QuestionI have a 2001 Chevy 2500HD with the 6.0L V8 and 4L80E Auto. The truck has 110K miles on it and is primarily driven on the highway. One morning 2-3 months ago when I started it up, (Garage kept in 65*F, and not much colder outside) it was running rough, misfiring. The check engine light started flashing. I decided to drive it in the neighborhood to see if the condition corrected itself once the engine was warmed up. Shortly after coming up to temp, the light went away, and the engine ran fine.
I took it to Autozone and had the OBDII code read, it was #7 Cylinder Misfire, but since the problem went away, I didn't do anything immediately.
About 3-4 weeks later the problem finally reappeared, but again it went away after driving around for 15 minutes or so.
The problem repeated again with the same symptoms after another 3-4 weeks of problem free driving. It again went away and the truck ran fine for another 3-4 weeks.
My luck finally ran out though, and now the problem has returned and it won't go away. The OBDII scanner still is saying it's a #7 misfire.
I started by replacing the #7 spark plug, and checking the resistance of the plug wire, which checked in spec. I then replaced the #7 ignition coil. After each attempt to fix it I would erase the code by unhooking the battery. I finally just decided to replace all the plugs and wires, as they were due for changing anyway. Still misfiring. I then reinstalled the original #7 ignition coil, and one by one put the new coil on each cylinder, but it's still misfiring.
I feel like I've at least narrowed it down to the fact it's not a spark problem. So I assume it's a fuel problem, a vacuum leak, or maybe a bad valve? It has the cheap plastic 2 piece intake manifolds GM has gone to, but the fact that the problem originally would disappear for 3-4 weeks leads me to believe it isn't really a vacuum leak.
Do you have any ideas I can try? Or does the nature of how the problem went away for long periods of time, and then returned give you any insight to what may be wrong?
Thank you in advance for your time.
AnswerHi Roger, I would go with the intake gasket. They have change the gasket because of problems with misfiring. Another way to test this is to spray carb cleaner at the intake port and look to see if it makes the rpm's to go up and the skip to go away for a second while your spraying. Also check the fuel regulator vacuum hose to make sure there is no sign of fuel in the hose. These regulators leak through the diapham and into the intake through the hose.
Hope this helps
Mark
