#6 & #8 Misfire @ Idle

[JonnyV2889]

Hey GM Tech,

 

Doing some work on a buddies 03 Tahoe 4.8L. He came to me showing CEL which scanned to be codes P0300 & P0430. The dealer quoted him 3k to fix those codes, a wheel speed sensor & do rear brakes. Long story short we ordered a new Y-pipe which will be here tomorrow, but yesterday he drove to the dmv and the thing died. Had AAA tow it to my place, cut the cat open & gutted them, then welded them back up to get him rolling again. He's put about 50 miles on since then, I'm expecting a CEL for the gutted cats, but my main concern is at idle cyl #6 & #8 are showing frequent misfires on my MAC mentor. It doesn't appear to be enough to trip a CEL light but it does trip on the mentor. Underload it doesn't read a misfire but very seldom. Put new plugs & wires on a few days ago, it appeared to be idling & running fine tonight minus the misfires picked up by the mentor. Had him pick up a bottle of injector cleaner, not sure where else to go with this or if I should even be worried? Most misfire per cyl I have seen between those two is 16 in a cycle @ idle.

 

Thanks

Jon

[JonnyV2889]

PS I want to say he has roughly 110k miles

[txab]

Leaking intake gaskets cause misfires

[cwendel4]

+1 on the gasket

Use carb spray and spray it around the intake gasket and see if they go away.

 

I would also check the two plugs and make sure they did not crack when you installed them or were cracked when you got them. I have had that happen before and a new plug fixed the problem.

 

Also, see if there is an oil leak that is leaking oil onto the porcillin of the plug, that can lead to miss fires happening as well.

 

But if it went away wit the new plugs, buy some good injector cleaner and run it through with a tank of fresh gas.

[minibike132]

What is the closed loop short term fuel trim on both banks?