Random Misfire and Cylinder 3 Misfire code after Intake Gasket change.

mikey334

I have a 2004 Pontiac Gran Prix V6 GM 3.8 Engine. I was loosing coolant, found out it was my Lower Intake Manifold Gasket. Changed it. And Was running perfect, no more coolant leaking. The next day i drove it and Hour and 32 min away, drove perfect all the way there until about 32 min left of the trip it went to misfiring and idling rough and check engine light went to blinking. I Stay the night there. Drove all the way back home in the condition. Got home ran code reader and got "Random Misfire Code" and "Cylinder 3 Misfire Code". I Just changed the spark plug and wires days before so i assumed it was a coil pack. Bought and Replaced Coil pack and NO CHANGED AT ALL. So i checked Cylinder 3 Spark plug and it was FOULED (CARBON FOULED, DRY,BLACK COLORED). I checked all the others and they were fine. So i bought a new spark plug to put in Cylinder 3 and fired up engine, still misfiring. I removed the new plug and it was FOULED also (DRY BLACK COLORED). I have no idea whats causing it. Could it be dirty injectors? Vaccum leak?? Asking because in order to change lower intake manifold you have to remove the Fuel injectors and rail, also the Intake manifold itself, confused. ANy help is appreciated.

imidazol97

What is the look on the shared spark plug on the same coil with #3? If it's clean, that would indicate the coil is firing correctly.

I would suspect the injector giving too much fuel. If you took injectors out once and replaced the o-rings, I'd go ahead and switch two of the injectors, like 1/3 or 3/5 to see if the fouling moved with the injector.

Could be a ring. What about the fuel pressure regulator and its vacuum tube? Any strong smell of fuel in the vacuum line there? Might be getting fuel directly into the engine through that vacuum line back to the throttle body, and hitting #3 hard with extra fuel more than the others. That might explain the random misfire that you got part of the time.

Take off that vacuum and let the engine run a few minutes to see if you get fuel out of the regulator where the vacuum line connects.

Are you losing any more coolant? Did you do an oil change right away and than another in a hundred miles or so--you can use cheap oil for those just to remove any coolant left behind.

Good luck.