Rapid Oil Pressure Change In 350 !

[Bob1942]

We have a 88 Chevy Van that I restored with a 350 small block I did Body, Suspension , AC , brakes, Interior everything !! ...I even rebuilt the engine bottom end (to factory new specs) this was due to low oil pressure ... We Drove the van 65,000 miles since then . Now it has a total of 180,000 miles on it ...Oil Pressure for the last 65,000 miles has been near the top of the range on the gauge ... It is a 60 PSI gauge and the pressure was running 3/4 up the gauge at idle ! and it would just reach the top of the gauge at highway speeds ! I would say 58 PSI on the 60 PSI gauge ! at 60 MPH... On our last trip of 1100 miles from Michigan to Dallas to see our daughter the oil pressure SUDDENLY DROPPED to about 15 PSI at idle and 35 PSI MAX at highway speeds and has stayed that way since then ... This rapid oil change has me worried ? What could cause such a thing ? Am I due for a new engine ? What caused such a rapid drop in oil pressure with no overheating or other symptoms? It runs great ! no oil burning no noises ?? . I just don't want to get half way to Dallas on our next trip and have my 350come "unglued " BOB in MICHIGAN

[MagicRat]

My '83 Chevy van with 70k original miles shows 15 psi hot idle and 35 psi hot at crusing speed on an accurate aftermarket mechanical gauge.

I consider this to be normal engine pressure for a small block chevy with a few miles on it.

Lets assume your oil pressure gauge is in good shape. Electric gauges can develop bad senders. Mechanical gauges can get kinked or damaged pressure tubes. Your problem might be gauge-related. Try changing it to be sure.

IMO 58 PSI is a bit too high. A bit more pressure than this (ie 75 psi) risks blowing oil filters off and twisting oil pump driveshafts. Consider yourself lucky that this has not happened.

These pumps have oil pressure relief valves, which is a simple check-ball on the pump held in with a spring. The tension of the spring determines oil pressure on an engine in good condition.

Often, high pressure oil pumps (which are often used in high performance applications) are the same as regular pumps, but with a stiffer check spring.
IMO the use of such pumps on street driven vehicles is misguided. Excessive oil pressure is not any better for the engine than normal pressure, it simply wastes power and stresses parts unnecessarily

The pressure you have now is just fine for your engine. My concern is that something is failing, so your problem may get worse, leaving you stranded or with a destroyed engine.

I think you should change the oil and filter first, and see if the problem persists. If it does, pull the oil pan off the engine and take a look inside.

Possibly the stiff check-ball spring has become fatigued (weaker) or has broken.
The oil pump may be loose or the oil pick up tube may be loose/leaking.
At the very least, try replacing the pump with a new one and a new pick-up tube.

A bearing cap (rod or main bearing cap) may be loosening. It may be helpful to remove all the caps and take a peek at the bearings for damage.

If any of the main bearing cap bolts seem loose do NOT just tighten them down. Replace them and torque to spec. The old bolts may have become over stretched and have become weakened. This occasionally happens when old fasteners are re-used during an engine rebuild.

[josephmiller]

i have a 1990 chevy k1500 silverado 4x4 with a 350 and a 700r4 tranny. the oil pressure when i bought it always ran around 30 to 40. it has recently been from 40 to 25. the oil has been changeda month ago and is 10w30. whats goin on should i switch to 10w40. ive been checking the oil and its reading right on the dipstick. WHAT shall i do?

[MT-2500]

Welcome to AF.
Lot of problems wit the oil pressure sending unit.
Get a manual test gauge and check the real oil pressure.
If it is way off from dash gauge replace the oil pressure sending unit and see what it does.
MT