Cylinder 3-4-5 misfire

[nkarasch]

My 99 GTP is having misfires on cylinders 3, 4, and 5. Cylinder 4 is having about twice as many misfires as cylinder 3 and 5. The only thrown code is for random misfire, it never specifies cylinders. It only causes problems when I am hard on the gas, gentle acceleration works without issue.

This problem has persisted for around two years now and hasn't gotten any worse. I've tried attacking the problem a few times and can confirm with near 100% certainty it is not a plug/wire/coil pack problem nor is it a vacuum leak. I took it to a GM certified mechanic and he didn't find the problem but suggested I change the injectors, is this likely the problem? How can three injectors go bad at the same time (I think it works this way?) ?

The best advice is to take it to a junkyard but any other advice would be appreciated, thanks!

[Fivefingerdeathpunch]

Possible ICM issue could be present.

If you can swap an ICM from another car just to try it, it would be worth it.

[nkarasch]

Possible ICM issue could be present.

If you can swap an ICM from another car just to try it, it would be worth it.

Thanks, I've never heard that suggestion. I'll have to make a junkyard visit in the near future.

[BoostedPontiac]

Ive got a couple sitting around if you'd want one. Just PM me.

[nkarasch]

I put a different ICM in today, no improvement. Further testing shows cylinder 4 misfiring at idle but under a load it either isn't or isn't severe. Cylinder 3 is a bit, but cylinder 5 goes haywire.

[BoostedPontiac]

How do the plugs on those cylinders look vs the non misfiring ones? You could also try switching the injectors around and seeing if the misfire follows.