The car is an '89 Iroc Z, 305 TPI, 5 speed, with about 60,000 miles on a rebuilt longblock. Everything else has about 202,000 miles with the exception of the alternator and waterpump. The car is absolutely bone stock right down to the GM muffler. The car runs excellent, has good power and gets great gas mileage. Idle is fine when the car is cold but after a 1/2 hour trip on the highway, you can detect a misfire, or multiple misfires at idle only, and the exhaust smells rich at idle. The unusual thing is that if I turn on the A/C, the idle smooths right out. I am not getting any codes, I replaced the original oxygen sensor with a new one from GM, and ran fuel injector cleaner through with no changes in the symptoms. The coolant temperature sensor checked ok over a wide range of temperatures. Anyone have any clue as to what is causing the misfire at idle only when the engine is hot without A/C on? Any and all ideas are welcome.

 

check your plugs and see how they are. also... you mentioned that it the miss seems to go away when you put the ac on? thats probably because the ac raises your rpms a little to work and you cant notice the miss when the rpms are a little higher than idle. like i said, check you plugs, wires, cap and rotor. i once had a miss at idle, checked the plugs and didn't really notice anything except that it looks like it ran rich. changed the plugs and the car ran perfect. does it feel like it bogs just a little when your comming off the line? make sure all the plug wires are seated right on the plugs and on the cap. take off the cap and see how the cap and rotor looks... it's always good to replace the cap and rotor around once a year or so... same thing with the plugs.

 

I didn't mention this in my original message, but actually changing the plugs (AC R43TS which is the factory plug according to my GM service manual) was the first thing I tried when the problem first appeared. Wires, cap and rotor were the next things that were changed since they were due to be changed anyway. None of these items made a difference in the symptoms. There is no bog off the line at all. Other than the miss at hot idle with the A/C off, the car runs flawlessly. Anyone have any other ideas?

 

Sounds like your coil is going bad... when they start to go bad they generally run crappier when it gets warmer. Try swapping it with a known good one..

 

Ok, I swapped the coil with the one on my friend's car and there is no change in the symptoms. Anyone else with any ideas?

 

ok it's a fule injected car. buy some fuel injector cleaner. run it though thier. and another way to test if the injector is bad. is go though each and every inject unplug it and if it get worse then that ones fine. if it stays the same the that ones bad. but try the injector cleaner first

 

got a few ideas you might try, i myself have an 88 iroc-z with a 5.7 tpi 98,396 original miles. i bought the roc june 2003 and tuned it to it runs flawless. i too had your same symptoms as well and changed everything out cap, rotor, plugs, wires etc. i found though that i know not many people on this site approve of them but bosch plugs and wires cured the problem right away. i saw that with ac delco plugs and the rapidfires they did not burn as hot as the platinums. i believe a hotter burning plugs assuming you have the stock ignition require this to achieve a more productive spark. like was also said make sure to change out your coil, get yourself an aftermarket h.o. coil. also check your pickup coil connections to and from the ignitiom mod. if there bad the spark will stay within a primary not secondary pathway. another thought is to change out your escm if bad will not advance your spark timing when the engine is in closed loop. i also wanted to ask have you changed out your maf relays especially the power relay. it seems that with your car there is more than one sensor or relay throwing the ecm off and cannot ignore or bypass the signle. final thought have you jumped the adl terminal to see for codes these ecms are very reliable in my book. hope the info helps tim