Hey Scotty, I got an 07 New body style GMC Sierra 1500 SLE with the 4.8 it's got 112k Miles and it's an automatic. was running smooth up until a couple weeks ago. I have a handy code scanner and it's giving me P0300 and with the misfires it's causing my first shift to be very hard. I started with replacing the spark plugs,wires and I even replaced the TPS and cleaned the throttle body as well as the MAF sensor. Took it to a prominent auto shop and even the guys there were stumped. What do you think is the problem
HI,
It might be worth it to do a compression test on each cylinder to see if they are all similar. It's a long shot, but if you've got nothing else to check, I'd give it a try.
|P0300 GMC code possible causes
Faulty spark plug (s)
Faulty ignition coil (s)
Clogged or faulty fuel injector (s)
Intake air leak
Fuel injectors harness is open or shorted
Fuel Injectors circuit poor electrical connection
Ignition coils harness is open or shorted
Ignition coils circuit poor electrical connection
Insufficient cylinders compression
Incorrect fuel pressure
Check the FAQ on this forum.
Btw, have you done the head gasket test?
One easy way to check for a misfire source, especially if you have spark plug wires is to attach a timing light to each spark plug wire and with the engine idling, see if the spark pattern is consistent. Do this for each one. If you have an electronically caused misfire, you'll see an inconsistent flash pattern on the timing light.
