redzr1

A couple years ago I had replaced the original optispark on my 92 auto Corvette with an AC Delco unit while trying to resolve a "rev limiting" that the ignition system was going into around 3000 RPM. The problem was finally diagnosed as a problem in the ECM instead of the original optispark being bad. I had also at that time put a new water pump in that had a lifetime warranty.

Now two years later I have decided to sell the 92 auto Corvette due to having purchased a somewhat beat 92 6 speed Corvette. In preparations to sell the 92 auto, I found that the water pump was leaking. I had figured that while replacing the water pump with a warranty replacement that I would swap back in the original optispark and save the newer AC Delco one in case the one on the 92 6 speed has problems. Unfortunately when getting everything back together with the original factory AC Delco optispark the engine ran very bad with a lot of misfiring and backfiring. I drove it for around 40 miles in this condition while trying to understand why it was running so bad.

My next step was then to go in and replace the old optispark that obviously did not want to work well with a unit from Chandler Motorsports so that I could still keep the one-two year old AC Delco one as a spare for the 6 speed 92. After putting the Chandler Opti on, missing of the engine was still being experienced but around 1/3 to 1/2 as bad with the original AC Delco Opti. Due to this I decided to verify that all the plug wires were installed in the right order, which I did three times to be sure nothing was wrong. With still no improvement in the misfiring, I then removed the Chandler Opti and put the one-two year old AC Delco opti back in and it ran just the same as the Chandler Opti. With both of these optis installed the car would only run an average MPG of 19-20 at 55 MPH, whereas when everything was running good this was more up in the upper 20s. Basically I don't think there is an issue with either the Chandler opti or the one-two year old AC Delco one that is causing the misfiring. I also swapped in a spare ECM that I had when the Chandler opti was installed and the performance of the engine did not change. I also checked the four wire cable that goes to the opti and all were 0.1 ohms as that is found to be one of the causes of misfires. Plugs and wires were replaced with new ones at the time of resolving the "rev limiting" and are thus less than two years old.

I would welcome any insight in what I should do to resolve what is causing the misfire. Is it possible that some of the plugs could have been substantially fouled during the 40 miles of driving with the original opti that was causing a lot of misfiring and backfiring. My next steps are:

* Swap in a new ACDelco coil
* Swap in a known good Ignition Control Module (don't expect this to resolve)
* Disconnect the injector on each cylinder to determine what cylinder misfire is present in and then check wires to any suspect cylinders
* Replace spark plugs, starting with those indicated as having an issue in the above test of each cylinder.

Regards,

Bill Hulka

redzr1

I put in new spark plugs and wires, which resolved the engine missing. All eight of the pluts were black, with one even having black residue peeling off as chunks. My theory is that when I put the original ACDelco opti back on, it ran the engine so bad that it fouled all the plugs. The plug wires were within +/- 10% of the new ones in resistance, sos I don't expect they were an issue but it was good insurance to change them at the same time.

mixalive

There you go..

kimmer

Bet you wish you did the p&w first.

redzr1

The water pump was leaking from the impeller shaft, so it had to be done no matter what. On the LT1 I would rather swap out the opti than change plugs and wires. Getting to cylinders 6 and 8 are a real pain with having large hands. It will probably take two weeks for my knuckles to recover from how banged up they got.