Buick 3800 Sudden Loss Of Oil Pressure

So my parents' 2003 Buick Lesabre randomly lost oil pressure. Like, he had driven it to Wal-Mart earlier that morning, it ran and drove fine, no noise or misfires or issues. A couple hours later they got in and went to go to over to my sister's, started it up and first thing he noticed was it was running poorly. Then the low oil pressure light came on and he immediately shut it off.

Oil level is perfectly full, not low or overfull. Oil has been changed every 3k miles. Oil pressure sender was replaced last summer (it was leaking). Put an oil pressure gauge on it, removed the fuel pump and injector fuses, cranked it and no oil pressure on the gauge. It also sounded like it had at least one cylinder low on compression.

So what the hell's the deal here? Did the oil pump randomly explode? Is the low-compression/poor running just from valve lifters not being pumped up?

I have never seen an oil pump be the cause of oil pressure loss.

Corollary:  If the pump died, barring extreme examples like super high revving engines with snout mounted pumps and lots of torsional vibration in the crank, it didn't die, it got killed by FOD, meaning the ultimate problem was not the pump.

THAT SAID.  I have seen not one but two Series II engines walk a cam bearing out, which will cause a a rapid-onset oil pressure loss.  On one occasion the bearing was fouling on one of the lifters, causing the valve to stay open.

At least it is super easy to check to see if by some miracle the problem isn't just the oil pressure regulator stuck open.  That's accessible by popping the extension housing off.

In reply to Pete. (l33t FS) :

I was just thinking of the time I saw a 5.3L oil pump randomly shatter a rotor. Truck was running fine, guy shut it off, came out and went to start it and it wouldn't crank. The rotor had shattered and chunks had wedged it so that it couldn't turn over. 

Walked-out cam bearing? Delightful.

In reply to NickD :

I had a 3.6l in a CTS come in running, albeit really noisily because it was chronically three quarts low on oil.  She'd been driving it that way for a long while and she finally had the funds to have us replace the timing chains.   When I had the timing cover off and I was rotating the engine to where the cam and crank marks were at, the primary timing chain calmly decided it was time to jump a tooth.  I was watching it when it happened...

Pulling the intake lower plenum, I should be able to look down through the lifter valley and see if there is a cam bearing exiting stage left, yes?

In reply to NickD :

Edited. Scratch all that.  Remembered a different problem.

You can only really see the cam looking up from the crankcase on those, and of course with a low oil pressure and loud squeak from inside the engine, you'd think main or rod bearings, so that is where you go first.

i'm so happy that where I work now, we DON'T dig deep like that.  It's always labor intensive and ends up costing more too.  I have a 24v 5.4 at work right now that jumped time, runs okay but sounds like hell on startup.  Hot idle oil pressure is 15psi, minimum acceptable is 25.  At that point you can expect the cam journals to be wasted, so it's engine time...

Pete. (l33t FS) said:

I have never seen an oil pump be the cause of oil pressure loss.

I would totally agree with you, except for the two times I saw, on an early Cad 4.1, the shaft that drives the oil pump gear slip down until it disengaged from the distributor drive.  A press fit shaft. Same basic oil pump as a billion other v8s, but Cadillac did it wrong, somehow.  Not really surprising...

In reply to Streetwiseguy :

That wasn't the pump, though, was it?  It was the drive shaft.

I've seen SBF shafts twisted like Twizzlers.

I'm thinking of the gut-instinct "oil pressure is low, must be the pump" mindset.  Which replacing the pump never actually works, because the pump supplies volume, bearing clearances make pressure.  (This offer may not apply to GenIII V8s because in replacing the pump, they probably also replace the oil pickup O-ring that is the actual cause of the low oil pressure)

Hmm.  Wonder if the pickup broke or fell off, somehow.

In reply to Pete. (l33t FS) :

Yes... but the shaft is a part of the pump, so I'm calling it a pump failure.  The long shaft worked its way down in the gear until it disengaged the drive.

The pump and oil pressure sensor is somewhat easy to take off on that car/engine.

So, the no oil pressure claim may have been a bit preemptive. Went to pull the gauge off and oil came out of the gauge hose. Hooked it back up and cranked it a second more and pressure hit 60psi. But it still sounds like it has a dead hole. Its got that hiccup when cranking like low or no compression. 

The oil pressure light when running may have been because it was running so E36 M3ty, like, nearly stalled.

But why did it run fine on a leisurely 3 miles to town and back, and then sit a couple hours and suddenly lose a cylinder. I didn't hear any valvetrain noise when cranking it, like a broken rocker or anything. Going to put a compression gauge on it and figure out which cylinder and then pull a valve cover tonight.

My Grand Prix went from running great to running like crap in 1 day. Eventually discovered 3 plug wires chewed through by ground squirrel. Repaired via pellet gun.

Well, compression on cylinders 1-5 is 150psi. Cylinder 6 is 90psi

Wasn't there a gasket that fails on these engines? Intake? 

In reply to NickD :

I've seen that happen once before.

#6 rod got shorter after the pool of coolant in the intake manifold got high enough that some got sucked in and the cylinder hydrolocked.

In reply to Jordan Rimpela :

The upper plenum itself sort of warps where the coolant passages are, because the EGR is right there too.

When my father got this car two years ago he immediately tore down the top end and put in the upgraded gaskets and the redesigned plenum, because that had happened on my sister's Bonneville. Coolant level hasn't changed in weeks (he checks fluid levels pretty much any time he goes anywhere)

Not sure if the photo will show this well, but 3 of the 6 pushrods have a weird wear pattern on the rocker arm end. 2I, 2E, and 4E have the usual hemispherical, polished finish. 4I, 6I and 6E have a cat's eye-shaped wear pattern and it has a rough, almost grained finish. But the rocker cups look normal.

Seems a little concerning

In reply to NickD :

I have only seen that one time, on a 3100 that the owner let the oil be milkshake for 20k miles because he was perpetually broke and couldn't afford the intake gasket replacement.

We replaced the rockers and pushrods and it kept right on truckin' for a few more years, until it got too rusty to drive because it was a Corsica.

In reply to Pete. (l33t FS) :

This isn't exactly painting a good picture of this engine's state of health

In reply to NickD :

Eh, slap some pushrods in it, and rockers if they look scored, and run it.

In reply to Pete. (l33t FS) :

Still gotta find my cause for low compression on cylinder 6. If this thing was run with a chocolate milkshake in the base, that bent rod theory is looking more plausible. But it ran without a hiccup for 20k+ miles since my father bought it.

In reply to NickD :

Perhaps the compression was low because air was escaping into a coolant jacket? 

So, the culprit is burnt valves. Really poor exhaust valve seat pattern. Which honestly never crossed my mind because its not something you really see much anymore