1991 GM released a new motor with stops, part # 16124952 made by Calasonic.

In 1990, bulletin 90-378-1A was released with the part number for L98 as well as ZR-1 programmers with capacitors to prevent logic loss. In 1991, bulletin 91-200-1A was released stating the corrected programmers for the early 1991 cars.

If you think you may have a logic loss on your AC system review this information and look for the presence of a Buler flat-pack motor worn tip or broken tip. If the indicator lights flash and you do not have the correct AC control, do the following test:

1. Turn the ignition key on, then turn on the AC on Auto.

2. Hold the up and down fan speeds buttons both in at the same time for about 5 seconds, then release.

3. In the digital dash, "-00" should appear . If a number appears, you have a system fault code. Press the FAN AUTO button to show any fault codes stored in the programmer. (Pg 1C1-2 of 1990 service manual.)

Code 01: Air mix door actuator circuit fault.

Code 02: air mix door actuator circuit fault.

Code 03: outside (ambient) air temp. sensor circuit open.

Code 04: outside (ambient) air temp. sensor circuit short.

Code 06: Inside car temp. sensor circuit shorted.

Code 07: Solar (sun load) sensor open.

Code 08: Solar (sun load) sensor shorted.

Code 09: low freon detected.

Code 10: UART or E&C data buss line failure.

Most commonly, a 01 or 02 code will be present. Sometimes there are no codes present."

SERVICE BULLETIN/RECALL - 90-378-1A and 91-200-1A

YEAR(S) - All

PROBLEM - It has been well documented that the LT5 commonly leaks oil out of the header studs side, and leaks down on the cat. If it sits around a couple of weeks, it would smoke badly after about 5 minutes of warm up. On each header side is a stud located in the middle of the header, where it mates to the manifold. The stud is about a 1 1/4" long and has a nut welded on. The stud screws into the block and into one of the oil galleries. With all of the heat generated here, the stud will fail to seal and an oil leak is the result.

CORRECTION - To fix the problem, use the following procedure.

1. Remove the front tire from the side to be fixed.

2. Remove the rear inner wheel well. It has an assortment of Torx and hex head bolts.

3. You will now be able to look at the headers and see the stud, situated between the first 2 and last 2 cylinders.

4. Use a 15mm socket to remove the stud. The nut is welded on to the stud.

 
5. Clean up the stud, and apply a liberal amount of high temp Red RTV to the threads.

6. Very carefully thread the stud back into the manifold. This is the hardest part, as it is very close quarters. Don't drop the stud, or it will fall into the deep crevices of the exhaust tubes.

7. Let the RTV set up overnight, and enjoy a leak free LT5.

8. As long as you have the wheel well out, you might as well clean the air bag crash sensor, located on the frame rail. The right side one gets a lot of AC runoff, which corrodes the grounding. Disconnect the battery ground before removing the sensor. The sensor has 2 different bolts securing it. (13mm and 10mm I think). Once removed, you can clean up the surface and apply some dielectric grease to the surface to keep the water out. The main grounding comes from the bolts, so make sure they're clean.

SERVICE BULLETIN/RECALL - None

YEAR(S) - '90 -'91

PROBLEM - Under hard acceleration you made feel a sudden lost of performance especially on hot days. This could be your air intake duct collapsing. This duct is thin and when hot becomes soft and under hard acceleration can suck close starving your engine of intake air. When you slow down this duct may open again and you start running fine.

CORRECTION - Replace the intake duct with a new one from GM. It's the same part number (#14104555) but seems to be heavier (thicker) in design.

SERVICE BULLETIN/RECALL - None

YEAR(S) - All

PROBLEM - Every once and a while the engine hesitates while accelerating.

CORRECTION - Of course there could be many reasons for this. But one that seems to be well known is AC platinum plugs. Owners have pulled plugs and found no platinum tip on the negative electrodes. Which when measured gave a gap of approx .058 thousandths. Sources at AC have said that the first couple of years they made the "so-called" 50,000 mile plugs they had a steep learning curve with welding the platinum "pucks" onto the center and side electrode. AC said that the newer double-plats are more durable.

SERVICE BULLETIN/RECALL - None

YEAR(S) - All

PROBLEM - Intermittent LT5 hot idle surging, hesitation, backfire or stalling.

