girvin02

I am ready to push my car into a lake, I am so frustrated. If anyone can help me out, I will be forever in your debt.

Here are the symptoms:

Car runs beautifully when engine is cold. Once engine reaches normal operating temperature (190-200*) still runs great, until I floor it - then the engine cuts out/hits a wall at exactly 5,000 RPM. Feels like it's hitting a rev-limiter and takes a second or so to hit the right RPM to shift into the next gear.

This has been an ongoing problem, so let me give you the full story so that you can give me an informed opinion...

My 1994 LT1/A4 Corvette currently has 92K miles on it. When the car had 83K miles, it started to run rough - bouncy idle, poor performance. My research, here and abroad, lead me to suspect it was a faulty optispark distributor. But before I invested the time/money into such a repair, I decided to replace all of the easier/less expensive items first.

One-by-one, I replaced the following items:
• Ignition control module (Duramax)
• Ignition coil (Duramax)
• Spark plug wires (AC-Delco)
• Spark plugs (AC-Delco)
• Fuel filter (AC-Delco)
• Air filter (AC-Delco)
• PCV valve (AC-Delco)
• O2 sensors (Bosch)
• Coolant temperature sensor (AC-Delco)
• Matched fuel injectors (Ford Racing 24# “blue tops”)

When none of the above items cured the problem, I decided it was time to replace the dreaded optispark distributor. Here's where I made a mistake, however. At the same time, I added 1.6:1 roller rocker arms. I now realize this was a bad idea (at least until I got the car running properly). When I added the roller rockers, I changed over to a LT4 knock sensor.

I cheaped out and purchased a rebuilt Opti that was made in Mexico and was not OEM. The car ran as it does now - great when cold, with a 5,000 RPM miss at full throttle.

After reading that the Mexican Opti's were inferior, I bit the bullet and purchased an AC-Delco unit, which, to my dismay, showed up in a made in Mexico box! I installed the new Opti, and the car runs exactlly the same.

So either I am the most unlucky guy in the world and got two bad distributors, or the Opti is not the problem.

I suspect the problem is one of the following, but don't wish to throw any more money at this car until I get some advise.

• Bad Ignition control module (Duramax)
• Bad Ignition coil (Duramax)
• My car doesn't like the LT4 knock-sensor
• Bad Optispark distributor (just shoot me)

Again, the car runs great when cold. I took it to the track a couple of weeks ago and it ran consistent 13.82's - 13.88's when I let it sit for 45 minutes between passes. But once it warms up, it cuts out above 5,000 RPM and runs in the 14's.

Please help!

93Rubie

I hate to say it but high temps and cutting out at high RPM tend to a be an ignition problem. Something is changing at only high temps and just because something is new does NOT mean it is good. I would test the coil, wires, plugs, ignition module, Opti-Spark, when hot. However, I have seen enough weird stuff to know better. It could be anything, given it happens only hot, you need to do your testing when it is hot. I would verify that your fuel pressure at WOT hot is ok to start. Also, I would verify you are not running against the rev limiter, maybe the transmission is holding a gear, when it should not be, when it is hot??? The stock tach can be inaccurate to use a good scan tool.

Also, see if you can get some good readings from all your sensors when hot and at WOT, you might need a friend for this. Verify, they are within specs.

For grins and giggles, did you put on new valve springs when you did the 1.6 rockers. If you did not, those old valve springs with 1.6 rockers could cause a problem.

Best thing to do is get a almost zen like mind and proceed logically. When that does not work, go and then back to the zen like state.

girvin02

Thanks for your reply 93Rubie. I also strongly suspect it is an ignition problem. I have confirmed that fuel pressure is good at WOT. I did replace the valve springs at the time of rocker arm swap. I'm pretty sure that the problem occurs well before the engine is close to the rev limiter (with my butt-dyno) and don't think it is a transmission problem. I guess I will be spending this weekend getting intimate with my multimeter.

AVL94LT1383

A Few things:
1. The 1.6RR could have put you in coil-bine.. prob. not but any time you push the lift on stock spings 500 or more you usually will. You symtoms are not indicating that but check. Especially if you did not change out to new springs that can take the lift as prev. post indicated.
2. Purcahse tuner cats and cable for your PC and look at tune along with purchasing datamaster for logging. This will tell the story with no more guessing.
3. If you have the orginal LT1 knock sensors put them back on. There is very mixed recogmendations for changing out sensors to LT4. Again the datamaster and tunercats will confirm what is working and not along with the ability to electronically correct issues on you 94 LT1.

girvin02

*** PROBLEM SOLVED ***

I can't tell you how happy I am right now. I have been fighting this issue for 2 years, but today I figured it out.

IT WAS THE COIL!

I tried to test the resistance of the coil but my cheapo multimeter isn't sensitive enough to perform this task. So I went ahead and threw in my old coil (original w\ 83K miles on it), which I had put on a shelf and saved, fortunately. I then drove about 10-15 minutes to the perfect testing grounds - industrial complex, after hours on a Saturday. I turned that complex into my own personal race track. After many hole-shots, I got the coolant temp up to 227 degrees and the oil temp up to 225. These temps are quite a bit higher than my car's normal operating temperatures, and the engine ran great the whole time!

Lesson's learned:
1. Never replace more than one item at a time, between testing, when trying to fix a poor running engine.
2. Never put another Duramax brand part on my car.

93Rubie

Good to hear. That first lesson you learned is a very important one,keep the variables to a minimum. #2 never assume "new" parts are good parts.