Cylinder 3 Misfire and Gas on Spark Plugs

Ok , i have a 2004 Pontiac Grand Prix GT V6 GM 3.8, my car didnt have any misfires until after i changed my Lower Intake Manifold Gaskets. After changing the gaskets and putting everything back together. The car ran fine. The next day i took it an hour and 30 min away, while driving there it started misfiring and shaking,rough idle at an hour into the trip. The check engine light went to blinking. The Next day i drove it all the way back home in the condition. Put code scanner to it, and received P0303 “Cylinder 3 Misfire”:

So i automatically assumed it was the coil pack because i just installed new wires and plugs a week before changing Lower Intake manifold Gasket. Bought new coil pack. Still getting rough idle and P0303 code. So i decided to take a look at Cylinder 3 Spark plug and it was dry black, carbon fouled:

So went and bought new spark plug, installed it, started car and still rough idle and misfiring. Took the new spark plug out and it had been FOULED also ( DRY BLACK AGAIN). So i was told to switch a coil pack around. I did it and the started car, took away some of the rough idle, car actually wasn’t misfring or shaking, would run smooth for about 5-8 seconds then start misfiring (Didn’t that until i switch coil packs around).

Next Decided to pull out the rest of the plugs from the back and all of them were WET with GAS!! and 2 Spark plugs are just dry fouled!! (Cylinder 3, Cylinder 1). Any ideas od whats going on now??

Any help would be appreciated AGAIN…thanks!

SN: If i can figure this out i will fully have my car back to running correctly.

I think you have to go back to the intake manifold. What were the symptoms that caused the need to change the lower intake manifold gaskets in the 1st place?

To access the intake manifold you have to take off a lot of hardware, much of it is fuel injection related. I think you were ok for a brief time until a heat and cooling cycle caused either the manifold to crack or the replaced gaskets failed. Now you are getting a too rich air fuel mix and the plugs can’t fire properly.

Chasing an ignition problem is getting you off track. I think your issue is something amiss in that intake manifold, possibly a crack somewhere.

You might look closely at the routing of your plug wires and injector wires to confirm they are correctly connected but chances are @UncleTurbo is correct.

To answer your first question, I had a small leak, had a coolant pressure test done and coolant was leaking from Lower Intake Manifold Gasket. Changed it and that fixed the leak. And all this happeneded after changing it. Im going to buy some more gaskets today and replace them. If that doesn’t work then i will go to the Intake Manifold itself and check it out. Will keep you all updated.

One more thing before i buy another gasket, im not loosing coolant ??? Could it still b e faulty gasket?

Air and coolant flow through the intake. What you have now is an air leak.

@knfenimore What you think is causing the air leak??

Think of it like a firecracker. What does it take to make it go pop? Light the fuse, if there’s powder inside, and it is tightly wrapped, it will go pop. So the first three to consider are

  • check for a good healthy visual spark at each of the involved spark plug
  • check the fuel pressure
  • check the compression

Still a no go?

  • coolant is getting to the cylinder and putting out the pop. Wouldn’t take much.
  • check that the injectors are getting pulsed
  • determine if the injectors are clogged

The plastic upper plenum may need replacing. They can cause your problem when removing and reinstalling them.

@“Rod Knox” What is the Plenum? Please Attach a picture.

Here it is.

Dry, black, soot-fouled spark plugs sounds like it’s getting too much fuel. Consider a stuck open fuel injector.

Did you replace the rubber gasket?