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