Another update.... after fixing the cat problem, finally got it dropped off to diagnose whats going on after having no codes. They verified some misfires on driver side bank 1, and the rich condition on bank 1. Bank 2 all is happy. They pull the spark plugs on bank 1 and find they are clean and look good, but cylinder walls have some blue discoloration on them, which they say is likely from overheating. Not once has the needle ever moved from straight up in the center.
So then they do a leakdown test, and everythign comes back within spec.. BUT the go on to a compression test, and tell me that I am only getting 90 psi on all 8 cylinders, and the motor is worn out... ??! This thing runs pretty smooth.. only sometimes a very barely noticeable vibration. If you hit the gas, truck takes off like a bat out of hell. Not burning oil.
I am losing my mind on this, its not adding up. I dont have a reason to not believe them. They tried two different gauges for the compression test, did it warm and wet to confirm. How can I be at 90 psi without eating gobs of oil and fouling the plugs? Much lower than 80 psi and the engine wouldn't even fire. But why does this show on all banks, and not just the problem bank? Nothing is making sense with this engine.
The only way I see a compression test coming back bad with a leakdown test good, is if the rings are OK when at TDC, but the cylinder walls are warped or "curved" going from top to bottom, giving the cylinder a shape like this (_), where you could lose compression only when the piston falls...
I really hope I find some other freak explanation for this, that doesn't demonstrate I just spent 15k on this motor in the last 30k miles rebuilding all timing systems, variators, supercharger, injectors, cats, fans, walnut blasting the intake valves, countless other diagnostics and tests.. to find out the motor is done.
With the technical issues these vehicles often present it's easy to get sucked into solving a problem without resorting to fundamentals.
A compression test may have saved a lot of time for hundreds of other such cases too, but let's make sure you're getting the correct readings first now.
FWIW, in serviceable condition, all 8 cylinders should be in the 180-200 psi range, within 5% of each other but you can probably live with +/- 10%.
With a cold motor:
Remove fuel pump relay or 20A fuse in under-hood fusebox.
Remove all 8 spark plugs (again if necessary).
Ensure battery is fully charged.
Remove air intake plenum (that's the plastic box which connects to the throttle body and both air intakes).
Fully open the throttle body butterfly and use a wedge, or other method, to ensure that the throttle is held fully open while cranking during the compression test(s).
Record the highest compression after at least FIVE revolutions. If it's still 90 psi (I doubt it) squirt several drops of engine oil down the plug hole and see if the compression increases at all (indicating worn cylinders or stuck piston ring etc if it does).
Repeat for all the other cylinders and let us know what the result is.
The engine is basically an air pump during a compression test procedure.
If I was leaning over your (or whomever is working on your vehicle's) shoulder I'd be scratching my head too but I'd double-check again:
90psi is less than half the what the normal compression test psi should be so either there's a restriction in the intake or your motor is so worn it'll be pressurizing the crankcase.
The throttle body DOES NOT have any direct connection to the gas pedal (no mechanical linkage etc). Thus, simply depressing the gas pedal during a compression test WILL NOT produce a valid test result. Are you convinced that the shop manually propped the throttle butterfly open during engine cranking? Any chance they're overlooking this and don't want to admit it?
If the 90 psi is indeed valid, remove the oil fill cap and crank the motor (with the spark plugs installed). With someone else cranking the motor you should hear a loud slurping sound as the crankcase is being pressurized due to compressed air leaking passed the piston rings. You'll likely observe some oil mist too so have a shop rag on hand.