Oil Pressure Question

[C10agent]

Hey Guys,
So I am a bit of a newbie when it comes to the engine..........okay everything. So I didn't trust the stock gauges so I hooked up a three gauge autometer knee knocker setup. Good thing is it doesn't overheat like the old gauge said. But, the oil pressure is throwing me off. So when it first started up and it is cold it is around 60 and the needle will bounce about 4 psi when idling. I let it run about 20 minutes and it would slowly lower until it ended up around 30 psi before I shut it off. Is that right??? I am freaking out that I have an issue with them motor, any help would be greatly appreciated. Thanks, Neil

[Tx Firefighter]

Perfectly normal and within range. A good rule of thumb is that a fully warmed engine should maintain 10psi of oil pressure per 1000 rpm. So, if your engine idles at around 800 rpm, you should have no less than 8 psi at idle. Likely you'll have a good bit more than that though, which is great. Just no less.

The pressure varied as the engine ran because cold oil is significantly thicker than hot oil. Therefore the cold oil generates higher pressures. As the oil gets fully hot, the pressure is going to fall lower.

[Old Truck Man]

Its probably ok. as long as its lubricating everything. I knew a guy who was insistant that his engine had really high oil pressure. So he would buy high pressure high volume oil pumps. The guage would peg the needle when cold at idle. and hold 70 pounds at idle when warmed up. and he burnt bearings left & right. The high pressure & volume was pumping all the oil into the valve covers when he really got on it. the pan was being sucked dry starving the bearings. He never understood high oil pressure doesent necessary equal long engine Life. Most of the Idiot lite dash gauges come on when oil pressure drops below 5 pounds. I drove a old car that when it was warmed up the idiot light would come on when it idled. Ran it for thousands of miles with no issues. It didn't smoke or use oil. I suspected it had been the reciepent of the common practice of a in frame ring job. When doing a in frame overhaul. The engine is left in the vehicle. The heads and pan removed and just the rings & rod bearings & gaskets replaced. The main bearings cam bearings & oil pump where never touched.

[Tx Firefighter]

What we are dealing with is that most folks who aren't experienced with older vehicles aren't familiar with what an oil pressure gauge is really supposed to act like. Modern cars swing the gauge up to a reading and it pretty much sits in the exact same spot the entire time the engine is running, and that is in no way accurate. The manufacturers have dampened the gauges so they never move nor actually read anything accurate really. Just that there is oil pressure of some sort.

I had a new a Dodge Cummins pickup that ate oil pressure sending until under warranty. That old Cummins just shook them to death. I'd start getting erratic oil pressure readings so back to the dealership for yet another sending unit. And I wasn't the only one. Tons of a cummins guys had the same issue. The oil pressure sending units just couldn't take the abuse from that rattly old engine. Finally Dodge did a service bulliten where they did something to the gauge system and from that point forward, the gauge just moved up to a spot on the dial and stayed there the entire time the engine was running. It no longer swung up and down depending on temperature or rpm. It was basically just an idiot light with a needle. That was their fix. I've notice in later years that stagnant oil pressure gauges are absolutely the norm in modern cars. Think about it, if Joe Consumer saw his pressure gauge swinging up and down he'd be back at the dealership having it looked at under warranty. Duplicate that by a few million cars sold and the costs would be huge. Instead they just dumb the gauge down.

[WIDESIDE72]

I bought a 91 olds custom cruiser wagon for $400 a few yrs back. 210k miles. They quit driving it because of low oil pressure. It was full of oil, po said he tried 20w50 with no change. The low oil pressure chime would come on after it warmed up. I drove it home and it never rattled. I figured whats the worst that could happen, lock up the tired old tbi 305?!? I installed an aftermarket gauge. Sure enough it had about 5 psi at idle after warming up. Im sure the bearings are gone. It has about 15psi cruising at 2k rpm so its not too bad. I still drive it around a little locally but wouldnt take it on a road trip or anything. Im building a vortec headed 350 to go in place of the tired 305. My plan is to have it in the road around the time i can buy cheap antique tags for it. (2 more yrs!)