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Old 12-24-2011, 07:57 AM
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No oil Pressure on '04 5.3 in a sierra pu.

Not mine, but a customer car for my neighbor's business.
2004 GMC Sierra 5.3

He has worked on a few of these trucks with little to no oil pressure and all he has had to do is to replace the oil pump.

This particular truck is being stubborn after the pump change. (And the recommended O-rings from the Chevrolet store)

223,700 miles

20lbs MAX when cold and even revving the engine
Once warmed up, the pressure drops steadily until the warning buzzer goes off and a warning in the dash display shows up.
Once warmed up----still 20lb max.

I suggested to him that perhaps the bearing were gone.
He mentioned that there was literally no debris in the oil pan (metal/sludge/etc) when he removed it.

This pressure stays the same (under 7lb warm idle with mechanical gage hooked up to the port) with mechanical or factory electric gage. but springs up to 20 tops when revved up a bit.

Something else weird----and I would have never believed it had I not seen it myself.
For some strange reason I have yet to figure out, one of the workers applied some air pressure to a line that runs from the MAF into the passenger side valve cover (about a 5/6 hose).

When air pressure is applied into this hose-----Oil pressure started creeping up. I am pretty positive that this should not be happening.
Perhaps we are just screwing with the computer's mind.

My neighbor is gearing up to replace the main and rod bearings in the truck now. And the owner is out getting prices for another engine.

Engine sounds great----makes no weird noises (except when the oil pressure drops and lifters start rattling), no misses and runs smooth.

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Old 12-24-2011, 08:34 AM
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I would be pulling a couple caps and checking bearing clearances. When the air pressure is applied, you are raising crankcase pressure, and forcing more oil into the pickup. I would also look for a crack or leak in the oil pickup tube, could be sucking air.
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Old 12-24-2011, 11:52 PM
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is he running a Fram oil filter?

I had one fail on me once, causing zero oil pressure.
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