P0300

[ThatThar]

This past week, I've been tracking down a misfire P0300 code. The original symptoms were steady/flashing check engine light, rumbling at idle, loss of power, and bad fuel economy. If you do a cold start, the symptoms do not appear until it warms up or until you begin driving. Over the past week, I have replaced spark plugs, spark plug wires, ignition coils, and the catalytic converter. All have helped the condition slightly, with the exception of ignition coils which greatly lowered the intensity of the rumbling, gave me a lot of my power back, and gave me a lot more fuel economy back. Still, however, the light is still there and it is definitely still misfiring. I'm out of ideas and I really don't want to take it to the dealership for them to diagnose it, but I will if it comes down to it. What's the next step?

[mikeluscher159]

Clean your MAF and IAC.
Any vacuum leaks?
Air and fuel filters new?
Any corrosion under the ICM?

[ThatThar]

The MAF was cleaned a few months ago. Air and fuel filters both have less than 5,000 miles on them. Where's the ICM and IAC?

[96 SSEi]

The MAF was cleaned a few months ago. Air and fuel filters both have less than 5,000 miles on them. Where's the ICM and IAC?

HI, THE icm IS the ignition control module -- the flat plate that the ignition coils plug into-- the idle air control is on the throttle body near maf-- it should have 2 screws

[ThatThar]

What's the test to see if the ICM and IAC are good?

[96 SSEi]

What's the test to see if the ICM and IAC are good?

hi, iac is a solenoid-- a plunger that fills a hole

In short, I dont know how to test but it should move freely
the icm-- I dont know-sorry

[ThatThar]

I tested for vacuum leaks with carb cleaner and I couldn't find any. I'm going to let the engine cool down before I test the IAC.

[ThatThar]

I replaced the IAC and I'm still getting misfires. I also cleaned the MAF sensor, still no improvement. So far I've replaced the catalytic converter, the spark plugs, the plug wires, the ignition coils, and the IAC. No dice.

[turbojon]

Have the intake manifold gaskets been replaced? If not, it is a good idea to have that done and certainly won't hurt. Mine were cracked and I believe they can contribute to misfires. Just an idea.

[ThatThar]

Is there a guide on the forums here about how to check and/or replace them?

[turbojon]

http://www.pontiacbonnevilleclub.com/fo ... =53&t=3591

[nos4blood70]

Could be a failed upper intake manifold, or an ICM.

[MKMike]

Everyone has provided all of the likely causes, so here's a link on just how to track down the trouble: http://troubleshootmyvehicle.com/gm/3.8 ... re-codes-1

[ThatThar]

I pulled fuel injector wires to narrow down which cylinders are misfiring and found that it's cylinders 2 and 5. They run off the same ignition coil and despite replacing all of the coils, I swapped them around to see if one of the new ones were defective, but it was still just the 2 and 5 cylinders. I'm going to try cleaning the harness connectors for the ICM.

[ThatThar]

After cleaning the ICM harness, it's still misfiring. I listened to the injectors again and confirmed that they are operating, but when I unplug the injectors for the 2 and 5 cylinders, I hear no RPM change. Should I replace the ICM?

[96 SSEi]

I would first swap the injectors to be sure its pcm fix- could be bad injectors?

[ThatThar]

I just borrowed my uncle's spark tester and it shows no spark to the 2 or 5 cylinders.

[MKMike]

The same voil operates both of those cylinders.
Here is the way to troubleshoot further:
[url][/http://easyautodiagnostics.com/gm/3.8L/ignition-coil-pack-tests-1url]