1996 Olds Cutlass Supreme SL 3.1L

Hello, DIY'er here looking for some guidance on how to proceed with troubleshooting this issue.

I have the above vehicle with 100K on it. It'* been in the family since new and maintenance has been done per schedule. (intake manifold done at 60K, number 1 coil pack replaced at 80K, plugs and wires at 65K)

It has developed the following condition:
Intermittent misfire (and sometimes dieing) which usually only takes place after the vehicle is driven for 40 minutes then shut off, then wait 30 minutes and re-start. Severe Misfire is at idle and goes away at about 1200 rpms.

Any other re-start condition (cold start or restart immediately after shut-down from long drive) there is no misfire.

Car runs strongly and smoothly above the 1200 rpm threshold.

Over the last four months this has only happened periodically making it difficult to troubleshoot. Over the last couple days it seems to be happening more frequently and is not only misfiring but dieing completely.

During this last episode yesterday the check engine light came on and it threw the P0300 DTC, 'Random/multiple cylinder misfire detected'.

I was a bit relieved to see the check engine light thinking the code might help pinpoint the misfiring cylinder or provide some more specific guidance as to the problem, but P0300 is not cylinder specific.

Searching the internet has come up with some possibilities with this engine, but given the global nature of the P0300 code I'm struggling with where to start the troubleshooting process.

- vacuum leak
- bad coil
- bad plug/wire
- plugged or bad egr valve (this was changed at 60k when intake manifold gasket was done)
- dirty maf (this is clean)
- poor compression in one or more cylinders (maybe burnt exhaust valve)
- clogged or failing fuel injector(*)
- failing fuel pump
- failing ignition control module

Assuming I can get the problem to surface again, where would you start troubleshooting? Maybe start with some carb cleaner in search of possible vacuum leak?

Thanks

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.