I have a 92 z28 with the 5.7 l98. In the past few months, getting a little worse as time went on, my car started running very odd when warm. Cold start is amazing, it starts and idles like a champ, but after driving for awhile at operating temp it starts to run a little rough, and it gets worse if i stop the engine then start it again soon after. Sometimes when it is at its worst, it will drop rpms really fast down to around 450 while braking and coming to a stop then it jumps back up and begins to run rough. This doesn't always happen. It'll happen for a day to a few days then quit for an amount of time i cant really measure, it's different half the time. I just want to know what you guys may think this issue is. Something that could be related, but possibly random, i was driving on the highway for about 40 minutes, when i stopped at a gas station for a drink. I went back out to my car to start it and the car wouldnt even crank. I lifted the hood for around 10 minutes and tried starting the car again and it started right back up and drove home, never had this issue again, but i believe it may have been a heat soaked starter. I should also note, that when i bought this car in 2014 it had a misfire, and after changing injectors and spark plugs, and some other small things it ran like a champ. I'm hoping this issue doesnt have to do with injectors or spark plugs because theyre new, but if it is the issue I'll gladly change them. Thanks for your responses, I'll be glad to answer any questions about the car to help pinpoint the issue. It's really began to bug me.

 

If the ignition module on the distributor gets hot can this be the cause? I think you need to scan for codes.. I had a bunch of 5.0 Mustangs and they would crap out. You had to keep an extra module ($90) in the glove box w the special tool just in case. I'll let the experts chime in.

 

Bump.

Also having similar issues. Would like to hear what other people have experienced and how they resolved the issue.

 

Had same type of issue with my 92 vert. Check egr and ignition coil

 

Could possibly be your egr valve being stuck open. Having egr at idle will cause motor to run rough.