I have a 2003 Chev Avalanche 5.3 L with 164K miles & a misfire on cyls 7 & 8. Here's what's been done to try to find the problem:
Swapped coils packs 1 & 2 with 7 & 8. No change
Swapped plugs 1 & 2 with 7 & 8. No change
Plug wires... No change
Fuel injectors...No change
Regardless of what's been done, the misfire stays with cylinders 7 & 8.
Any & all ideas accepted. Mechanic & I are baffled.

 

would check compression, check for vacuum leaks around intake is it actually missing or running bad or just giving a check engine light while still running ok?

 

Vacuum leak at rear of intake. (Common)
Worn cam lobes. (Seen this many times)
Remove valve covers and check valve movement with dial indicator.

Broke valve spring(s). (Requires a close visual check)
Broken valve guides. (Not so common, but does happen)
Broken teeth on plastic covered timing gear. (Real challenger)
There are other causes, but they are not commonly found.

Happy Hunting!

RR

 

Thanks for the quick responses.
Alen: We've checked for a vac leak & found none. Intake gasket was replaced 2-3 months ago. Compression check is on the list of "to checks". It idles somewhat rough (worse at operating temp). Definite miss under light to moderate acceleration. Runs fine on hard accel. According to the Snap On OBD II analyzer, it is actually missing on 7 & 8. If so, I would expect it to idle really rough which it does not.

Rooster: Good suggestion about the timing gear. I know the belt (or chain) has never been replaced. If it turns out to be the gear, good idea to replace the belt/chain at the same time?

 

Rooster: Good suggestion about the timing gear. I know the belt (or chain) has never been replaced. If it turns out to be the gear, good idea to replace the belt/chain at the same time?

Timing gears and chain are replaced as complete set. Not real expensive, but is quite labor intensive.

The intake manifold can have an internal leak. Good possibility it got warped when gasket was replaced.
Encountered this problem multiple times while operating a repair shop. Intakes must be tightened in small increments until proper torque is reached, and following correct torqueing sequence.

RR

 

How would one detect an internal intake manifold leak? The intake on this thing is plastic.

 

Alan & Rooster: It seems we found the problem. Just wanted to follow up & let you know what we found. Both coil input plugs were dirty & not providing voltage to the coil packs. Gave them a good bath with contact cleaner & unpluged & pluged back in several times (with a good bath each time). After a few times, cylinder 8 started firing normally. Repeated the process on cyl 7 and lo & behold, it started firing normally. Maybe it will last, maybe not, we'll see. Thank you both for your suggestions.

 

Good to know you found a culprit.
Electrical gremlins are the most difficult to find.
Consider putting dielectric grease on those plugs to gain time before next corrosion problem.
Thanks for reporting!

RR

 

Late to the show (as always ) but I had a 2000 Silverado with a random misfire code. Never had any symptom other than the CEL. All the mechanic did was disconnected the battery and let it sit. Reconnected the battery and all was right with the world.

 

Rooster: Having had a boat, I did exactly that. Why both back cylinders & at the same time is the real puzzler.
BTY, Did you know boat is not a word but an acronym for Break Out Another Thousand? Be Well Guys