05 GC 4.7 Cylinder #5 Misfire
This might be long winded... This is an 05 GC with a 4.7 and 88k miles
Two thousand miles ago I had the local shop change out the spark plugs as part of the 90K mile service. A few weeks ago the CEL came on and I brought it back into the shop because it was popping codes 300 and 305 for cylinder #5 misfire. They changed out the spark plugs after I asked them with the OEM recommended copper core plugs. In addition they changed out the coil for the number 5 cylinder, all at their cost. Needless to say the CEL came back on.
The car now sits at the stealership because it is under warranty. They just called and said that the #5 cylinder failed the pressure check. Service rep states that the pressure is 90 psi. They had not got into the engine to see what is causing the pressure loss.
I have searched the forums and have not really seen what the most common cause is for this issue. I did see a few posts that said something about carbon build up on the valves.
What do you guys think? I am worried that the warranty will not cover it. You know how they operate.