Misfire Help

bsaera

Original poster
Member
Jul 6, 2012
2
Need some help with 2005 Envoy. It's the 4.2l SLT with around 78k miles. I have a misfire in cylinder 4, code p0304, I've change coil and spark plages, also moved them around to other cylinders to see if it changed cylinders. It always has cylinder 4 misfire. Looking down the spark plug holes I noticed a lot of carbon build up, did a compression test and all except 4 were around 190 and cylinder 4 was about 90. Tried seafoam through the vacuum line and in the gas. Did not work so I got some GM Top End Cleaner. Removed the spark plugs poured some in and let it soak for a day. Sucked it back out with my hand vac and the liquid was black. I poured in 32oz across all 6 cylinders but only 16oz came out. Put everything back together and it ran great. No check engine light! This lasted for three days, about 100 miles, now the check engine light is back on and it seems to run worse than before. I'm looking for some suggestions.*
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
CaptainXL said:
Hello member! What's the code?
:nono: The code was given. The OP did 90% of the work already. Read the full post and give decent advice. This isn't trailvoy. :frown:


bsaera said:
Need some help with 2005 Envoy. It's the 4.2l SLT with around 78k miles. I have a misfire in cylinder 4, code p0304, I've change coil and spark plages, also moved them around to other cylinders to see if it changed cylinders. It always has cylinder 4 misfire. Looking down the spark plug holes I noticed a lot of carbon build up, did a compression test and all except 4 were around 190 and cylinder 4 was about 90. Tried seafoam through the vacuum line and in the gas. Did not work so I got some GM Top End Cleaner. Removed the spark plugs poured some in and let it soak for a day. Sucked it back out with my hand vac and the liquid was black. I poured in 32oz across all 6 cylinders but only 16oz came out. Put everything back together and it ran great. No check engine light! This lasted for three days, about 100 miles, now the check engine light is back on and it seems to run worse than before. I'm looking for some suggestions.*
Sounds like you eliminated the easy and cheap causes. You're left with rings, bad valve (there are four per cylinder), or a head gasket leak. Next diagnostic to run is a leakdown test.

Any other unusual maintenance history, like neglected oil changes or running hot? Did you buy it used and don't know the maintenance history?
 

aspevacek

Member
Jul 4, 2012
11
Run a compression test and a wet compression test on the low cylinder. See where the numbers go. I noticed you had already ran a compression test adn it was out of spec versus the others generally engine builders will give a total of 10% to 20% between cylinders. The carbon should not cause that much of a difference unless it is keeping a valve from fully closing or slowing a valve that is closing. have you hooked a vacum guage to the intake to see what it looks like at idle and as you rev the engine a bit? how did the plug look that came out of the low cylinder?

I know I have asked a few questions but they were things i did not see in the original post.
 

CaptainXL

Member
Dec 4, 2011
2,445
the roadie said:
:nono: The code was given. The OP did 90% of the work already. Read the full post and give decent advice. This isn't trailvoy. :frown:

Um. I. Think we have a misunderstanding. The op had a code and fixed it but now they have a new code. Just trying to find out what the new code is.
 

CaptainXL

Member
Dec 4, 2011
2,445
the roadie said:
I figured it was the same code, because low compression, if not fixed, is always going to throw a code eventually.

My motto is this. Never assume anything. It makes an a$$ of you and me. Bad troubleshooting skills Bill :biggrin:. I was justified my response. Just the facts maam ... just the facts. From dragnet .:thumbsup:
 

bsaera

Original poster
Member
Jul 6, 2012
2
Thanks for the help. Code stayed the same each time it came on (misfire 4). I did the wet compression test and no change in PSI reading. I purchased the Envoy used with around 20k miles, before that I don't know the history, but since then I've taken very good care of it (oil changes every 4k miles and never overheated). All the spark plugs had small amounts of oil on the threads (which I've read is ok) and cylinder four plug seemed to be blacker than the others, the tip and the first few threads. One thing I thought was strange but maybe it's nothing: after my last round of seafoam I put in new ACDelco plugs, two weeks or so later I pulled them all back out to pour in the GM Top End Cleaner and the spark plug from cylinder four had an orange ring at the base of the white shaft.

Just when I finally accepted the fact that I need to take it to someone (fixing rings, valves or head gasket is not something I'm going to tackle in my garage), the Envoy fixed itself! Woke up this morning and the check engine light was off and I couldn't feel the misfire.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Have you had a lot of rain lately?
The orange on the porcelain is probably rust.
Cylinder #4 is often the one that gets water in the plug well. (Water drips down between the hood and the cowl.)
After the water boils/dries out, the miss will go away.
 

freddyboy61

Member
Dec 4, 2011
276
That wouldn't explain the bad compression reading on cyl. 4. It probably was a hung up valve, maybe some debris causing a poor seating situation. Hoping it stays fixed.
 

Opeth

Member
Mar 25, 2012
177
Just went through this fiasco about two months ago with #6..... The damage is probably already done, but check your intake manifold bolts. Mine were loose, not even hand tight which I assumed caused a lean condition in that cylinder and burned my valves. I had to get a whole new head installed, to the tune of $4k....I had around 74k miles on it too.
 

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