Getting code P0306

jbones

Original poster
Member
Dec 5, 2011
658
Engine light on, two different auto stores scanned P0306, cyl 6 misfire. I checked most of what I could coil, plugs all the preliminary stuff each time having the auto store clear the code, always came back a few days later. Ended up taking to stealership, they did a top end clean and FI clean last Friday. Light came back yesterday, had it read today, and same P0306. I searched the forum and read the threads on the issue, sure hope it's not the valve in #6.

What Dwayne Lanes did.
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CaptainXL

Member
Dec 4, 2011
2,445
The leakdown test confirms crud on the intake valve. Just keep running fuel injector cleaner at every fillup. Use Chevron Techron Concentrate Plus, BG 44K or any other fuel injector cleaner that has PEA (Poly Ether Amine) in it. Note if the misfires start getting better or less frequent. Then you know you are on the right track. The dealer did notice that the misfire went away after a treatment, so keep at it.
 

jbones

Original poster
Member
Dec 5, 2011
658
CaptainXL said:
The dealer did notice that the misfire went away after a treatment, so keep at it.

Good point!

But at what point, or test determines if the valve is bad. Because the dealer is alreadly mentioning $3k to fix a bad valve, Seems a bit fast, and costly in the trouble shooting.
 

CaptainXL

Member
Dec 4, 2011
2,445
jbones said:
Good point!

But at what point, or test determines if the valve is bad. Because the dealer is alreadly mentioning $3k to fix a bad valve, Seems a bit fast, and costly in the trouble shooting.


Hard to say. I would give it a few months unless the check engine light is constantly blinking which indicates imminent damage to the catalytic converter will happen.

The P0300 is interesting. This indicates that more than one of the cylinders is misfiring. This could indicate water in the fuel or just crappy fuel in general. How often does the truck get driven? Try adding some isoheat to the tank and see if it improves.

About the only thing I can think of that you can do that might help (aside from cleaning the fuel system) is to perform an engine oil flush. Especially if you notice a bunch of top end valvetrain clattering.

How many miles on your engine and what type of oil do you use? Are you the original owner? I am assuming that if you are not then the engine was maybe abused in some sense? What's the maintenance history?

Another thing to think about. Since the misfire is a P0306 this could indicate that the fuel pressure may be low and the #6 injector could be stuck open in which leaking fuel may create a buildup of crud on the top of the valve. f you have the means I would borescope the intake port and take a look at the valve. An injector balance test should also be performed before any drastic measures such as replacing the head or doing a complete grind and seat valve job. I highly doubt you have an out and out physical valve problem at this time. The dealers fuel injector service confirmed that.

Keep running injector cleaner, flush oil if possible and drive it like you stole it (if you normally just putt around town).
 

jbones

Original poster
Member
Dec 5, 2011
658
CaptainXL said:
Hard to say. I would give it a few months unless the check engine light is constantly blinking which indicates imminent damage to the catalytic converter will happen.
Engine light is steady.
CaptainXL said:
The P0300 is interesting. This indicates that more than one of the cylinders is misfiring. This could indicate water in the fuel or just crappy fuel in general. How often does the truck get driven? Try adding some isoheat to the tank and see if it improves.
Drive every day, fill up often @ Costco. Even got a speeding ticket a few weeks back.

CaptainXL said:
How many miles on your engine and what type of oil do you use? Are you the original owner? I am assuming that if you are not then the engine was maybe abused in some sense? What's the maintenance history?
Was a lease vehicle, I purchased it with 16,000 miles. 72,000 now, gets regular oil changes, I use Castrol GTX 5-30, I cleaned the TB at 40,00. No other engine maintenance issue since owning.
 

CaptainXL

Member
Dec 4, 2011
2,445
Assuming all your maintenance is good which it sounds like I would check the last 4 bolts on the intake manifold and make sure they are secure. A search on google brings this problem to the top.

