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2003 chevy trailblazer_ls
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Discussion Starter · #1 ·
First I would like to thank everyone that makes this site possible-it is invaluable! I had a code p0306 #6 cylinder misfire at 75k miles, no mods, regular maintenance. It had a rough idle but performed good above idle. I put in new plugs and swapped coil packs but still had a miss at idle with no change in RPMs when disconnecting the #6 coil pack. Checked compression which was about the same as the others. Checked the fuel pressure at the tap 55 psi. Did not notice any intake leaks and planned on cleaning the TB next. I assumed that is most likely the fuel injector so I planned on replacing the #6 injector. While tearing it down I noticed the last 4 bolts on the intake manifold were finger tight with the back two halfway out.
I plan on changing the valve cover gasket but leaving the injectors alone since I am confident that this is the problem. I have yet to put it back together but if anyone wants pics just let me know. It can be a tricky tear down. Thanks Doug
 

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Discussion Starter · #5 · (Edited)
Solved photos attached.

:thumbsup:One would think so but it did correct the problem. Runs like a watch! It was showing a misfire on the #6 cylinder, not sure why it didn't trigger a p0171. I will add some photos of the different parts during tear down to hopefully help someone else for tear down and bolt location. I did have a lobe on the cam shaft # 5 intake cylinder that had discoloration. No edges to catch with a razor knife. Not sure whats up but will probably have to throw a cam shaft at it. You can see the dark areas around the #6 intake on the manifold too! Thanks Doug
 

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Thanks for the pictures. I am getting the same code for the #1 cylinder misfiring. I am in the process of removing the intake manifold, a real pain in the forearms ( cuts and scratches).
 

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Discussion Starter · #8 ·
Loose intake

No problem, I hope the photos help. The intake bolts were finger tight at best around the #6 cylinder. I know what you mean about the scrapes. You almost need a shark suit to work on it! I tried the coil and plug first with no luck and thought it was the injector but once I started taking off the intake I knew it was the problem. Let me know if you have any questions. The only special tool I really needed was the fuel line removal tool. $15 at Autozone....Good luck!
 

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I finally got it taken all apart. I had missed one of the intake manifold bolts behind the tube. That is when I found the pictures. I swapped the number 1 and 2 injectors and cleaned the throttle body. I got it all put back together ( and no extra bolts), still same rough idle and wont go over 2500 rpm. I now have 6 new codes, I am researching them now.
 

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Discussion Starter · #10 ·
I'm no expert but a plugged cat converter may limit rpm's? I ran a compression check compairing to the other cylinders and checked fuel pressure at the fuel filter. I assume you swapped or unplugged coil packs to check drop in RPM which would narrow it down to a particular cylinder? Hope this helps-let us know what you find.
 

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2005 saab 9_7x
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I also have a miss at idle on #6

I also have a miss at idle on number #6. I've logged some data and it got worse after swapping plugs last year. I forget exactly which ones I bought, maybe it was the Denso's, but I notice a bad miss at idle and turned out to be number 6. So I moved #6 plug for #1 plug and it stayed at #6. Then I swapped the same coils, and it stayed at #6.

I seafoamed a couple times, which helped, but still had the miss on #6. Finally, I went back to the factory plugs (new) and the misses on #6 dropped dramatically. They are still there, but not nearly as bad as before.

The last thing I wanted to try was swapping an injector from 6 to another spot. I've seen injectors stumble at idle, but once the rpms raise, they smooth out. So that's my next mission.

I will check to make sure all of the bolts are tight first. Hopefully it's just that, but we'll see. Any other help or advice is greatly appreciated.
 

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I would think that you would end up with a p0171 (lean) before it would even bother triggering a misfire. I wouldn't chalk this problem up as solved just yet.
You might not see a lean code because the fuel trims would be able to compensate at idle since it's only 1 of 6 cylinders misfiring. But if you get repeated misfires in one cylinder due to a vacuum leak, that would show up first before the lean code.
 

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2005 gmc envoy_slt_xl
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Thank you.

Thank you for the info and pictures.

Suspecting my #6 misfire was caused by the same cause, what I did today was to follow your info and picture and located the last two bolts for intake manifold and tightened it.

I do NOT have to remove anything! Just needed to work in the tight space to locate the bolts and tighten them -- that is all I did. First feel and locate the bolts then take your time to use right size socket and adapters to connect to them and tighten them.

Hope this info helps anyone who wants to do this.


First I would like to thank everyone that makes this site possible-it is invaluable! I had a code p0306 #6 cylinder misfire at 75k miles, no mods, regular maintenance. It had a rough idle but performed good above idle. I put in new plugs and swapped coil packs but still had a miss at idle with no change in RPMs when disconnecting the #6 coil pack. Checked compression which was about the same as the others. Checked the fuel pressure at the tap 55 psi. Did not notice any intake leaks and planned on cleaning the TB next. I assumed that is most likely the fuel injector so I planned on replacing the #6 injector. While tearing it down I noticed the last 4 bolts on the intake manifold were finger tight with the back two halfway out.
I plan on changing the valve cover gasket but leaving the injectors alone since I am confident that this is the problem. I have yet to put it back together but if anyone wants pics just let me know. It can be a tricky tear down. Thanks Doug
 

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2007 chevy trailblazer_lt
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Thank you for the info and pictures.

Suspecting my #6 misfire was caused by the same cause, what I did today was to follow your info and picture and located the last two bolts for intake manifold and tightened it.

