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After 2 days in the shop, mechanic doesn't know why cylinder is misfiring

johnpfw

Full Member
Joined
Oct 18, 2008
Messages
15
Location
Las Vegas, NV
2008 with about 86k on it.
Engine light is on with P0304 code (misfire on cylinder 4).
Plugs and wires were replaced 3 weeks prior.
Switched coil for one from working cylinder. Same code returned after 125 miles.

Brought to local mechanic.
He had it for 2 days but doesn't know what the problem is.

He was able to confirm:
Misfire only happens at idle, increase the revs and no misfires reported.
He opened it up to check the springs and lifters and they looked OK on visual inspection.
He also confirmed that he could remotely close the cylinder in question which he says means the lifter isn't stuck.

:needhug:

Any ideas?

 
Cracked porcelain on the new plug when you installed it; it's very easy to crack one and not know it. Put a new plug in it; it's a cheap way to verify one more item is not the issue.
 
MyBigToy said:
Cracked porcelain on the new plug when you installed it; it's very easy to crack one and not know it. Put a new plug in it; it's a cheap way to verify one more item is not the issue.

This.

New does not always mean good.

Cheap and easy test is to just get one new plug and swap that one out.

Next, move the spark plug wire to another cylinder to test that, as well.

After that, I would consider a compression test to rule out a valve/lifter issue.

What is your oil pressure at idle and at higher RPM?

A failing lifter may not work as well with lower oil pressure and work better as the oil pressure increases.

Just some thoughts.
 
Something I just noticed.....
The new spark plugs I installed a month ago (ACDelco professional iridium 41-110) shows up on OReilly and AutoZone websites as 'Part doesn't fit your 2008 Avalanche'.

The original plugs were Ac Delco 41-985.

Could this be the problem or be contributing to the problem?
 
Sure, especially if the gap is wrong. Generally different spark plugs are designed to fire at different levels. Some fire "hotter" than others. Might be a timing deal, I am not that knowledgeable about the different ratings of the plugs but certainly, if you get the wrong plug that can affect its performance.

 
OK
Went back to O'Reilly auto parts store and got correct plugs. They said I can return the others once I get them out.
Installing them this afternoon.
Will report results back on Monday morning as to whether the misfire is still there.
 
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