So I finally get the car running again after months of being in various stages of repair...

Symptoms:

Intake manifold pressure fluctuating... Had been idling around 28-29 kPa now I can't get it below 32-33
Fixed the exhaust headers and got rid of the leaks... AFRs still bouncing around 1-1.5 lambda
Crankcase pressure through the roof (blowing gases out of any open hole... PCV, breather, oil cap, etc) Oil is being forced up and out of the dipstick tube and also through the PCV tube into the upper intake, oil being sucked into cylinders 1, 2, 5, and 6 via the intake plenum
Compression test: (keep in mind I'm running 11.4:1 CR) Cylinders 2,3,5,6: ~220 PSI. Cylinders 1 and 4: ~65 PSI

Engine has almost never been run lean. Mostly so rich it makes your eyes hurt.

I'd believe blown rings in one of the 2 bad cylinders but both cylinders on the same coil pack (DIS)? I kind of need this thing on the road by August because I think I have a wedding to go to (invitations haven't been sent yet) and I would prefer to drive this to my buddy's wedding...

 

Hate to tell you but unless you've got a couple valves adjusted too far down (easy to check) you've got AT LEAST head gasket problems, possibly holes in a couple pistons.

There's only so many ways air can get out of the cylinder- past the valves, past the head gasket or past the piston. Time to figure out which.

And, uh, 11.4:1 compression on pump gas? Not advisable. Detonation does nasty things. Like, oh, for instance... push head gaskets and break pistons.

 

Um, valves are NON-adjustable on 3100 cylinder heads and no other cylinder has this problem. Didn't have this problem until late 14 and had run on this setup for better than a year until whatever happened. Failed head gasket? No cooling system problems and I would tend to think that I wouldn't have blowby into the crankcase... Hole in a piston? I seriously doubt it. Especially 2 of them. Wouldn't have ANY compression, I would tend to think. The engine doesn't really smoke through the exhaust (except where it's ingesting it through the intake and that isn't much), no funky noises, nothing. And yes, it still runs.

Running 91 octane straight gas. And tuning for it. Very little timing and very rich on fuel (because the AutoTune in Tuner Studio refused to work while I was driving because the AFRs were bouncing so bad before I had the headers fixed). I'm good. At least in 4 of the 6 cylinders. And all 6 for over a year until things started going to crap. Only put 87 in once and that was waay long before this problem reared its ugly head. Ended up replacing the FPR because the ethanol ate it.

Decided that between the low compression and the screwy starter mount, I'm going to get another short block. And having it rebuilt at a machine shop. King main and rod bearings, new cam bearings, new piston rings. Might go with the thicker 3100 MLS head gaskets to drop the CR a pinch, but it will still be over 10:1. Although having the block decked .015" will probably negate that...

Now, if someone could tell me the difference between plain, moly and chrome piston rings I'd appreciate it. And why I should use what rings.

 

Well then I guess you don't have a problem since you've concluded it can't be any of the things I said it was.

Except for the inescapable conclusion that the air is getting out of the cylinder SOMEHOW and there's only so many ways that can happen. ALL of them mechanical.

Maybe you get lucky and just find a bent pushrod. Or a broken rocker. Something easily replaced. But no matter what, if you want to fix it you ARE going inside that engine because only MECHNICAL problems cause low compression readings.

"Plain" rings are cast iron (or ductile iron) faced. The ring is all one material. (We're talking about the top compression ring, since that is where most of the differences are between different types of ring sets)

"Moly" rings have a groove cut in the face of the top ring which is filled with a soft metal compound made of molybenum alloy. Allows for quick and tight sealing of the ring to the bore. "Plasma Moly" are the same thing but the process used to adhere the moly to the ring face gives it a tighter bond and it's better in situations with high cylinder pressures (boost/nitrous).

"Chrome" rings have a very hard facing material that is much more resistant to wear. Allows longer ring life in dusty environments. For a car that doesn't go off road into dusty dirty environments these are not the rings you want to use- things have to be machined JUST RIGHT to get them to seal up.

No matter how worn out your rings are they are not going to drop cranking compression from 220 PSI to 65 PSI. If they're BROKEN or the piston is missing chunks, yes, that would do it.