P0306 ( cylinder #6 misfire) --> low compression.

[Opeth]

2006 Trailblazer 4.2I, 74k miles.

I'm going to write a trouble shooting thread since there seems to be quite a few of these pesky misfires plaguing our platform, and as suggested by James. This has been a painful and time consuming process that is still not resolved, now I am left at the mercy of my original selling dealer because of the extended warranty I bought. I will update this as I am from the dealer.

Symptoms started as a slightly out of normal idle that just seemed a little rough, barely noticeable (no surges and stayed right at 600-650 rpms) I figured I should think about doing a tune up with new plugs and TB cleaning. A week or 2 later while sitting in the car wash, the idle seemed a bit worse and next thing I know my CEL started to flash. I blipped the gas and it went away.

New ac delco plugs 41-103 were ordered and installed that same weekend which was two weeks ago. All gapped carefully to .042 and all coils were handled gently, and plug boots were reinstalled with dielectric grease.

Upon first start up, truck seemed to have a smoother idle but later in the evening the CEL once again started to flash and the idle seemed quite poor again. Drove the truck home, and upon parking the CEL stayed on resulting in a P0306.

I swapped coil pack #2 with #6 and plugs from #3 to #6 to rule out either of those two. Disconnected the battery overnight to reset the PCM, CEL returned shortly after the truck had warmed up the next day with P0306 again. Ruled out plugs and coil packs.

Stumbled upon a post elsewheres about a member who had this same issue and found that his intake manifold bolts were loose near cylindr #5 & 6. I discovered the same issue, they were maybe hand tight at best, tightened them down but CEL still remained going on and off.

Last Friday stumped and not wanting to throw away money at parts I didn't know would fix the issue or not, I gave it to my dealership since I had bought an extended warranty which I was lead to believe was "bumper to bumper."

After an initial first improper diagnosis by them stating it was a bad coil even after i told them it wasn't, I was called Monday and told the compression on #6 was more than 70% lower than the rest. They couldnt say what the culprit was nor knew if the warranty company would cover this and needed authorization to do a tear down to diagnose the issue and would most likely have to have the claims rep come and look at the head.

This is where my truck stands, hopefully I will hear something today and will update this as I am.

[Trail X]

So, it sounds like your misfire is more stubborn than some of ours. My CEL won't come on for months, and then randomly pop up. I'm hoping my issue is not the same as yours, but admittedly I have not done a compression check yet. But mine is not annoying except when creeping along with my foot lightly on the brake (like at a stop light).

Keep us updated!

[Opeth]

Just received an update. After head removal cylinders 1-5 looked great, #6 has tons of carbon build up and looks like death. Good news for me ruling out any signs of abuse or neglect on maintenance. I have always been up to date and fluids always changed, but I'm sure the warranty company will try to find something to get out of paying.

Its now a 24-48 hour wait for an inspector to come look at the engine to determine if they are going to cover my claim and rule this a stuck valve. Something internally obviously failed to cause this serious of a carbon build up.

I never sea foamed the engine either, I had a very bad experience with that stuff a few years back and have been Leary ever since. I feel if I had used it, it would have just covered up the real issue at hand. If it worked, it probably would have helped remove some of the carbon but this was described to me at rock hard cement carbon build up, but that is only a speculation.

James-
Even when the CEL was blinking the truck felt like it had its normal power, it would stay on for a day and then go off. But it usually did come right back on later during the day, so yes more frequent than yours. Did you seafoam your engine?

[Opeth]

Warranty inspector came in and photo'd the engine and head this morning. They requested quotes for a whole engine replacement and / or new head replacement.

I would assume the latter of the two since the head is already torn apart and engine replacement seems to be a huge chore. I would think they would have drop the entire frame from body to fit that beast in there. Service advisor didn't say what they ruled as failing part, I will ask tomorrow.

[DirtyBacon04]

Just curious what your bad experience with seafoam was. Usually any negative report i hear about seafoam is people using it too late in the car's life and revving it real high to see more white poofs... then boom. Was your experience with your TB or previous vehicle?

[Opeth]

1999 cavalier 2.2ohv 53kmiles automatic 4T40e... I don't know how it happened but i was slurping the liquid through one of the vacuum lines that was under the throttle body, ( recommended location by several other similar car owners) I got the normal results with white smoke, wasn't bouncing it off the rev limiter but revving it to 1500-2k. I went to go for a drive, the car was now slamming into every gear under normal acceleration. It ended up killing my MAP sensor which was located on the intake manifold. Sensor friendly my ass...

That car had a stupid ass PCM tune and relied on variables like no other car I had seen before, and to be honest I dont remember what it was anymore. Been out of that tuner car scene for a good 5 years. It was such a pain in the ass to tune them I ultimately did an entire engine and transmission swap which required gutting my car, and swapping everything from a totaled donor car with the 2.2L Ecotec engine, 5 speed Getrag and slapped on the GMPP Supercharger kit.(interior wiring harnesses and all)

[DirtyBacon04]

I've always used the vaccuum line from the brake booster. I didn't know if there were any other lines to use, but that one seemed universally applicable for sea foam.

