Misfire problem

[jetson22]

I have a 2013 Denali XL 6.2L with 285k. About a month ago I was going down the highway and all of a sudden it started to really miss to the point it would hardly run. I scanned it and had multiple misfires. I had it towed to my mechanic. He has checked lifters, swapped coil packs, swapped plugs, (wires and plugs new less than 6 mth), swapped injectors, it has spark and compression is good. it seems that no matter what it is always cylinder 4. He rebuilds and tunes LS all the time. Anyone have a similar issue or advice?

 

[strutaeng]

Does your truck have AFM?

Has he done an injector balance test?

Could be wiring harness issue as well.

 

[Marky Dissod]

... 2013 Denali XL 6.2L with 285k. About a month ago I was going down the highway and all of a sudden it started to really miss to the point it would hardly run.
I scanned it and had multiple misfires. I had it towed to my mechanic. He has checked lifters, swapped coil packs, swapped plugs
(wires and plugs under 6 months ago), swapped injectors, it has spark and compression is good.

it seems that no matter what it is always cylinder 4 ...

Could be wiring harness issue as well.

Specifically, cylinder #4 wiring harness issue ...

 

[JoeBlobs]

Check for broken/missing exhaust manifold bolts.

 

[Fless]

Fuel pressure readings?

Post the short- and long-term fuel trims when it's misfiring.

 

[jetson22]

Yes it does have AFM (total crap} Yes to injector balance test. Has checked harness, manifold bolts and I don't have the number but fuel pressure was good. He has not been to come up with anything. Says he could go further in with some new lifters etc but was not sure if that would be the fix. I am considering a salvage motor or just trying to get rid of it. Any advice?

 

[JD12]

i have the same p0304 code on my 07, mine has vvt but no afm or dod…if you find the right resource, the p0304 has thirty plus possible causes…

i have new iridium plugs & wires on all cylinders, and i change cyl 4 every 3k miles…its not the coil. sometimes the cyl 4 plug is fouled, sometimes its corroded, sometimes its fine.

the highest probability for the misfire is the valve, the camshaft lobe is probably wearing unevenly. the lifter may or may not be collapsed or collapsing on occasion. it could be the ECU but its most likely mechanical. deleting the vvt camshaft and hydraulic lifters will cost you some gas but save you the headache in the long run.

 

[West 1]

I had one that had carbon build up on the valve stem causing it to stick a little open. Finally figutured it out by doing a leak down test on that cylinder. It was leaking out the exhaust valve, I pulled the valve cover and pulled up on that valve and the leak down sealed up. I pulled both heads and had them rebuilt. I could have done a band aid repair and just cleaned up that one valve but the heads were ready for a rebuild.

 

[Fless]

You can use an inspection camera to take a look at the valves and the cylinder walls, turning the engine over by hand to watch the valves to see if they seat.

 

[Doubeleive]

You can use an inspection camera to take a look at the valves and the cylinder walls, turning the engine over by hand to watch the valves to see if they seat.

how does it pass a compression test if the valves are not seating?
I think something has been overlooked here, based on the reported issue and checks performed
which if the mechanic builds and tunes ls engines doesn't make sense

 

[Fless]

how does it pass a compression test if the valves are not seating?
I think something has been overlooked here, based on the reported issue and checks performed
which if the mechanic builds and tunes ls engines doesn't make sense

My response was directed at @JD12's issue.

 

[Doubeleive]

Yes it does have AFM (total crap} Yes to injector balance test. Has checked harness, manifold bolts and I don't have the number but fuel pressure was good. He has not been to come up with anything. Says he could go further in with some new lifters etc but was not sure if that would be the fix. I am considering a salvage motor or just trying to get rid of it. Any advice?

at this point I would consider a 2nd opinion, like literally, nothing makes sense if all those checks were performed. this guy might have a fish on the line so to speak, just speculation...

 

[Doubeleive]

My response was directed at @JD12's issue.

ahh, I was wondering lol

 

[Doubeleive]

and it's been a month already with no reply, derrrrp

 

[mikez71]

It seems like there's a lot of single cylinder and random misfire threads.
And a fair amount never get to the fix... or don't tell us about it..

I have occasionally heard that you could have leaky valves that still compression test OK.
Leakdown test might show something a compression test won't. ? idk.

 

[Fless]

It seems like there's a lot of single cylinder and random misfire threads.
And a fair amount never get to the fix... or don't tell us about it..

I have occasionally heard that you could have leaky valves that still compression test OK.
Leakdown test might show something a compression test won't. ? idk.

Leakdown would be part of a compression test. That would help determine if the rings are leaking, or the valves, or the head gasket.

 

[mikez71]

I would not assume they did leakdown if they only said "compression test" ?

 

[West 1]

On the one I repaired witht he carbon build up it was very hard to find because it came and went, it was never a 100% misfire. The carbon build up had it sticky so it would work perfect and then not close fully for many revolutions. When I compression tested it it was perfect. Some issues are just very hard to find or diagnose. Sometimes you just get lucky and find them.

 

[SLTlogic]

I have a 2013 Denali XL 6.2L with 285k. About a month ago I was going down the highway and all of a sudden it started to really miss to the point it would hardly run. I scanned it and had multiple misfires. I had it towed to my mechanic. He has checked lifters, swapped coil packs, swapped plugs, (wires and plugs new less than 6 mth), swapped injectors, it has spark and compression is good. it seems that no matter what it is always cylinder 4. He rebuilds and tunes LS all the time. Anyone have a similar issue or advice?

Tell your mechanic to pull valve cover and check cylinder 4 rocker arm if it’s extremely loose then you have a stuck afm/dod issue and to temporarily get it going. Do as follows. Remove intake manifold , valve cover and cylinder 4 Rocker arm and valley pan. find oil port for cyl 4 lifter and stick a slim flathead into it and hammer till pushrod comes flying out (literally) reinstall everything and order a $25 afm disabled from amazon and plug in before any attempt to start it. It’s a temporary fix u till you do a full afm/dod delete swap out lifters, cam, trays non afm valley pan etc. don’t waste money replacing with another set of afm lifters etc

 

[mikez71]

On the one I repaired witht he carbon build up it was very hard to find because it came and went, it was never a 100% misfire. The carbon build up had it sticky so it would work perfect and then not close fully for many revolutions. When I compression tested it it was perfect. Some issues are just very hard to find or diagnose. Sometimes you just get lucky and find them.

What do you think caused the carbon buildup on one valve only? Leaky injector or worn plug?