CORRECTION - Intermittent LT5 hot idle surging is a chronic symptom of lost intake manifold vacuum pressure. The MAP hoses are notorious for swelling up and compromising the seals and/or filling up with oil residue and fouling this sensor. IMHO, there is some kind of a relationship between the MAP and idle air control. If the MAP sensor is sending the wrong reading to the ECM, it may try to compensate by increasing (or conversely decreasing), the IAC opening, thereby inducing a fast idle/surging (or engine stalling) condition.

Please check those MAP hoses for loose fit. If swollen or easy to pull off the fittings, put some clamps on all hose fitting and/or replace with higher quality (thick wall, fuel grade) rubber hoses. You might also consider replacing the MAP sensor itself, since it could have developed an intermittent failure mode internally.

Thanks to Dennis Duchmann for this info.

YEAR(S) - All

PROBLEM - You find you can't shift into gear and your clutch pedal feels funny.

CORRECTION - For months you drive happily along winding through the gears and having the time of your life. Then, one day, you can't shift into a gear and your clutch pedal feels funny.

So what's wrong?

You have fallen victim of the infamous clutch slave failure. What happens is that the seal within the cylinder has worn and fluid will begin to leak out. It does not do this in a major way usually, but rather in very small undetectable amounts. The clutch will feel normal until all the fluid has leaked out of the reservoir. Then you will experience a very light clutch feel with it engaging further and further to the floor until you can't shift.

To tell if you have a failure there are two quick checks to do to verify if it is the slave. The quickest is to jack the car up on the left driver side (use jack stands) and look at the transmission. On the left side there is a small black metal cylinder that connects to the transmission. That is the clutch slave. Run your finger along the bottom and if there is fluid there, then I would strongly suggest the slave cylinder has failed. Next, remove the ECM module and lift it up. Under there is the reservoir for the clutch slave cylinder. If that is empty then all signs are you have lost the slave.

If you are in warranty then have the dealer replace it. A word of caution here, there may be some dealers that will only fill the reservoir up and not change out the slave. You will be able to go for perhaps several months before another failure. Be sure the dealer does indeed replace the slave cylinder.

To replace the clutch slave cylinder yourself then the task is fairly easy. There are two bolts holding on the cylinder. You will need more than likely two universals to get to the top bolt. Once removed, you will remove the line going up to the reservoir. All the fluid will be gone so don't worry too much about that.

Next, take the new cylinder and attach it to the reservoir line. Go up to the engine bay and fill the reservoir with brake fluid as specified by the manufacturer. The cylinder will then need to be bled. Turn the cylinder upside down where the bleeder screw is facing up. Open the screw and allow fluid to fill the cylinder. Close off the screw but leave the reservoir open.

When installing the slave cylinder to the transmission you will need a bit of muscle but not much. Put both mounting bolts in the cylinder and guide it up to the holes. You will need to push a bit as the plunger will need to be retracted some. Push the cylinder (retracting the plunger) onto the transmission and get both bolts started. Once both are started then you can tighten them down. Do not tighten one side fully, then the other. Make a few turns on one side, then a few on the other until both are tightened to recommended torque specifications.

Now that you have installed the slave all you will need to do is to break the internal plastic retainer and pump the system up to pressure. Replace the top of the reservoir and simply pump the clutch. Pressure should return quickly.

Thanks to Jim Milstead for this information.

YEAR(S) - All

PROBLEM - Not really a problem but a way to lower ET's by allowing a quicker clutch engagement.

CORRECTION - The clutch slave cylinder modification is used by several aftermarket suppliers. According to literature, the modification allows a quicker clutch engagement giving the owner lower ET's and less clutch wear due to less slippage.

To modify the clutch slave it is highly advisable to use a new slave cylinder. Take the slave cylinder apart taking extreme care not to break the round plastic retainer inside the cylinder that holds the plunger in. Also, there is a small clip inside that can be removed and replaced with no special tools but you will have an easier time of it with a pair of compression pliers with the ends being needle points.

Once the cylinder is disassembled, look into the fitting where the reservoir hose attaches. Inside you will notice a small brass restrictor. This can be drilled out to the size of the larger hole (drill out until you see no more brass). Clean out the cylinder thoroughly and put it back together. Be very careful again not to break the plastic retainer on the plunger (this will break off once you have installed the cylinder). Once you have gotten it back together just install as described in the file on Clutch slave.

Thanks to Jim Milstead for this information.

