Originally Posted by 2kg4u
I think the ICM is the major suspect, particularly since this seems to be heat related.
Vacuum leak is a definite suspect.
I don't think it is related to the fuel pump since that would create problems under load at higher speeds worse than at lower speeds like yours is.
A bad plug or wire should only give you a single cylinder misfire, not a random misfire.
You might want to check the fuel pressure regulator. Pull the vacuum hose off, plug the end with your finger and see if the idle speed changes. Smell the hose for gas.
Here is an excellent series of articles on how to test various components of your engine that could be causing your problem. It includes how to test the ICM.
Part 1 -3.1L, 3.4L GM Index of Articles.
Use a wood rod or a mechanics stethoscope to listen to each injector to see if it is firing. Use a noid light (you can make one from a 12V bulb, google it) to see if each injector is getting a signal. Throw a fuel system cleaner (I use Seafoam) into the gas tank.
Thanks for the reply. That is a great website, very well done. The ICM test process on there is for a no-start condition though.
I sent him a question to get some clarification on one of his processes and he replied that he thought the problem on my car might be a failing fuel pump or bad crank sensor based on his experience.
Thinking I'd be clever, and because the ICM isn't that hard to remove, I decided to take the ICM into the parts store to have it tested, and if it tested good I'd borrow their fuel pressure gauge to check out the fuel pump.
I watched them test the ICM on their machine and it came back as bad so I purchased a new one.
Installed new ICM and the car started right up, CEL cleared itself apparently as it was no longer lit (I don't remember bothering to clear it when I pulled the P0300 code). I then road tested the car to try and reproduce the misfire at idle after a drive. Drove 8.5 miles to lowes and the car ran great. I was in the store for maybe 20 minutes and the car started fine but shortly out of the parking lot I was stopped at a light with the car facing downhill and I thought I could feel just a slight miss, but then I got on the freeway and drove 8.5 miles home with no misfires on the freeway or at any of the lights between the freeway off-ramp and my house.
Parked the car at home, shut it off, and then set the timer on my phone for 30 minutes. Went to restart after 30 minutes and it went right into the misfire at idle deal just like before except this time I could not even increase the rpm'* with more throttle, it just died and would not re-start. The CEL did not come on.
Maybe the car will start in the morning after it has cooled off as it has in the past. I'll get the fuel pressure gauge in the morning and see if I can get the car to repeat that misfire performance with the pressure gauge hooked up.