The Stovebolt Site Forums The Shop Area Engine Shop Normal Oil Pressure

I have a '57 235 truck engine in my '47. Finally got the carb and coolant issues worked out and it runs like a top. Since I got the carb issues worked out and it idles smooth at 500 RPM, I noticed that the oil pressure is steady at just above 10 PSI after warming up. Is this low of pressure normal at idle? It goes up to 40ish at 2000 RPM.

That's a little low at idle, but not alarmingly so. I wouldn't lose any sleep over it.

What weight oil are you running? That sounds a lot like you're running some sort of water-thin multi-viscosity oil that flows through the system faster than the oil pump can compensate for at low speed. 500 crankshaft RPM means the oil pump is only turning 250 at idle. That's barely above cranking speed.
Jerry

Jon-
It's going to be a daily driver and was supposedly rebuilt and drove 1000 or so miles before the truck was garaged to be tore down for a full restore. Kind of makes me wonder if it was indeed rebuilt or possibly rebuilt incorrectly, or who knows. The valve train looks pristine though so something must have been done to it.

Jerry-
Guess it would have been helpful to include the viscosity, sorry...I'm running 15w-40 Rotella T4. Sounds like I may need to upgrade to a Melling m-45 pump?

While you're in there installing the Melling pump (great idea, BTW!) take a little time and inspect the rod bearings, including checking the oil clearance with Plastigauge. A new set of bearings will be cheap insurance, especially if the current ones are showing any wear, or if the oil clearance is over .002". Good luck!

Any oil pressure at all indicates the pump is supplying more oil than the bearings need. If not, the leakage from the bearings and other places would make the pressure zero. The idea of having oil pressure is to assure that everything that needs oil gets it- - - -with a bit to spare. Extremely high oil pressure is just wasted energy- - - -power used to drive the pump is power that can't be used to turn the transmission! Most "idiot light" switches turn the light on at 7 PSI, so a lot of folks without a pressure gauge have no idea how low their oil pressure really is at a hot idle.
Jerry

I worried about the low oil pressure in my 216 for years. One day I checked it with another gauge and found out my gauge was off.

I normally use straight 30 weight VR-1, but it is warmer here (for more of the year) than it is in some places. Jerry's correct about the idiot light and that's why I said I wouldn't lose any sleep. Plus I've heard since I was a teenager 10 psi per 1000 rpm is appropriate. I don't know if that is true, but it has been easy to remember so I've always used it as a guide. My 235 by the way is about 25 psi at warmed up idle, but that's with a new (Melling) pump and I had them grind the crank to get bearing clearances of .0015" and the cam bearings were new.

Back in the late 1950's when the first 265 Chevy V8's began getting enough mileage on them to need an overhaul, Dad's shop did a few "in the frame" rebuilds- - - -rings, rod bearings, valve job, etc. which was the normal minor overhaul that had been done on cars for decades. Several of them had problems with the idiot lights flickering at a stoplight. Most of them were Powerglide equipped cars. They finally figured out the oil pressure loss was from worn camshaft bearings, which couldn't be changed without a complete teardown. There was no such thing as a high volume oil pump back then, which would have solved the problem, or at least hidden it!
Jerry

Soooo....the 40ish psi at 2000 RPM was an anomaly. I've been fiddling with the oil pressure for the last half of the day and now the pressure reads 10-12...all the time. Pulled the dizzy, chucked my priming tool into my drill, and pressure reads 16 at the highest speed (checked with two pressure gauges). Oil pump is shot.

Gotta love getting problems solved (carb and radiator) only to have another problem present itself! Better in the shop than on the road though.

Jerry-
That was YOUR idea from a previous thread!
Going to order the Melling tomorrow. I'll definitely get the plastigauge out and check clearances, hopefully everything is okay.

I'm pretty sure the M45 is a bolt-in swap. You might have to tweak the inlet and output lines a bit to make everything line up, but nothing dramatic. I don't remember exactly when Chevy swapped from an oil pump pickup screen mounted to one of the main bearing caps to one that threads directly into the pump, but I'm pretty sure the pump itself is the same. Whatever pickup is there now should be a direct swap to the new pump.

BTW, in 60+ years of building engines, I've seen exactly three "worn out" oil pumps that didn't have a catastrophic failure like ingesting a big chunk of metal or something similar. It's far more likely that you've got some badly worn bearings that are consuming more oil than the pump is able to provide. Also, try removing the pressure relief valve in the cover of the old pump and check for a piece of debris that might be holding it partially open. I've seen that on several occasions.
Jerry

I had been meaning to post about this question for a while. I run Rotella 15W40 in my 261, and noticed that at hot low idle ~500 rpms, the oil pressure drops to about 15psi. Give it gas to a fast idle and then start accelerating, and it jumps right back up to 30psi. Good to know that is sufficient oil pressure. These 6's sure do purr at 450-500 rpms.

I agree with Jerry. When I bought my ‘52 with a rebuilt 235, my oil pressure dropped to less then 5 lbs at idle when hot. I spent more than a year chasing the issue before I pulled the motor and disassembled. The rebuild turned out to be really shoddy work (bearing clearances were all over the place from too tight to .010” too loose) that required me to have a competent machine shop regrind to everything to the next size bearings.

The Melling M-45 is a direct replacement for all 1955-* 235 & 261.

Yep

My Dad brought a couple extra gauges over that he had lying around. "Make sure the gauge isn't bad, all that new Chinese stuff is crap." he said. I tell him I already ordered a new pump and will just replace it, can't be the gauge it's brand new. "Won't take but 2 minutes" he says and heads over. Both of his gauges read 32 at idle, and increase into the 40s with RPM increase. Dad smirks, goes about disconnecting his gauge. My new Chinese oil pressure gauge is junk. Ordered the Melling pump this morning so at least I'll have it on hand if the existing pump goes sometime or if I have to get in there for something else.

Your Dad is a very smart man! A real good example of KISS!

Glad to hear you solved your issue!

Oil pressured gauges work just as well on air pressure as they do with oil. A compressor with a pressure regulator on it makes it easy to check the accuracy of a gauge. That's how I test a compression gauge before using it on an engine.
Jerry