I do NOT have to remove anything! Just needed to work in the tight space to locate the bolts and tighten them -- that is all I did. First feel and locate the bolts then take your time to use right size socket and adapters to connect to them and tighten them.

Hope this info helps anyone who wants to do this.
I'm getting a P0171 code that clears itself (lite goes out) after 3 or 4 drive cycles. Today I got a misfire on cylinder #6 code. I've been monitoring the sensors with an Elm327 OBDII tool connected to my laptop. The only thing notable that I've observed is that the long term fuel trim goes to a high positive value when the RPM falls below 1000. This would seem to indicate a vacuum leak. At higher RPMs the fuel trim hovers around zero.

The short term fuel trim is centered around zero all of the time regardless of RPM. (I don't really understand this as I am new to analysis of the data)

From reading this thread, I suspect that my intake manifold bolts are loose around cylinder #6.

The pictures are very helpful. Thanks for the posts. The temperature is supposed to get up to 50 degrees tomorrow, so I will attempt to locate and tighten the bolts. They look like they are going to be very difficult to access, but the quoted posted stated that he didn't have to remove anything to get the job done.

I have a couple of questions that would be helpful to know the answers before starting:

1. Where is location of cylinder #6 ? (I would suspect that it's the cylinder closest to the firewall)

2. Does anybody know the required socket size required to tighten the bolts?

Jim
 

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I believe the cylinder #6 is the one closest to the firewall.

I think the socket size is 10mm (I could be wrong here?). You can try to put your hand from the side of intake manifold and feel/locate the screw before you use the socket to locate it and then use the flex & extension bar to reach it. I can at least tighten up the two close to the firewall.

Some Update on my P0306 (Cylinder #6 misfire) --

I am still getting P0306. I don't feel anything abnormal. Engine runs very smoothly idling and/or high speed.

What I did?

- I replaced all spark plugs, tried to tighten up the two screws (close to firewall) for intake manifold.

- I switched #6 and #2 spark coil, but I still got P0306 after driving for hundreds (?) of miles. I was expecting getting P0302 if the #6 spark coli was bad. But it seems the spark coil for #6 was good.

- P0306 always appear after a long high way driving and then occurs during local idling -- once again, I don't feel anything abnormal, the truck just drives very smoothy (local or highway).

- I also tried to start truck so it re-learn idling. Let it start to run idle for 3 minutes, and shut it off, then start it again let it idle for 3 minutes. (I did not disconnect battery before I did this). The P0306 appears again today. I was in NYC, and I just used ODB-II tool and deleted the code. I drove back from NYC for about 220 miles, and the P0306 has not set so far.

- Next I still try this if the P0306 occurs again: I will disconnect the battery (or ECM) and reconnect to reset ECM. Then try let engine relearn idle again to see if it helps.

By the way, for my trip back from NYC, the MPG finally gets 18 (I reset the MPG once I got on interstate) with some traffic jam in Connecticut -- You know what I am talking about if you know this area.

Please let me know if you have any suggestion on how to deal with this mysterious P0306 code? To me, I am not convinced that the truck has any performance problem -- it seems a ECM related code mis-firing/reporting. Any suggestion is greatly appreciated.


I'm getting a P0171 code that clears itself (lite goes out) after 3 or 4 drive cycles. Today I got a misfire on cylinder #6 code. I've been monitoring the sensors with an Elm327 OBDII tool connected to my laptop. The only thing notable that I've observed is that the long term fuel trim goes to a high positive value when the RPM falls below 1000. This would seem to indicate a vacuum leak. At higher RPMs the fuel trim hovers around zero.

The short term fuel trim is centered around zero all of the time regardless of RPM. (I don't really understand this as I am new to analysis of the data)

From reading this thread, I suspect that my intake manifold bolts are loose around cylinder #6.

The pictures are very helpful. Thanks for the posts. The temperature is supposed to get up to 50 degrees tomorrow, so I will attempt to locate and tighten the bolts. They look like they are going to be very difficult to access, but the quoted posted stated that he didn't have to remove anything to get the job done.

I have a couple of questions that would be helpful to know the answers before starting:

1. Where is location of cylinder #6 ? (I would suspect that it's the cylinder closest to the firewall)

2. Does anybody know the required socket size required to tighten the bolts?

Jim
 

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2005 chevy trailblazer_lt
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For what it is worth!

I have had a constant (daily) PO300 or PO306 CE light on for months, Started using Marvel Mystery Oil (don't laugh) in my TB crankcase since 11/2013 and with my code reader no PO300 or PO306 has shown up during several checks. even my mechanic was surprised, He told me with 180,000 miles on it, I should just drop in a new engine. Also GM has admitted to the fact that the head design was defective for 2005 and some other years, something to do with the valves not closing properly in the combustion cycle. Perhaps the Marvel of this mystery is that the carbon is getting cleaned up. :undecided So this is what it's worth.
 

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2006 chevy trailblazer_ls
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Engine Code P0306

to Douglas Case - thank you for the description and excellent photos to assist the repair for this error code. You are a great MAN among men!!
I look forward to tackling this job as soon as the weather clears.
Fred Schmidt
 

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2006 gmc envoy_sle
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I had a code p0306 #6 cylinder misfire at 86k miles. Replaced the coil pack and seems to be fine now after a week. Thanks for posting all, appreciate the info and less guess work. :thx
 
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