I am new to these cars and I guess I have a lot to learn! My "new" 126K mile '94 Z28 runs pretty good for several minutes, then will develop a miss while cruising, a miss/surge under hard acceleration (any RPM), hesitation from an idle, and will hunt up and down (slowly) for an idle all the way up to 2000 rpm. The co-worker I bought it off of said that a GM dealership diagnosed it as a bad fuel pump. Well, I just got done installing a NEW fuel pump about an hour ago and it still does it! I also tried another MAF, but to no avail. Doesn't set any codes (no SES light anyway), and when I disconnected the TPS sensor the idle levelled off, but at the same high 2000 rpm. I don't know if that is just what it normally does in "limp" mode or if it is indicative of something. Whatever it is, it apparently happens once everything is heatsoaked. Any ideas? Thanks,

 

Try this, disconnect both of your O2 sensors, this will cause the car to constantly run in open loop, that is most likely why it runs fine at first, since for the first X minutes until the O2s warm up it is in open loop (a set table of air/fuel ratio values), it is most likely running like crap because once it tries to use the O2s to adjust the BLMs according to the TRUE A/F ratio in the exhaust, it isnt getting a correct reading from the O2s. If it runs fine when they are disconnected get a new set of O2s, you only have 2 of them in a 94 by the way.

 

Jon: I'll keep the O2s in mind. Don't know about these cars, but my Buicks always go into closed loop after just a few minutes, and the symptoms I'm having in my Z happen after about 15 minutes of driving.

Based on further research on this board, I replaced the coil and IAT (only $25 total) and it seems to run a "little" better, but that may just be my wishful thinking. Still runs like crap. Next most recommended item is replacing the Optispark. Even a recent article in GMHTP states two of my symptoms as possible bad Opti signs: long cranking times (happens to me more often than not), and poor performance after warmup.

Thanks, Jay

 

There is a good chance it could be the opti, but I usually like to rule out the cheaper options first, two new O2s are only like $60 compared to about $300 for the opti, and you can check the O2s for free, no way to test the opti.