Oil Pump, problem or not?

I have an 88 Chevy C1500. It started showing low oil pressure about a year ago and I went with a heavier oil and oil pressure go a bit better.

Just changed the oil and now, when I start the engine cold, I show normal oil pressure. As the engine temp increases, the oil pressure goes down. When engine is fully warm, oil pressure shows 0 and “check gauges” light comes on.

Decided to put even heavier oil in but guy at parts house said it was probably the sending unit, not a worn out engine. I tend to agree because I drove it more and never hear any ticking or valve noise. Too cheap to replace the sending unit.

From the symptoms above, does thus sound right? Could the sensor somehow fail as engine temperature increases? Everything I read says oil pump failure is very rare.

Thanks in advance,

I think you’ve got a worn engine and just don’t want to accept that fact. I suppose it is possible for a sender to be bad like that but not likely. They are pretty cheap so you could just replace it or put a mechanical guage on to check actual pressure.

@KCPhoto You’ve taken too many chances already.

You need to hook up an oil pressure gauge and verify the actual pressure. Compare that to what is listed in the service manual.

Perhaps you do just have a faulty sending unit, but you may have a worn engine. The fact that heavier oil improved the reading on your dash mounted gauge bothers me. That seems to indicate your engine is worn.

Thanks guys! I’ll try to find someone with a pressure gauge to check it out.

Reach behind the engine, next to the distributor and find the sending unit. If it is oily it bad.

How many miles? Sounds like a worn out engine. A shop could check the pressure, or you could put in a new sensor just to eliminate that possibility.

If you have normal pressure when the oil is cold, I doubt if there is anything wrong with either your sending unit or pump. I think you have worn bearings that are letting the warmed oil through too rapidly to build up any pressure.

I think the way a shop does it, they unscrew the sender unit, and install their own oil pressure guage there temporarily, then they start the engine and see what their own presumably known-good guage says. This is the first thing to do I think, as the experts above have mentioned. At that point you’ll have independent confirmation one way or the other of the oil pressure situation, without relying on the currently installed oil-pressure sender unit. Cross your fingers, you may just need a replacement sender unit. Best of luck.

1988? How many miles?
I side with those who suspect a well worn engine. You should not only check your oil pressure, but also do a compression check.

Lack of ticking from the valvetrain is not a sign that the engine isn’t worn out. As a matter of fact, noise from the valvetrain isn’t a sign that it IS worn out.

Engines lose oil pressure due to wear because the sleeve bearings and their corresponding surfaces wear. The increased spaces that result reduce resistance to flow for the oil, and unless the fluid being pumped has resistance to flow the pressure cannot be maintained. Typically this shows up at idle.

Everything you’ve read is true. Oil pump failure is very rare. An oil pump is typically just two gear-like impellars interlocked that spend their lives being constantly bathed in fresh oil. There’s not much there to wear out.


The sensor’s diaphragm can develop a pin hole leak. At start up the 50+ psi can push the diaphragm against the spring and indicate substantial pressure but as the oil passes through the diaphragm the pressure will equalize above and below the diaphragm and indicate ZERO pressure. Although engine wear is likely the cause the sender might be failing and it can be checked for FREE.

Also, what voltage is indicated when the engine is running? The gauge ground can deteriorate and reduce the negative potential, resulting in a reduced reading on all gauges.

@RodKnox oil pressure can only be checked for free if OP does it himself.

No shop will give away that diagnosis for free.

I had a 74 pickup that did the same thing, pressure would show normal on cold startup then bottom out when hot…changed the sending unit and all was fine so this one may have had an internal leak…but a friend had this problem with an older Ford 302.

Hooking up a pressure gauge had 45 psi on cold start up then as the engine warmed up would drop to 4 - 5 lbs so this was engine bearing wear, but never made any noise top or bottom.

He got a lot more miles out of it by running straight 40 weight oil and a can or 2 of STP oil treatment. Idle pressure was now about 15 to 20 psi. You are NOT to do this, have the pressure checked and be done with it and find out what it is.