2012 Tahoe 5.3 with Misfire on 4 ...

[wjburken]

A quick question after rereading some of the posts here.

How did the initial mechanic determine that the AFM lifters were fine?

Personally, given all the issues that folks have had with AFM lifters going bad, I would have considered replacing the lifters when he had the head off to check them, but that’s just me. I mean, he was already there.

That being said, I would check everything involved with #4. Start from square one. Swap plugs with #2 and see if it moves. Swap wires with #2, swap coils, etc. I still think there is value in checking you coil harness as well. There was a guy on here a while back with similar issues, but on the other side of the motor. He ran in circles doing everything except check the harness and that’s what it ended up being. When trouble shooting, you have to rule out stuff, not just dismiss stuff. For work and on my vehicles, I’ve had too many brand new parts that were DOA and too many seemingly unlikely things end up being the cause to not look at everything.

 

[wsteele]

A quick question after rereading some of the posts here.

How did the initial mechanic determine that the AFM lifters were fine?

Personally, given all the issues that folks have had with AFM lifters going bad, I would have considered replacing the lifters when he had the head off to check them, but that’s just me. I mean, he was already there.

That being said, I would check everything involved with #4. Start from square one. Swap plugs with #2 and see if it moves. Swap wires with #2, swap coils, etc. I still think there is value in checking you coil harness as well. There was a guy on here a while back with similar issues, but on the other side of the motor. He ran in circles doing everything except check the harness and that’s what it ended up being. When trouble shooting, you have to rule out stuff, not just dismiss stuff. For work and on my vehicles, I’ve had too many brand new parts that were DOA and too many seemingly unlikely things end up being the cause to not look at everything.

I agree.

Not knowing exactly what has actually been done in what exact sequence versus what is believed to have been done makes advice pretty iffy.

Frankly I was surprised to read his mechanic friend pulled the head to check the lifters. I mean, I think most would be fine with a quick inspection of the rockers functioning on that bank to rule out a lifter collapse as the misfire cause. Also, he probably would have just heard it clapping away.

The other problem is the complexity of how these systems work and how that might confuse perceived results. For instance, the P0300 is a permanent code. So if cleaning the MAF had actually addressed the multiple random misfire, a successful completion of the trip criteria so the ECU would clear that code likely would still be awaiting the ECU's decision.

It kind of becomes a game of blind man's bluff. Probably very frustrating for the OP.

 

[Leevon]

Maybe he meant "pulled the head cover"? I think that's what GM calls them, the OP is Canadian...I'm reaching though.

 

[wsteele]

Maybe he meant "pulled the head cover"? I think that's what GM calls them, the OP is Canadian...I'm reaching though.

I think GM still calls them valve covers, but good point about the lost in translation effect. Could be a simple case of misnomer. Good/bad nomenclature definitely affects the Blind Man's Bluff quotient.

 

[Leevon]

Better yet they come up with this lol:

 

[Ivanhoe]

Hello all,

If he does it for a living, he likely took a good look at the valves. Pretty hard for a mechanic to pull a head and not recommend freshening the heads, unless they look pretty darn good.

I think I would still try that cold engine compression test.

There really isn’t all that much to it, fuel, spark and compression. Oh, and of course, everything happening at the right time, with the right mix.

A compression test was done at the time and all good.

I pulled #4 spark plug and it was black. And took some readings

 

[Ivanhoe]

Some pics of reader

 

[wsteele]

Can you select injector pulse width on that unit in live data?

 

[wsteele]

Also, if you find the time to dick with that scanner, see if it will display fuel trims.

Are the upstream O2 sensors original?

 

[Leevon]

I just bought this 2007 Tahoe with 258,000 miles (see sig link). It had a misfire at idle, rough idle, and P0300, PO304 just like yours. Found severely fouled plugs, #4 was the worst. We replaced the plugs and did a top engine clean (intake clean). She's running like a champ now. I wonder how closely your friend examined the valves. Another possibility could be plugged cat which can cause PO300.