Maf/misfire?

Paradigm49 · September 11, 2009

Thankful that there is a forum such as this.

Bought a 2007 GMC Sierra 1500 z71, with the LMG 5.3 flexfuel engine.

3 days after purchasing vehicle, took the truck on a slightly bumpy road, but not what i would consider off roading, but enough that it jarred the truck a bit (wanted to see what it could do---former f150 owner---will never go back)

Truck is running with what feels like a misfire, but not throwing any codes. SES light does not come on. The only way i can get the SES light to come on is if i am cleaning the TB, and the MAP sensor seems to get a bit soaked in the cleaner, although today i had done that and let kept the throttle body pryed open with a screwdriver to let the excess fumes out. Had the truck scanned for error codes once by a friend of mine that is a mechanic, and the error codes that came up were P0300, P0442 and P0401. Had those codes cleared and they have no come back. However, upon re-scanning the computer, the diagnostics computer that my mechanic has will not connect to the computer (3 times now to be exact)

Anyways, to make a long story short, I have replaced the plugs and wires, plugs twice actually after realizing that the gap setting on the plugs was already correct from the start, therefore screwing up the iridium tips. Didn't make any sort of difference. Took the truck to dealer, they stated that it wasn't throwing any codes and it wasn't detecting any misfires. They did replace the MAP sensor, which personally i don't think was necessary, and didn't fix the problem. Also---in replacing the plugs, noticed that the rear plug on the passenger side seemed to be firing correctly, but had a little bit of oil around the base of the threads of the plugs.

I did some investigation of my own. I can disconnect the MAF sensor while the truck is running, i can start the truck with the MAF sensor disconnected or completely removed from the tube. Cleaned MAF sensor, no change at all. Truck is running very rich (massive amounts of carbon on tailpipe)

I am no mechanic by any means, but i do have quite a bit of mechanical ability, but i'm completely at a loss as to what could be wrong, and when the truck was delivered, i could barely feel the engine running it was that smooth, so somehow i have a feeling that i am partly responsible for this, but it is still under warranty but i would be welcome to any suggestions or ideas.

Thanks!

spock · September 15, 2009

I am interested in this also. I have P0442 and the rough idle.

Jennabear · September 16, 2009

The P0442 is a gas cap code. Tighten it back down, disconnect battery, reconnect and it should be gone.

P0300 is random misfire and P0401 shouldn't show up as you have an 07. That code is for EGR and 07s don't have EGR.

Paradigm49 · September 23, 2009

I was pretty sure that the truck didn't have an egr.

I've checked a few things on my own, and found it to be somewhat interesting.

The truck will start and run without the MAF connected, i can actually remove the entire air filter/MAF sensor, and tube that connects to the throttle body and the truck will still run, which last i had checked, it's not supposed to do.

Decided for the hell of it to clean the throttle body approx 2 weeks ago, and after cleaning that, the truck would start, but the transmission was bucking and right before the truck would shift into 2nd, 3rd or 4th gear, the tires would chirp and the finally switch into gear, but it wouldn't be a smooth transition, it would be very pronounced and very rough. Never figured cleaning the throttle body would do that, and that is the first time i've ever had any vehicle do that.

Took it to the dealership again, the replaced the PCM, made a small difference, but not a ton.

It's back up at the dealership today again, service engine soon light is back on, so i will see what is going on with that. Personally i think that the MAF is bad.

But i did notice a few odd things.

The wire going to the fuel injector on the passenger side--2nd injector from the back, the wiring is differently colored than every other injector, i also noticed some bare wire and what appears to be a burn mark on that wire, not sure if it's causing any problems, but just found it odd.

Haven't had any of the evap or egr codes come back as of yet, but having those codes on a truck that doesn't have an egr in the first place tells me that something was defintely a bit screwy with the PCM.

Any ideas?