I have a P0301 misfire on Cylinder 1 on a 2004 pontiac grand prix with a 3800 series II engine. It is present at idle and disappears under higher RPMs. I have changed the spark plugs and wires. I have swapped the fuel injector (misfire stays with #1). I have swaped the coil (misfire stays with #1). I have replaced the internal manifold gasket and plenum gasket as I already had a small intermittent antifreeze leak and it had no effect. I have cleaned the mass air flow sensor. I have checked the EGR valve. I have changed the MAP sensor. I did a compression test and the cyclinder had good pressure and held the pressure. The only time the misfire disappeared and it was only for a short period of time was when I had a full tank of gas but it returned when used only an 1/8th tank. Since the misfire is only related to cylinder #1, I wasn't thinking it was a weak fuel pump or a clogged fuel fliter but that the issue was related to the cylinder itself. Any suggestions on solving this misfire is greatly appreciated!!:

Check your fuel pressure just so we can eliminate it.

Well, new plugs and wires, swapped injectors and coils.........I'm not thinking fuel pressure if it'* only one cylinder......got me thinking engine mechanical.........

But the misfire is there more at low rpms than high rpms?

Have you done a spark output test with a spark tester? Could be an ICM problem?

At 55mph, while cruising up a slight grade, do you feel a slight misfire as you lightly accelerate?

This misfire only occurs at low RPM and nother in motion. It is more noticeable when in gear than in park. The misfire will disappear as soon as any acceleration occurs.

There is no misfire going uphill at 55 MPH with slight acceleration.

Took out the ICM and went down to the local chain auto store and had them test the ICM. They said it tested ok. To rule it out, an ICM from a '97 grand prix was put in. So far no misfire is being detected.

It appears the misfire has been solved. If it occurs again, I will post again. Otherwise consider it case closed.

Thanks for the help.

It could have been the ICM, they can test to be fine, but still be malfunctioning.

The misfire has returned. Same symptoms as before. It seems that the replacement ICM has failed. Question is what would cause the second ICM to fail? I don't want to keep putting ICMs in just to have them fail because of some other issue. Any thoughts??

Bad grounds, and or cracked coil packs can cause issues where the charge shorts and makes its way back to the ICM which can fry it.
I have not experienced that myself, but I read about it here.

https://www.reddit.com/r/MechanicAdv...oing_bad_2005/

I'd get a multi-meter, set it to ohms, and check the ohms of each coil pack, the resistance should be close to the same for each, there is a spec resistance mentioned, but it was in my manual, and I lost it.. If you have a manual you may be able to find it in it.
The reason I suggest that is from what I have read, a failing coil pack can kill the ICM, that is a new one on me, but others say it has happened to them.

Wait, we don't know the second ICM has failed, just that it possibly sees the same symptoms as the first ICM and is reacting the same as the first one.

I'm with Mike: check fuel pressure.

Id start with fuel pressure, id inspect the wires to cyl 1 fuel injector, as well as wires at the icm