Have about 900 miles on new motor and this problem started about a week ago. Car runs great until it reaches normal operating temp then it misses bad when you take off from a stop or if you give it gas in any gear. I just put a new opti in, put in another MAF which neither helped. It's not plugs or wires I don't think because it runs great until warmed up. It will even run good in closed loop for a while but then will start running like crap again. I can turn the car off and let it sit for a few minutes then start it up again and it will run great for a few miles before missing again. Any Ideas?

 

Thats what mine did when the opti went bad after 2000 miles, But I would have to let it cool down a few hours. The optical disk wasn't running true due to the bearing support being cracked and the disk was rubbing on the plastic of the optical sensor. I imagine that the dust created after a while resulted in the optical sensor not being able to detect the pulses and the car would start missing really bad and within a few mins it was undriveable. Let it cool down and I could go another 20 mins or so. I kept telling myself it wasn't the opti cuz it was so new (had another one fail at 200 miles) so I did the coil and the ignition module, none of which helped. Don't rule out parts because they are new.

Bill

 

The miss was happening with the old opti and I put the new one on and it still does it so I am not ruling it out but I don't think thats my problem.

 

Have you looked at the O2's? Like you mentioned it has to be something that is in the closed loop equation. Hows the coolant temp sensor, have you scanned the car to check for some eronious (sp?) readings from one of the ECM inputs?

 

I looked at my datamaster files and compared to when car was running good and the only difference I see is the AC pressure used to be at 70 and now its at 140 although I don't know what that would have to do with my problem, Also the car doesn't throw any codes when missing. I swapped the coil and Ignition module and alternator yesterday because I have a lot of extra parts around and have been trying just about everything. I will swap out the O2's today.