YEAR(S) - All

PROBLEM - LTPWS sensors in high speed running can brake loose and cause your tire to become unbalance or unstable. The LTPWS sensor plastic case is simply attached to the stainless strapping band with plastic tangs and is no way up to the centrifugal forces that ultimately come into play at high speeds. Merely the act of breaking loose inside a mounted wheel and tire assembly at high speed can cause further damage, potential quite serious, to the tire casing.

CORRECTION - If you are going to do a lot of high speed driving like in the Pony Express 100, you should remove the sensor from each of the tires.

Thanks to Dennis Duchmann for this information.

YEAR(S) - All

PROBLEM - Many ZR-1 owners have experience over heating problems more so in the warmer climate areas of the country. It can't be pointed out as really a design problem since in most cases the problem can be cured by following some simple maintenance practices.

Let's hear from one expert.

The truth is, during normal street driving, your cooling system will be just fine...that is, if it's properly serviced and in working condition.

What a lot of people might not fully understand is that our Corvettes are "bottom feeders" and rely heavily on clean, clear unobstructed airflow through the radiator and operational fan motors.

The term "overheating" itself is somewhat a problem to begin with. The first question out of our mouths when a customer says the word "overheating" is "how do you know?".  Most everyone believes their engines are overheating when they go above 230 degrees or in our case "the needle goes WAY up there!" In reality, some may call 210 overheating if their gauge is slightly amiss ...

The truth is that until your car either boils over or pegs the needle causing the ECM to cancel the secondary throttle, your engine is NOT actually overheating. Running too hot for our liking? You bet!

The Corvette will actually get it's hottest under normal circumstances while idling or while driving slow in stop and go. This is simply because air is at minimum flow through the radiator. This is considered normal. If your Corvette gets hotter when operating at highway speeds, then it is not normal. There is a restriction, inadequate air deflection into the radiator, or a plugged radiator itself.

Here's some suggestions.

First, look up into the cavity and look for the obvious. (No, the Radio Shack bag is NOT suppose to be there.)

Next. Remove the top of your radiator shroud and look down in between the AC condenser and the radiator. CAREFUL! There may be animals in there! See all those shade tree leaves, rocks, gum wrappers, sand etc? Guess what. One of these days we're going to invent a new procedure to judge these waxers and many will suddenly not over heat any more....:-)

Here are some pictures of one such radiator:

Radiator from top.
 

Radiator front view.
 

Radiator side view.
 

Radiator bottom view.

Now, drain the coolant and remove the hoses. Pull the radiator out and take a good look at the bottom rows of fins. Hummm, know why you can't see through them? 'Cause they're plugged solid just from normal driving. You see, every single little piece of pebble that gets pulled underneath your front end gets sucked right up by those AC Delco fans right up into your radiator. Anything larger than a spec? Gets caught right there into the fins. When they're totally plugged? Then they go higher... and higher... Radiator replacement is the best cure here.

Here in the desert wasteland we see temps of 125 in the shade, 160 (and higher) road temp. Yes our cars run hot (240 at the ECM sensor) but that's just reality. We take steps to get ready for our summer just like many of you do for winter.

Not too long ago, I had the opportunity to purposely take one of my engines on the track and peg the temp gauge. Soon the "Check Gauges" light came on. A few moments later, the secondary power canceled. I continued....(because I was going to overhaul the engine anyway). I drove another two laps and still had no other problems.

Upon disassembly, I noticed the bottom of each cylinder's head gasket has some distortion but still did not let go. Now, what did you call "overheating" again?? :-)

The bottom line is simply this. New technology has raised the working cooling temps beyond where we are comfortable. Do LT5's (and most any engine) perform better at lower temps? Absolutely!! T-stats and radiators are highly advisable if you live in a warmer climate. Are they required to make up for inadequate GM engineering? Hardly.

Last point. The AC condenser may also become plugged and therefore obstruct incoming air. Check it out by putting light behind it (when the radiator is out) and then look at it from the front. Also, the original cooling fans in our cars have holes drilled in their cases. If you drive in wet weather or have a habit of shooting water into your radiator area, you probably have cooling fan motors which are damaged or not operating a maximum efficiency.

The updated motors, AC Delco #15-8404, (both motors are the same number) have a new sealed motor assembly and higher operating rpm's. In addition, there is an accompanying new fan blade assy, AC Delco #15-8468. We can buy these parts in bulk but I don't have the prices handy. If you are interested, drop me a note.

Thanks to Jim VanDorn from Auto Masters for this info.

