2005 Suburban, 5.3 Vortec

About a week ago the oil pressure started fluctuating significantly, varying with RPMs. Previously pressure was always about 40 PSI. Now is is rarely that high when the engine is hot and sometimes drops to almost nothing.

Hoping for the cheap fix, I replaced the oil pressure sensor (a pain if you don't have the intake manifold off)...no change.

So...likely either the pump or the main bearings.

Any idea how I would be able to tell which one?

 

Rarely the sensor, usually sludge in oil sensor screen. Clean/replace screen, clean bottom end with seafoam or equivalent, use synthetic oil and good oil filter. Will take awhile to clean out all the passageways of sludge from conventional oil but it can be done. You may experience sludge clogging the screen again as the engine is being cleaned and for this reason I know some guys that have removed the screen altogether...

 

Thought about that. Happened to me once with a Volvo I purchased used...had about 3/4" of sludge throughout the top end. Crushed the screen twice before I had it cleaned out.

Vehicle has about 286k on it. It has only had non-synthetic in it about three times and gets changed when the factory 'change oil' lights come on. I've been using K&N HP filters on it exclusively for more than a decade.

I have noticed very recently an RPM-variable whirring noise that hadn't been there before. Could be an indication that the pump is going.

 

Well I can't speak to that - my only experience has been the screen on the oil pressure sensor.
First time it happened to me I had a heart attack - light came on said stop engine no oil I thought the worst. Got it to a dealer and he said it was just a clogged. screen. Did the whole Sea Foam and synthetic oil regimen and be durned if the light came back on again but knowing what I know now wasn't it all concerned and cleaned screen once again.
BTW my truck has 234k on it...

 

Thanks for the info.