My '89 Iroc Z is a TPI 305 5 speed car with about 203,000 miles. The car is completely stock except for a Hypertech chip which was installed in 1990. The plugs, wires, cap, and rotor were all recently replaced. The car runs and accelerates perfectly, hot or cold, with one exception. When the car is fully warm, like after a 25 minute drive on the freeeway, at idle, there is a slight intermittant misfire. If I turn on the A/C, the misfire will go away. Also, if I disconnect the connector with the tan wire that you disconnect when you are setting the timing, the misfire also goes away. Does this sound like the Electronic Spark Timing (EST) module is not doing its job and needs to be replaced? Is there any other dianostic procedure that should be performed? Any other ideas on how to cure the rough idle when fully warmd up?

 

If the mis is only at idle with no load on the engine and it isn't causing any problems with driveability or fuel milage I wouldn't be too concerned. I have seen brand new cars fresh off the lot with an intermitent mis and have had no other broblems at all.

James @ F-Rated Performance

 

Have your injectors cleaned, early signs of clogged injectors always show up at idle first

 

Thats not a bad idea, but word to the wise. Don't get clean happy, every time you clean the injectors some of the protective coating on the windings of the injectors gets washed away.