NOTE: Check your mixture of Anti-Freeze and water. Try to mix in the highest percentage of water possible depending on your climate. Also try adding a bottle of "Water Wetter" form Red Line.

One of the very best things you can do as mention above is to clean your radiator. Here's a procedure member Marc Haibeck has:

I remove the radiator, and use water to blow it out.

Removing the radiator is not easy. I can provide tips for the two hardest parts of the job. After removing the six difficult to access 7 mm hex fasteners in the front, relocate the clip nuts.

Slide them off the radiator cover and place them on the panel that the cover mates with on the car. This will allow the fasteners to be reinstalled from the radiator side, making the reassembly much easier. For alignment of the clips with the holes, it will be necessary to trim a 1/4 inch strip from the flange before installing the clips. Thanks to Jim Milstead for this great time saving trick.

After working hard to pull the radiator cover off the area where all of the lines emerge, it will be even harder to push the cover back over the lines. To make it easier, make the lower rear edge of the opening for the lines larger. Trim off a 1/2 inch wide by 1 inch high section from the bottom rear edge of the slot. This is not necessary but it helps tremendously.

Also, the cover is of course, made of fiberglass. It is a good idea to wear gloves when handling it.

YEAR(S) - All

PROBLEM - You may notice an oil leak on your garage floor and/or your oil pan is wet.

CORRECTION - Check and tighten your oil pan bolts. You may even find some missing bolts. Check Service Manual for proper order and torque.

YEAR(S) - All

PROBLEM - Under the plenum you will see a crankcase ventilation cover. Check the bolts for tightness. Many owners have found these to be very loose which can cause sludge buildup in the "V" area on top of the engine block. This sludge may also clog the drain hole and damage the starter.

CORRECTION - Check and tighten the crankcase ventilation cover bolts. You may also want to replaced the gasket as well.

YEAR(S) - ALL

PROBLEM - The 3rd brake light or CHMSL stops working even though your other brake lights are working.

CORRECTION - It's unlikely that all the bulbs are burnt out unless over a long period of time each has burnt out. If you own a '90 - '92 ZR-1 and suddenly find that your 3rd brake light is not working you may want to first check your brake switch. The reason being is the early ZR-1s brake switch has in fact, "two" switches inside. One just for this 3rd brake light. In most cases where an owner has found a non-working 3rd brake light the problem was a bad switch. For later model ZR-1s '93 -'95 the 3rd brake light does not have a separate switch and therefore if you do find your 3rd brake light not working you indeed may have bad bulbs or a wiring problem.

YEAR(S)- ALL

PROBLEM - Secondary actuators not opening all the way or the new actuators from GM don't open correctly.

CORRECTION - In the original LT5 engines from GM it has been found that some of the actuators are maybe upside down causing the actuators not to open fully. The fix is to just turn them upside down.

According to Gordon Killebrew: "If the actuator lever, that extends from the center opening of the actuator and is attached to the bladder within the actuator, is sitting at an angle, it is 180 degrees out of correct position (on backwards). The actuator lever end that is attached to the actuator diaphragm should be pointed to the center of the engine. When installed correctly the lever is in a straight line with the port throttle arm."

However to correctly determine if they are opening correctly or not is to remove the plenum and use a vacuum gauge to measure how much vacuum it takes to open them. Check your Corvette Service Manual for correct specs.

The second problem is the new actuators from GM (the diaphragm piece is red) are not built correctly. The problem is the spring is not centered on the post in the back of the can. The actuators either have the spring on the post or off to the side. The one's that are off to the side bind as the arm is drawn in. They are either good (spring on the post) or bad (spring off to the side). There's no in-between.

NOTE: that the arm must retract into the can all of the way to the taper on the arm for full opening of the secondaries.

YEAR(S)- 1995

PROBLEM - Vibration occurs at about 95 mph with a 4.10 axle ratio and at about 120 mph with the OE axle ratio. On most cars there is about .010" of play at the
u-joint when the transmission yolk is pushed up and pulled down. I learned that GM became aware of this problem on a few '95 Corvettes.

CORRECTION - They solved it by using a yolk that fits tighter on the transmission output shaft. With the new yolk, the play at the u-joint will be about .002". The yolk is not available as an individual part either from GM or the company that makes it, Spicer. GM used it on the drive shaft assembly for all '96 Grand Sports. The part number for the GS drive shaft is 12557818. My experience has been that on two cars it solved the problem. On two cars it provided about a 75% reduction in the vibration.