I have a ’92 LT1 auto. and I need help desperately. About 500 miles ago my engine started to miss and then I even had problems to start the engine. I bought a new opti-spark and I also found a broken wire from the connector that attaches to the ignition coil. I thought I had solved my problem.
After repair the engine starts and runs without load up to about 3700 rpm very good. Then all of a sudden the engine starts to miss and at the same time my tach needle starts jumping up and down. Up to now I changed the spark plugs, ignition wire and the ignition module. I haven’t changed the ign. coil yet. It must be an electrical problem! Why is the tach needle jumping up und down (about 500rpm)? There are no codes set and I don’t know where to look after anymore. The repair manual doesn’t tell me very much either. Driving a corvette this way here in Germany or in any other place of the world isn’t very satisfying. We have no adequate dealers or shops nearby that can help me with this problem.
:confused:

 

I had this problem, a stumble at 4000 RPM. Turns out it was a combination of a bad opti (rusted inside rotor, and broken bushing/bearing?), old plugs, and an coil to distributor ignition wire that arc'd from the distributor boot to the water pump housing.

But you say you have a new opti....so hopefully its not that

Maybe you just need a new fuel filter or something that simple. I replaced mine just to be safe when I first got the car.

Next problem I had that caused stumbling was low (37psi instead of 41 to 46 psi) fuel pressure. This caused problems when driving long distances in say 5th or 6th gear around 2000 to 3000 RPM. What happened there is during normal driving (no accelearation) it would stumble and shake. A new fuel pump fixed this.

When my O2 sensors went bad, I also got stumbling(until my plugs eventually fouled, and the car would hardly idle)

I would next look at the O2 sensor readings from a scan tool. the mV should fluctuate between about 50 and 900 mV during closed loop operation.

Maybe you need new O2 sensors?

If you don't have a Scan tool then pull a spark plug: see if the white ceramic insulator below the electrode is black. If so, you're probably running rich, normally caused by bad O2 sensors.

Hope my experiences with my 1993 hope you solve your problem

 

Optispark, $300 or so.

 

thanks for your quick reply. Can you name me a good scan tool for up to about $300? I'll try that with sensor as soon as I can. Thanks again!!

 

Some of the members have the Autoxray for about $150- $200 depending where you buy it. I got mine for $159 + tax. Works great.

 

For what it's worth,
I own a 92 Coupe and when I bought it, it was the most ill performing LT1 on planet earth. After going through the normal routine of replacing all the ignition components and fixing vacuum leaks, I finally broke down and replaced the injectors. Believe it or not, the injectors were 99% of my problems. At the time, I had roughly 80K miles on the original injectors. When I pulled the old ones out, I noticed that 2 of them had been previously replaced.

Some might argue that fuel problems won't cause an erratic tachometer reading. In my case, the tach was very erratic when the injectors failed. I don't think that the coils were breaking down, I think that the pintels were worn out and fuel delivery was very inconsistent.

Food for Thought,
Purp