2010 5.3 oil pressure

Working on my dads truck, it is throwing the po521 oil pressure sensor code.

Replaced the sensor, the screen below it and changed the oil. It had synthetic oil and it was like water. Went back with conventional oil with no luck. The guage at times will just drop to zero. Cycle the key and it will come back up.

Installed my manual guage and it was reading 20lbs at idle and 35 at 1600rpms.

My question is do anyone know what the pressure should be on this engine.

Has no noise and 162,000 miles. Im thinking the guage is the issue.

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Listen to me! I buy ALOT of 2000-2006 Chevys, I buy a bunch that people think the engine is bad or oil pump is shot, drop the oil pan, replace the o ring where the pickup tube goes in the oil pump. It will fix it, just got done fixing a 2005 with a 6.0, would hold no oil pressure when cold, some will lose when they get hot. If it's a 2 wheel drive, it's very easy, the crossmember unbolts and the pan drop straight down, and they sell that o ring at the parts store.

Re:

It is a 4 wheel drive, ive worked on cars for a living, so it would not be a big issue for me.

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You have to remove the front diff to get the oil pan off on a 4x4, but most of those engines always hold around 35-40 lb of oil pressure when hot at an idle.

Re:

Well the o-ring would explain the drop in pressure.
What do think is causing the pressure to drop to zero on the dash gauge but not with the manual guage. It happens you can stop the engine and start it back and it goes back up.
Now granted it will only go up to the first mark.
I wonder if it has to do with the crank case pressure, allowing it go back up when you cycle the key?

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I don't know, but most gm gauges I have seen go bad usually just quit working and peg out, not really do what yours is doing, but 20lbs on the manual gauge is lower than normal, I may be wrong, but I bet the o ring fixes it.

Re:

Thank you sir, i agree most guages will do what you said. But anything is possible. I will make plans to drop the oil pan.

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Here is just a reference on the oil pressure, here is my 305k mile Tahoe, 40lb hot at a idle. And that engine has never had a bolt turned on it.

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Re:

Dads guage is different style. From what you are saying, the engine is same from 2000 up to 2010.

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Drop the pan and see what you find. I had one that had jelly looking sludge in it....when it warmed up the sludge would cover the pickup and lose oil psi. That was one tough motor. Far as I know it's still running. No telling what hellish life it had to be sludged up that bad.
It was a 07 4x4. I didn't have to completely remove the diff, just a bolt or 2 to get it rocked down out of the way. It wasn't a terrible job

Got the oil pressure problem fixed, turned out to be the o-ring. Only thing i could see wrong is it just flatten out, no visible tears in it.

Thank you guys for the help.

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. There is no telling how many of these engines that have been took out and junked because of that 5 dollar part.