Misfire P0303 - Need some advice

[ChadinSLO]

Does have AFM. The issue, when it occurs, disables stablitrack and AFM. Didn't notice tapping sound. Both of these are currently working as issue has not presented itself since the mechanic swapped plug, wire, and coil. The leak test was performed after the swapping so leads me to belief there is an issue but currently not showing up in codes that have been reset. Car is running good ...we shall see. Will talk to another shop on Monday and ask more questions of current shop who performed diagnosis thus far. @j91z28d1

 

[j91z28d1]

if you're certain it's not a stuck afm lifter, which should tap while miss firing. there is a tsb, @dojetman has posted and done to his own truck. a bad design valve cover in the pre 2011 I think it is year, they suck in a lot of oil over the years and the rings get stuck and cause bad compression, fouled plugs and all kinds of not great stuff. there's a kinda fix, of replacing with a updated valve cover and add a catch to stop sucking oil in thru the pvc system. some stuff you put in the cyl to break up the gunk that stuck the rings, disabling the afm mode to hopefully make them last longer. then keep some fresh plugs in it. ride it out and hope you get down the road a few years before it's starts missing because a afm lifter went bad and ate the cam.

it may or may not be worth the time and effort. what advice given depends on what you can do yourself and what you'll need to convince a shop to do and what the cost is.

 

[ChadinSLO]

Thanks @j91z28d1. Someone else I talked to today mentioned the possibility of a stuck AFM lifter and then I read your post. Is it possible that the AFM lifter "unstuck" with the stuff the mechanic did? Almost sounds too good to be true. Drove the Yukon 63 miles today here and there and it's like it never happened. No codes. Idles great. Weird?

 

[j91z28d1]

I've seen some posts talking about theirs did it and then unstuck a few times. others have been about to put Marvel Mystery Oil in the oil and got theirs to unstick for a while.

if that's what it was, it will mostly do it again. if you can catch it while it's working and disable the system in the ecm. that might make it last longer. but the general consensus is the right way is a full delete kit. involving replacing the cam and pulling the heads.

I don't think it's anything they would have done testing it thou. it could be as simple as it's running OK enough it feels fine with new spark plugs, but you're still down compression in that cyl. it's all hard to say even when you have the truck to diagnose in person. it's just all random guessing over the net.

if you get bored, there's a million threads here you can read thru and see what might match your situation test.

 

[DBYoda]

Your mechanic is probably recommending an engine replacement just to cover his back with any future issues that could pop up on a 200,000 mile motor. The very least you could do is pull the head and have the valve seals repaired and maybe you'll be good for awhile...

Repaired my seals, lifters, head gasket, coil packs, wires, plugs and fuel injector AND still getting p0303. 2005 Tahoe. I’m still stumped too.

 

[Fless]

Just to rule it out, has a cam/crank correlation procedure been done recently? If not, it's a quick and easy thing to do with a bidirectional scanner. Also recommended if either the crank or cam sensor has been changed.

 

[DBYoda]

Just to rule it out, has a cam/crank correlation procedure been done recently? If not, it's a quick and easy thing to do with a bidirectional scanner. Also recommended if either the crank or cam sensor has been changed.

For me , no sir . I will bring it up