What can happen is that the computer detects a lean condition in the cylinder (too much air) because of a leaking intake manifold and it adds fuel to compensate which would foul the intake valve and possibly the injector.

https://www.google.com/search?q=p0306+trailblazer
 

Opeth

Member
Mar 25, 2012
177
I went through this last year.... Loose intake manifold bolts and a stuck/non- seating valve. Head replaced under extended warranty $4k bill because all but 3 head bolts snapped off. My out of pocket was $300 for non covered fasteners and liquids and deductible.

G'luck
 

jbones

Original poster
Member
Dec 5, 2011
658
CaptainXL said:
Assuming all your maintenance is good which it sounds like I would check the last 4 bolts on the intake manifold and make sure they are secure. A search on google brings this problem to the top.

What can happen is that the computer detects a lean condition in the cylinder (too much air) because of a leaking intake manifold and it adds fuel to compensate which would foul the intake valve and possibly the injector.

https://www.google.com/search?q=p0306+trailblazer

I'll go back and check the intake bolts as mentioned , checked the TB bolts earlier, GTG.

Hope what Opeth had eludes me, my warrenty is expired, never purchased the extended.
 

jbones

Original poster
Member
Dec 5, 2011
658
So didn't have time to check intake bolts before I went to work, I'm assuming I have to remove the PCM and intake cover first. This weekend I guess.

But I did manage to get a few short turns on the opposite sides cam shaft cover bolts. Coincidence? But the engine check light was off when I went to work. I thought you had to clear the error code. Did the light turns of the bolts tell me I have a bad gasket?
 

CaptainXL

Member
Dec 4, 2011
2,445
The intake manifold bolt torque should be properly checked, not the camshaft cover. I wouldnt just "hand tighten" these bolts. In fact no bolt on an engine should be hand tightened, ever.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,028
Brighton, CO
The only other thing I can add to the above is SeaFoam. Have seen that stuff work wonders!
 

jbones

Original poster
Member
Dec 5, 2011
658
Light is off, and not throwing a code right now. I think I'll wait it out to see what happens. If light returns i'll check manifold bolts, and go from there.

Maybe the top engine clean and FI service did the trick, and the light returning for a few days was the truck passing a stone :sick:(left over carbon build up)? Any way I'll wait to see.
 

navigator

Member
Dec 3, 2011
504
jbones said:
Light is off, and not throwing a code right now. I think I'll wait it out to see what happens. If light returns i'll check manifold bolts, and go from there.

Maybe the top engine clean and FI service did the trick, and the light returning for a few days was the truck passing a stone :sick:(left over carbon build up)? Any way I'll wait to see.

I've seen with lights, they come on and stay on for awhile and then even if you haven't resolved the issue they go off. If the issue hasn't resolved itself it will come back in a few days. Hopefully your service and running it has cleared it up.
 

jbones

Original poster
Member
Dec 5, 2011
658
navigator said:
I've seen with lights, they come on and stay on for awhile and then even if you haven't resolved the issue they go off. If the issue hasn't resolved itself it will come back in a few days. Hopefully your service and running it has cleared it up.

You called it good!

Darn light back on today.
 

jbones

Original poster
Member
Dec 5, 2011
658
As I mentioned the light returned, same code. Picked up some Seafoam and Chevron Techron Concentrate plus. Will try this after I verify manifold bolts tighness tomorow.
 

CaptainXL

Member
Dec 4, 2011
2,445
jbones said:
As I mentioned the light returned, same code. Picked up some Seafoam and Chevron Techron Concentrate plus. Will try this after I verify manifold bolts tighness tomorow.

Don't make the mistake of dumping Sea Foam down the brake booster hose to clean your top end and pistons. Its just NOT good for that, it DOESN'T work worth a damn, and its risky because you could damage your engine. Most of the smoke you see in videos is just the pale oil burning off....not dirt and grime. See the following MSDS which tells you what is in Seafoam. It's basically made of mineral oil, isopropyl alcohol and naptha.

http://www.mta.ca/administration/fa...Engine treatment - SeaFoam Sales Co. 2010.pdf

Use it in the fuel only if you ever do use it. Techron Concentrate Plus or BG 44K would be much better to clean the carbon out of the engine.
 

