Derl

1989 Silverado 2WD, 350 cu. in., 1/2 ton
I have owned this pickup since it had 8,000 miles on it in 1990. It now has 222,517 miles and runs like a Chevy. Really runs Good.
The problem I'm having is the oil pressure slowly drops to Zero as the engine warms up. I have replaced the Oil Sending Unit twice and the gauge in the dash once. It still does the same thing.
Question: What else do I need to replace? Or what do I do now?

Thanks in advance for your replies.
Derl

MDTAHOE

You need to perform a mechanical oil pressure test. ( a gauge screwed directly into the sending unit port).

SWHouston

Like MDT said, do a mechanical reading on it, that's the only way to eliminate possible electronic/electrical problems.

But beyond that...
You may have your Oil Pump (pop-off) regulator slightly stuck open, or...
the other most likely thing that will cause this, is worn Cam Bearings.

You could try running some real heavy Multi-Vis, to see if you could help it a little if you don't want to start pulling things apart.

PS: IF you go for the Oil Pump, don't try to "fix" it, just get a Rebuilt or New Pump and stick it on. I gotta admit though, I don't think that's it. And, IF you're that close to the Rod and Mains, why not !

Derl

This is a Special Thanks to MDTAHOE and SWHouston for your quick replies to my question. Here is what I found out;
In 1997 my oil pressure gauge on my truck quit working, I knew the oil pressure was OK because I didn't hear any lifter noise and checking inside the valve cover I had oil still running through every thing. So I had a mechanic replace the oil sending unit. After that the gauge still didn't work, so I thought it must be the gauge on the dash. So I have went from 1997 until now without really knowing how much my oil pressure was.
Last weekend my son and I made a trip to the junk yard and found an '89 with the oil pressure gauge in it. We took it out and was very careful with it as we put it in the dash of my truck. It still did the same thing. Works good when the engine is cold but goes to Zero when it gets warm.

After putting the junk yard gauge in last weekend, it concerned me that my oil pressure might not be correct. So I made an appointment with my mechanic to have a mechanical oil pressure check like you had suggested. When I pulled the truck into his shop this morning and raised the hood he asked me what I thought was a stupid question. He asked me to point to the oil sending unit that I had replaced. So I pointed to the one next to the distributor at the back of the intake manifold. Then he said sometimes these older engines have two one on the back of the intake manifold next to the distributor and the other one is at the rear of the engine close to the exhaust manifold. He checked and mine had both. He replaced the one at the rear of the engine close to the exhaust manifold and the oil pressure gauge worked. I have 35 to 40 psi with a mechanical oil pressure gauge and on my gauge on the dash I have 45 psi. I sure is good to have that working again. My mechanic said the one on the back of the engine is for a pressure light and the other one is for the gauge. Who’d thought?
WOW, a lesson in replacing parts on a 350 Chevy.
Thank yall very much for your help.

SWHouston

Very glad you got the problem fixed !
But, I'm real surprised to hear it had two senders...
very unusual to me.