1999 Blazer 4.3 just started losing oil pressure when warm

I have a 1999 Blazer with the 4.3 engine. It has 109,000 miles on it. I have been changing oil regularly. Yesterday it was in the teens temperature-wise here. I drove to work with no problems (5 miles). I went out to lunch and no problems. On the way home, I noticed as the engine got hotter, the oil pressure started dropping. Normally it was running 40-50 psi. At 35MPH, it was down to 20 and dropped when I slowed down. If I put the truck in neutral and increased RPM, the pressure would come back to 20 or so.

This morning with it cold, again I had 40 psi. I had the sending unit checked and manually they also got the low pressure when it warms up. Since this just started yesterday, any idea what it might be? They are talking replacing the engine and I was wondering if they have missed something else that might be causing it. I am just wondering what caused the sudden pressure loss in one day.

Can you see any leaks? Is the motor making extra noises?
I'd replace the oil pressure sensor before handing it over for motor replacement and not by the guys that said the motor needs replacing either.

How much oil shows on the dipstick? What does the oil look and feel like? Does it look like the oil is being thinned out by gas, etc? What type, what brand, and what weight of oil was used on the last oil change – how long ago?

No leaks other than a slight one that comes with age. It is not using oil at all. Oil change within 1,000 miles. Oil is full and typical color. No foam or discoloration that I can tell. I have seen oil where I had a head gasket leak in another vehicle and I had foam and had coolant in that engine oil. It is not burning it, at least no smoke out the exhaust and not losing oil.

It does not look like it is being thinned by gas at all. I have Valvoline Max-Life in it using 5W-30 and the AC-Delco filter. As I said above, changed within 1,000 miles. Oil is up to the proper level. Engine is not making any noises like valve knock or anything. Only indication something is wrong is the pressure itself.

It just acts like something is opening up a blow-by valve when it gets hot. Would a pressure sensor act up at temperature but not when cold? I am afraid I have a crack internally that is allowing the pressure to escape back into the pan when the engine warms and expands.

I dropped it off at a local Chevy dealer for them to look at it and get a 2nd opinion before anything is done. Whatever it is just happened yesterday. It was cold here getting down to 10 degrees or so. I do have coolant in it as I just topped it off over the weekend when I had to replace the battery.

The place I took it to this morning wanted more for a used engine with 69,000 on it than Chevy did for a rebuilt engine with warranty!

You could also have a restricted oil filter, it could be sticking oil pump pressure valve/checkball.

Filter may be it, sender unit partly clogged, or the oil cooler lines could have just started leaking alot underneath- they all do at some point at the crimps. Then if this baby has never had a lower intake gasket replaced theres another possibility - coolant sprays into the valley and never see it in the oil but lifter tapping starts and oil pressure drops. Age also causes as the oil pump wears and clearances open up so pressure drops.

It is not likely to be a wear-related problem. If it appeared suddenly as you say, it is more likely to be a component failure. I'm assuming it's 4wheel drive. I'd start with the simplest component that could cause this. There is a 40 psi bypass valve in the engine block right above the oil-filter adaptor. Remove the adaptor to access. There is a 20 psi bypass in the remote oil filter housing. Either bypass could be stuck open due to a chunk of something stuck in it, or the spring could break. Clean or replace these two items before you move on to checking the oil pump.

A plugged filter would Not show up as low oil pressure.