Yet another low oil pressure post

[jestock]

I’m a long time lurker but first time poster. I’ve read through all the low oil pressure posts, but didn’t find one about the issues I have with mine.

Quick backstory first: When I got my avalanche, the previous owner removed the check engine light and jacked around with a lot of other stuff. I’m not sure what else they may have torn into, but I don’t think they kept up with regular maintenance.

2005 1500ls 2WD with the 5.3l.

On a cold start, oil pressure is around 42psi and stays there until it gets up to temp then slowly starts going down until the low oil pressure message appears on the dash. I’ve verified it with a mechanical gauge and it does, in fact, drop. Revving the engine will make it climb to a max of around 34psi. I also get a DTC for the knock sensor in bank 2, but that could very well be due to it running when the pressure is low. There is no loss of power, so I wouldn’t think it would be the cam bearings. Rod bearings have a clearance < 0.005” and main bearings are also well within spec (except #5).

Yesterday when at temp pressure tanked as usual, but when I revved the engine to about 2k to build pressure, it remained around 20psi at idle for a good 20 minutes. I shut it off then started it again around 10 minutes later and it tanked to around 5psi again. Couldn’t get it to stay at 20psi again like it did earlier.

I have tried the following to no avail:
Replaced o-ring on the pick up tube
Replaced oil pump
Replaced rod bearings (journals looked perfect, so did a roll in)
Replaced main bearings (all except #5 because I didn’t want to drop the transmission; journals looked perfect so did a roll in)
Replaced timing chain and gears (since I had to pull the timing cover to replace oil pump figured I might as well while I’m in there)
Added 1 quart of Lucas oil and filled the rest of the way with 5w40

Is there anything else that I haven’t thought of? At this point replacing the engine is the only thing I can think of. It has around 204k on the cluster, but I think someone swapped it because carfax reports 240k.

 

[ronthehandymanga]

Cam bearings? looks like you have done the other suspects.

 

[jestock]

I thought that too, but wouldn’t that cause the engine to run rough?

 

[ygmn]

No,
bad cam bearings - (1-2 thousands greater clearance) would be hard to detect engine running worse but oil pressure could drop.

Questions & things to check:
What OIL weight and are you sure level is correct,
Is oil dipstick, correct?
Oil pump ok even though new? New parts fail too. what brand?
How is oil pick up tube sealed?
Which oil filter?
How bad is #5 main clearance?
How did main and rod and any other journal bearings look?

Oil Pressure is caused by the resistance of oil pump flow - Which is trying to get back to TANK aka Oil Pan.
SO things to look for are
A - those things after the pump outlet that LEAK oil back to the pan.
B - Those things before pump inlet that reduce oil flow into pump.

things that prevent pressure from increasing:
Journal bearing (mains, rods etc) clearance too large
block Oil Gallery holes are cracked and/or leak oil back to pan.
Air in oil
Pump Cavitation (starved for oil - pump will create air) pick up tube leaks air into pump
Other less viscous fluids mixing with oil - antifreeze, gasoline, seafoam, etc.
pump clearances too large so does not enough pump flow rate to keep up with all the journal bearings.

To me if this is not a pressure gage issue and actual pressure is low - time to remove engine and inspect as really an enigne running under 20PSI is not a good thing for bearings.

DId you measure oil in a different spot other than where sensor is mounted?

 

[jestock]

I used 5w30 then switched to 5w40 with 1qt of Lucas oil to see if that would make it any better, which it did not. I’m using an ac delco filter (pf46e, if memory serves correctly). Oil level is correct. Oil pump is working correctly; it’s a melling. Pick up tube is sealed with a green o-ring; original o-ring was orange. Old main and rod bearings were pretty worn, but I’ve seen worse. Journals looked perfect though. I’m not sure how bad #5 main is; I never dropped the transmission to look and can’t see it without dropping the transmission and pulling the main seal.

I pulled the pick up tube and performed a pressure test on it to confirm there wasn’t an unseen crack/hole or clog. I also cleaned it to visually inspect it. I do have a new pick up tube that I bought, but didn’t think it was worth pulling the pan again since the old tube was fine.

I only measured pressure at the sensor hole, but measuring it above the oil filter wouldn’t be a bad idea. I’ll need to get an adapter so that I can check it from there. I’m thinking cam bearings at this point; either way it’s going to involve pulling the engine and either doing a swap or a rebuild. It’s going to suck either way, but it is what it is at this point.

 

[jestock]

As it turns out, my problem is with the crank journals. The main bearings were absolute garbage and I didn’t realize the journals were that bad when I rolled in the new bearings. I would have saved myself a ton of work and frustration had I just pulled the crank, but a lesson learned hard is a lesson learned well, or so they say. I’m going to polish the journals on the crank and mic them and go from there. Thanks everyone!

 

[jestock]

Thought I’d provide an update for anyone facing a similar problem; As it turns out, the #5 main bearing was spun. While I didn’t want to pull the transmission, admittedly out of sheer laziness, it had to be done in order to remove the main cap for #5. Journal surface was fine - not perfect, but good enough to use as-is instead of having it turned or replacing it.

Although I had already replaced #1 - #4 main bearings while attempting to resolve my low oil pressure issue, I pulled the main caps off again to inspect the bearings to ensure none of the others spun, became caked with babbitt, or anything else since I was already down there. Main journal #3 (the one with the thrust bearing) was less than ideal, but I deemed it good enough for now. The other main journals looked and felt fine. Rod journals looked pristine, so I left them alone since I had already put new bearings in them.

I removed ALL of the side bolts and loosened ALL of the main caps so that I could torque them down properly with the recommended factory pattern, especially since I don’t know who else was in there before me. I replaced the rear main seal while I was in there, loosely put the flywheel back on and confirmed that the crank rotated freely without significant resistance.

I still need to reinstall the rear transmission crossmember mount, plug everything back in, reinstall the driveshaft and oil pan before filling with oil and testing it, but I’m pretty confident that it will be good to go.