Good compression, cylinders dead?

[Phantom240]

Okay guys, here's the skinny. I have a 99 Yukon Denali with 129k miles. Cylinders 5 and 7 are dead. I ran a compression test on that bank, and came up with 205psi on #1, 195psi on #3, 200psi on #5, and 198psi on #7. From a compression standpoint, this motor is strong as an ox. When running, it smokes a bit and smells like burning oil mixed with tons of fuel. The fuel smell could be explained by the dead cylinders. I haven't popped the valve cover off yet to check for valve operation, so I guess a valvetrain issue still isn't out of the question. Are there any other pointers you guys could give me to help me diagnose the problem?

 

[Phantom240]

I did some thinking today, and came to the conclusion that my problem cannot possibly be valvetrain related. If I had a bent valve, compression would be garbage. If I had a flat lobe on the cam, I would either get backfiring through the intake (exhaust valve), or no compression (intake valve). Thus, I believe my issue to be fuel related. In reading about these Vortec motors, I've seen that the injection system is a pile of steaming rhino dung (to put it lightly), and are prone to many types of failure. It is my intuition that either A) the fuel pressure regulator is bad (less likely), B) the lines running to the poppets have ruptured (somewhat likely), or C) the two poppets are stuck open, flooding the engine with copious amounts of fuel.

Agree or disagree?

 

[SunlitComet]

if you have not upgraded to the new injector system, you might as well get that and your manifold gaskets redone with the better designed parts.. sure yo don't have a burned out dizzy cap or leaking valve seals fouling things up?

 

[Phantom240]

The leaking valve seals is a definite possibility. I don't really know of a way to test valve seals aside from popping the head off. As for making sure I'm getting spark, I'm fairly certain, though I can get under the hood with a spark tester to make absolutely sure. The ignition components in this thing seem to be OE. I think the plugs show excessive wear, judging by the corroded mess on the ground tab, and what seemed to be excessive gap. I've realized that it may just need a tune up, but I'm doubtful. Here's a picture of one of the healthier looking plugs. If you look closely at the ground strap, you can see the buildup of corroded garbage created from the spark jumping the gap over the years.



EDIT: Oh hell, I just realized that 5 and 7 are fired sequentially, meaning that a crack in the distributor cap could muck up the whole operation, rendering those two cylinders dead.

 

[SunlitComet]

pretty good plug actually running a dirty tho. try out a spark tester and lets get thinking about some possible maintenance. what do you know of the maintenance on the drivetrain?

 

[Phantom240]

Yeah the first two plugs didn't look too bad. Worn and dirty, but otherwise the coloration looked fine. This one came out of one of the two dead cylinders, #7 specifically. It's obviously fouled badly. I have no idea how long this engine has been running in the current state of disrepair. The gap on these plugs doesn't look bad to you guys? I mean, when I put new plugs in my Prelude, the center electrode was a bit longer. It looks like the center electrode on this plug has worn down considerably.

I pulled the plug wires off of 5 and 7 last night, and 5 had a hard time producing a spark. I had to just about put the head of a bolt in there to get it to arc, telling me that the path of least resistance is likely somewhere else in that distributor cap, which promotes the theory that it could possibly just need a tune up... and whatever is causing it to burn oil.

I know as much as you do about the maintenance.

I bought the truck at an auction for a whopping $1700. According to a sticker in the windshield, it came from another auction house, and the title shows it was traded in at a dealership in Florida.

So once I get it running and fix the sloppy front end, I'm going to take it to get the transmission, transfer case, and differentials serviced. This is only the second vehicle I've owned with an automatic, and the first 4WD... so I'm not gonna play around with it lol.

 

[SunlitComet]

different plug makers look different but those are platinum and are pregapped to .060 when new and should run about 100k miles. what gap can you measure on those now? would you like #'s for a stock parts tune-up?

 

[Phantom240]

I priced everything out, and checked availability. However, I don't know which exact plug came in this thing as factory equipment. Was it the AC Delco 41942?

 

[SunlitComet]

now it is 41-993. it will be a platinum pre-gapped delco.

 

[Phantom240]

Coolbeans. Gonna pick everything up tomorrow, and see what she does.