[Opeth]

Finally going somewhere with this....both Exhaust valves were burnt on #6 cylinder causing them to be stuck open. I wasn't told a reason why this failure occurred, I'm wondering if the loose intake manifold bolts I discovered caused a vacuum leak and lean condition burning them out. I never had a DTC come on for a lean condition so who knows.... All other 5 cylinders looked great I was told.

So I am getting a new head installed to the tune of $3500, I'm only responsible for $100 deductible plus head bolt kit since the warranty doesn't cover "bolts."

[navigator]

could they not just do a valve job with new gaskets and head bolts and put her back together?

[Opeth]

could they not just do a valve job with new gaskets and head bolts and put her back together?

GM states to replace the whole head if failure occurs versus repairing it. It probably can be done by a reputable machine shop with right equipment. A thorough cleaning, new guides, valves, machining the valve seats and decking the head might be the same cost as a new head in the end.

I saw a re-man. on ebay for $500 which could be an option as well. I'm not floating the bill on mine so I'm happy for a whole new head.

[fishsticks]

Agreed....your warranty is paying for it. Go for brand new.

$3500 sounds like you'll get new cams and phaser as well. I imagine the repair procedure calls for replacing rather than reusing the timing chain too.

[Opeth]

Agreed....your warranty is paying for it. Go for brand new.

$3500 sounds like you'll get new cams and phaser as well. I imagine the repair procedure calls for replacing rather than reusing the timing chain too.

I would assume so, I guess I'll see the nitty gritty when I pick it up by the end of the week. I have a loaner vehicle till Friday. They tried giving me a mini van and I said hell no, was promised mid sized car like an Altima by Enterprise....So I complained and settled on me paying $20 and driving a 2012 Toyota sequoia

[Opeth]

Whelp.... New head installed yesterday and I still won't be receiving my truck back till next week now. I can't be mad because the tech is being thorough on the job and said it just doesn't feel right to him. He feels there is a timing issue, even after he's triple checked the chain and cams. He feels the clock spring on the end of one of the cams is faulty, not sure if that's the correct term but it's the size of a hockey puck on the passenger side cam at the front of the engine. He's reached out to GM for assistance and they can't seem to lend anything useful. He said he's going to go through it with some other techs Monday to see if anything seems a miss.

I mentioned the cam position solenoid and he's already said he checked it, no broken screens and clean.

And now I have a 2012 GMC Terrain to play with...

[navigator]

wonder would something like a head change need the PCM to do a relearn? Then again the battery likely has been disconnected the whole time so it would get reset but it might need to run a little to do the relearn.

[Opeth]

Oops, forgot to update this. Got my truck back last Tuesday finally. Apparently the tech had the timing off by one tooth, which he and another tech were able to finally figure out and rectify. The new cam timing part was not needed but since the extended warranty authorized the part they still installed it anyways.

Truck runs like a dream, smooth as butter and no vibes. Only thing I am unhappy about is that they drained my synthetic oil and replaced it with conventional without asking me. The tech broke 14 of the 16 head bolts and nearly all the exhaust manifold bolts. Bolts were able to be extracted from the block after he removed the head. Total clocked time was 33.8 hours to do all the work, or so the bill states. My extended warranty didn't cover bolts or fasteners so I had to cover all new fasteners to a tune of $318 including my $100 deductible. They paid $3,978.... Not going to complain

[fishsticks]

Curious, does you bill state if you new head was fully loaded including cams or just a bare casting?

[Opeth]

Curious, does you bill state if you new head was fully loaded including cams or just a bare casting?

Head ended up being a GM remanufactured, not aftermarket. Carries same warranty as a new head, so i didnt complain. Cams had to be transferred over from the old head.

[Trail X]

Curious Cody, when you had the misfire, did it come and go? Or did it just come one day, and never leave?

I'm really hoping that my periodic misfire is not this... funny thing is, I have a compression tester... just have yet to take any readings with it.

[Opeth]

Well before I changed the spark plugs it was just a little hesitation and if it sat in park idling long enough, the CEL would begin to flash. Blip the gas pedal and it would go off. It didn't always do this either. I changed the spark plugs with Delco's and it felt smoother for a bit but the vibs in park while idling were still there.

Eventually the CEL started to flash again and finally threw a code. A day later the code would go away, but would return once the truck got up to operating temp to idle down near 600 rpms. It then became very prevalent every day at some point. When accelerating it even started to flash before I finally brought it in to get checked out.

If I didn't change the plugs I dont think I would have really found the problem, but I'm not sure.

Wouldn't hurt to finally get that tester out and check.

[Trail X]

Flashing CEL? I'm not familiar with that operation of the CEL. I would guess it to be a more severe warning than a steady light.

I went through a stage where I got a steady CEL every few weeks, but lately I only get one every 4-6 months. Always P0302.