2005 Chevy Express 2500 - 4.8L #4 Misfire

[oldsmobile39]

Had a Chevy express van come in for a cylinder #4 misfire. Ran normal diagnostics- spark, fuel, injector pulse, etc., which all came back normal. Ran a compression check and found no compression for that cylinder. Removed cylinder head and found a warped valve at the stem area- no evidence of contact made at the head, and no overheating issues or abnormal discoloration or the metal.

Has anyone ever head about this, and any reason why? We're replacing the valve today, but I'd hate to have it happen again. Any infor would be greatly appreciated!!!

[ricmorin]

Which engine?

[liljoe]

What was the service history before the mis fire started. Is it a regular cust, or a newbie?

[oldsmobile39]

Sorry- its a 4.8L V8 withan automatic transmission, and cargo body. Approx. 60K miles on it, too. Regular customer- a commercial account to boot.

[ricmorin]

I've read about some issues about valve guides too tight and cam.lifter problems. Also sludging issues. Look carefully.

[steven kiser]

check valve guide. i would think that this was run low on oil and the valve stuck and was hit by the cylinder or the lifter locked. i've seen lifters throw the retainer clip and the seat cock and momentarily jamb up. a valve just can't bend. if it was a restricted exhaust the manifolds would show heat bluing. if the cylinder was spotless and the others show some carbon i'de think hydralock but that usally bends valves. i've never seen a valve bend without a reason. valve guide may have dropped and caused this. i would see this in escorts a lot. they would pop out and dance around a bit and somehow pop back in.

[Tim Martin]

I have seen a lean mixture do this to a valve. The combustion temperature can get high enough with a lean mixture to pull a valve rught up into the head. Coolant on a hot valve will also do this as well as a piece of carbon getting loose.

hth