So the process started with three (in a row) sloppy sloppy cold starts. It wouldn't run without feathering the throttle, but after 3 minutes it smoothed right out and ran fine.

Clearly an intake gasket. Oh yeah, the patient is a '02 2500HD w/ 6.0 pushing 225,000.

So we dive right in, pull the throttle body and all the rest of that crap, swap intake gaskets, and of course clean everything up as we go. Who woulda thought mud-dauber wasps would think this was a neat nesting site?

Scrubbed the faces of the injectors with a BRASS brush while I was there.

Re-assembly went fine, but now we've got a tick. Not a hound-dog tick, but a tick in the engine; sounds like (and later confirmed to be) #5.

Pull spark plug wire, which separates. Install another. Good spark.

Pull compression check. Everything 180~195 EXCEPT #5, which is 210.

New plugs all around. Old ones Bosch double platinum tips, new ones NGK plain ol plugs. Still missing #5, still high compression. Did I mention the ports were vacuumed out while the intake was off?

Ran some tranny fluid, water mist, Mercruiser Power Tune, and more through there trying to knock out any carbon buildup. No change.

Pull and have cleaned all injectors and both rails. Put 'em back, with new plug wires all around too. No change.

Swap #5 & #1 Coil packs. No change.

Pulled injector wires #5 have varying voltage of 8.5~9.5V. SAME AS #3.

In military lexicon, Whiskey Tango Foxtrot. I have fuel, fire, and compression; yet it's a miss?

Anyone else seen this? I could pull the valve covers or even the heads but every ingredient is there for this thing to be cooking with gas... so what's up??

Puzzling, to say the least. Oh, and the miss is there 100%, all the time, throughout RPM range.

 

not sure if its possible but with your compression being so much higher could the exhuast valve not be opening? pop the valve cover off and see if both valves are opening properly

 

When you cleaned the injectors did you swap them around to make sure the one that was in Number 5 didn't go back in the same hole??? If not swap it with another and see if it follows.

Also put a noid light on and make sure the injector is being commanded to open and close properly on that cylinder. I assume you have checked that the connector is seated properly on that injector since you tried to check voltages.

If it does not follow the injector, pull the valve covers and check the rocker arms and springs. Slowly turn the motor over and make sure all the valves open and close properly. Make sure none of the rocker arms cam apart.

 

Check for a clogged cat also.

 

I would swap injectors between cylinders and see if miss changes. The higher compression coukd be due to carbon buildup but would not cause a miss. You could have a lifter not properly opening valve.

 

Use a bore scope and see if something fell in that cylinder.

 

I had a problem VERY VERY similiar to this. Turned out to be a weak valve spring.

 

!^ I didnt even think of that. Makes total sense. Sometimes you dont think of things unless u have experience with it.

 

Ok, so reading my own diatribe to start this convinced me the problem wasn't there before the intake gaskets, so maybe I should re-do that part.

Darn it, the problem stayed. Maybe even got worse. And GM wanted $42 for ALL the injector o-rings when I just needed the one ($0.69) for the left rail to the return line. It ain't GM Blue nitrile, but we'll see how it holds up.

So with the problem still there I started hunting borescopes. Boy they've gotten cheap; might just buy one to have around. 10M w/ 9mm dia tip for $47? Runs off USB cable. Anyone ever tried one?

Oops, back to the miss hunting. Pulled the left valve cover today just to look around. Springs all look great. Funny, the #5 intake valve had at least 5/16" of clearance, because I could measure it with a piece of key stock. Just didn't seem right, so I pulled that rocker arm and with a vice grip was able to extract the pushrod.

And if a picture is worth a thousand words, I'll shut up and let it explain itself! Anyone got a stock LQ4 pushrod spare in the glovebox? Or should I try to straighten this one? I'm thinking the #5 intake wasn't opening at ALL, or if it did, it would be real real rich, but we'll try a new pushrod and see what comes up next.

 

you may want to check to see if that valve/piston isnt damaged. i have never seen a pushrod bend without a valve hitting a piston or something else