2006 4.8 Silverado p0308 Misfire on Cyl #8

[STL_G8GT]

Hey all -

2006 silverado, 4.8l v8, 170k miles, oil change about 1k ago with a high mileage 5w30.

2006 Silverado showing p0308 pretty regularly - idles a bit rough. Hooked up HPTuners with misfire count and 8 goes crazy (hundreds of misfires) - with a handful of misfires in cyl 7 and a couple here and there otherwise. Idle is a bit rough. I do occasionally hear a bit of clatter/tick from front of block but believe it to be timing chain related and not connected to this issue (used hose to ear to block to pinpoint).

I swapped plugs and coils and wires and the problem stayed in 8. All of the plugs look identical - not wet, not oily, just clean with an offwhite hue. I pulled the valve cover and not only is it clean in there for 170k miles, but the springs appear to be fine. I physically attempted to spin them, etc, with no movement.

I did a compression test on the motor and got this

1: 120
2: 125
3: 125
4: 130
5: 135
6: 135
7: 120
8: 118

I then tried a wet test on 8 with 2 tbsp of 5w30 in the cylinder and got the same results.

I have not yet checked intake manifold gasket, swapped injectors, or performed a leakdown.

Anyone care to offer a suggestion or a path from here?

[TooManyHobbies]

Check the wires on harness going to coil 8?

[STL_G8GT]

Spent some quality time with it today...

Tried a leakdown test - but only had the air compressors regulator to go on (so no leakdown % to report). Even still, cyl 8 was a shade more noisy than cylinder 6 (my test) - and it seemed to be on the intake side moreso than exhaust.

I cleaned all of the injectors and swapped 8 and 4 - the misfires continue on cyl 8, however. Also of note is that I swapped 7 and 1 to see if the occasional misfire on cyl 7 moved, and it also stayed in place. Cyl 8s injector was dirties of all, probably because it's the end of the line on the fuel rail and on the far side.

Lastly, HPTuners showed a p0300 stored in history for the truck, not the p0308 - however when I watch the scan, I'm only seeing it on three cylinders, and only 7 (about 50 misfires) and 8 (about 500 misfires) were showing up.

After cleaning injectors (and moving those two), performing leakdown and compression tests, I cleared out dtc history before an idle test here in the driveway and 7 isn't reporting any and 8 is never exceeding 30 in the history.

The miss is only at idle. If I rev and hold anywhere above 1300 or so, the misfire stops occuring.

[STL_G8GT]

Check the wires on harness going to coil 8?

Wiring looks peachy at the harness and so on

[-Ray-]

I'll ask one of my GM tech buddies after my conference call.

[STL_G8GT]

I'll ask one of my GM tech buddies after my conference call.

Appreciate it.

At this point I'm thinking intake gasket, burnt valves, collapsed lifter/cam pitting.

Regarding intake gasket, I'm not showing any lean codes, which I would think I would if unmetered air was getting in past the maf, correct?

Also, rail fuel pressure at idle shows right about 60psi. Added to the first post that all plugs look the same - not wet, not oily, just an off-white hue and generally clean.

[-Ray-]

Pete,

After talking to a couple of the techs they suggested two things.
1) Try and see if you can get a tech II to check crankshaft variation. I'm not sure if they meant relearn, but I hesitate to say relearn.
2) possible burnt valve. Pull the head, especially if you've had a recent exhaust manifold leak.

my suggestion is to have the heads rebuilt since a wet compression test didn't yield better results.

[STL_G8GT]

Pete,

After talking to a couple of the techs they suggested two things.
1) Try and see if you can get a tech II to check crankshaft variation. I'm not sure if they meant relearn, but I hesitate to say relearn.
2) possible burnt valve. Pull the head, especially if you've had a recent exhaust manifold leak.

my suggestion is to have the heads rebuilt since a wet compression test didn't yield better results.

Ray - thank you for taking the time to ask "the guys" - I appreciate it!

1) I can do a crankshaft relearn (CASE) with HPTuners. I saw that the process sets the speed at which the reluctor wheel passes the crankshaft position sensor, thereby telling the pcm that there is a miss if that time has any variation.

2) I moved a MAP torch (unlit, of course ) around the intake while watching a scan and it never changed the behavior of the misfire or idle speed. For $35 I will replace them when I pull the intake to check the head. I am leaning more towards a valve like you said.

My lt fuel trims are showing about 8% for bank 1, but near flat for bank 2 (where #8 is). When I first started the truck after it had been sitting for about a month, I got a p0053 code - which is heater circuit for bank 1 upstream o2. I cleared that and it hasn't come back - but I will pull upstream o2s before the head comes off.

Again, thank you for asking them and for your input.

I'll do the CASE relearn, then swap o2s and see if the misfire or the fuel trims switch sides. After that is done, nothing is left but to pull the head and check the valves, pushrods, and lifter. I was initially hoping it was a broken valve spring, but no dice. I guess now I'm hoping it's a burned valve or collapsed lifter.

Will keep you updated!

[STL_G8GT]

Valvetraon looked good...


Cylinder that was misfiring looks ok, just like the rest...


#8 exhaust lifter... Looks like Kim Kardashians face without makeup...


I scoped that lifter hole and found a cam lobe that was scored and pitted on one side of the lobe.

I'll pull the other head and cam tomorrow. The truck has a topper on it, so it won't go in the garage... 40mph winds made the coolant drain a real circus.

We may sell the truck after I fix it, so that'll decide if I do cam swap or just new factory cam and both lifters for cyl 8.

[TooManyHobbies]

Yeah that face is not too pretty, but at least you found the issue.