jbones

Original poster
Member
Dec 5, 2011
658
CaptainXL said:
Don't make the mistake of dumping Sea Foam down the brake booster hose to clean your top end and pistons. Its just NOT good for that, it DOESN'T work worth a damn, and its risky because you could damage your engine. Most of the smoke you see in videos is just the pale oil burning off....not dirt and grime. See the following MSDS which tells you what is in Seafoam. It's basically made of mineral oil, isopropyl alcohol and naptha.

http://www.mta.ca/administration/fa...Engine treatment - SeaFoam Sales Co. 2010.pdf

Use it in the fuel only if you ever do use it. Techron Concentrate Plus or BG 44K would be much better to clean the carbon out of the engine.

I did a bit of reading on the Seafoam and video watching after buying last night, and decided to scrap the idea.


I went back to checking intake manifold bolts, all were loose, I tightened, took back the Seafoam, and purchased two bottles of the Chevron Concentrate Plus. Hope the very loose bolts were the culprit, light is off for now, dumped a bottle in, fingers crossed.

It almost seamed the truck ran so much better after the tightening, could be wishful thinking, but it really did seem more responsive.

I forgot to mention, I'm from Kalamazoo, I used to race my 70 Malibu on 28th street, a few life times ago.
 

seanpooh

Member
Jan 24, 2012
461
Are the intake maifold bolt fairly easy to get to? Every time I think of checking the tightness, my mind just thinks that I have to remove something out of the way to reach the bolts...
 

jbones

Original poster
Member
Dec 5, 2011
658
seanpooh said:
Are the intake maifold bolt fairly easy to get to? Every time I think of checking the tightness, my mind just thinks that I have to remove something out of the way to reach the bolts...

Not too bad, you can see the majority looking in at different angles, once I got a warm fussy where to blindly manipulate the socket, a piece of cake. The one I still need to check appears to be blocked by the PCM. I unclipped and hung off the side, and looked as if I would still need to remove the bracket? But don't take my word for, I didn't look all that hard, I had somewhere to go. Hope to check that last bolt out tomorrow.
 

CaptainXL

Member
Dec 4, 2011
2,445
jbones said:
Light back on yesterday, and off today. WTH!

Then it seems to be fuel or ignition related since it comes and goes. Swap out the coil and spark plug with #1 in the short term.

Long term have an injector balance test done and get a fuel rail clean if needed and replace any faulty injectors.

Failing that have a leakdown test performed. Actually you should probably start there and do the leakdown test first before this leaves your pocketbook empty trying other things. The goal is to get a baseline that the mechanicals inside the engine are operating correctly. Then you can proceed to the electrical and electro mechanical stuff like coils and injectors.
 

Opeth

Member
Mar 25, 2012
177
Just go right for the leak down test, after all you've posted it's leading to a seized/ stuck valve. Everything you've done is exactly what I did, hate to say it but it might just be inevitable.

It's not an easy job either to remove the head, my GM bill was around $4k. Nearly all the head studs will snap off and exhaust manifold bolts too. If it is the head, might be easier to offload the truck and get into something else if you don't have the cash before that light starts to just stay on.
 

CaptainXL

Member
Dec 4, 2011
2,445
I think it would be worth trying some solvent flush as well in the oil. Not a lot of money wasted there. But yeah what opeth said it sounds like the valve might be stuck.
 

jbones

Original poster
Member
Dec 5, 2011
658
l'll get the leak down test done again, but after top end clean & FI clean it passed when the dealer re-checked (within specs they said ), and they mentioned all injectors where good.

Looking at the service manual what of the listed causes; such as, loose of damaged accessory drive system components, that may send a misfire code without actual having a misfire, or bad MAP sensor?

What type of solvent flush in the oil you recommend CaptainXL. I originally moved away from the Seafoam thing, and have been running Techron Concentrate plus about every other two - three tanks, so I wouldn't imagine seafoam in the gas tank would produce any result the current stuff isn't already.

But yes, leak down test sounds good, would like to see if any improvemnet since I tightened the intake manafold bolts.
 

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