I have a 2004 Grand Prix GT1, the car has a 3800 Series III engine with 148000 miles.
I have been having a problem for a while now, and I just can't seem to figure it out. Every once in a while (maybe every 2 or 3 months) the car will develop a misfire. The problem comes on instantly, meaning it was fine when I turned it off last time, but after I start it, there is a noticeable cylinder misfire. While the problem is present you can feel the misfire just before the vehicle stops, while at idle, then again as you start to accelerate. While driving there is no misfire and the car runs like a top. The problem will last anywhere from a day to a week, then just as quickly as it came on it will go away. Sometimes it will throw a code indicating a cylinder misfire. (Sometimes P0300 sometimes P030X, X being the cylinder.) The cylinder is not always the same.
While the problem is present I have performed the following:
Cleaned the MAF sensor
Cleaned the EGR valve
Cleaned intake
Tune up (plugs and wires)
New MAP sensor
New fuel filter
Used Seafoam injector cleaner in the tank
None of which has corrected the issue.
I am at a loss, does anybody have a clue what to look at, or is/has anyone else ever seen this problem?
