The Stovebolt Site Forums The Shop Area Engine Shop Low oil pressure

Hi guys I have a major dilemma. I have overhauled a 1957 GMC 270. Runs really strong. Had the cam bearings installed and all the machine work done at competent machine shops. Problem is after the engine is up to operating temperature the oil pressure falls off to near 0 lbs. I plasti gauge all the mains and half of the rods. All within specs. I have to admit I was cheap and did not replace the oil pump. The engine sat for many years, but only had 32k miles. I took the pump apart and cleaned it well and in stalled it. Cold start up pressure is 60psi or better. Up to operating temp to zero psi . I also have to admit the engine was on my bench for over a year and I could have missed something or left something. Something is wrong anyway. Have had two mechanical oil gauges on it same result. Any ideas would be most helpful. I will answer any and all questions Thank you. Dave

What oil viscosity are you using? The cold oil pressure would indicate clearances are decent but the drop from cold to hot oil pressure would indicate that the oil is too thin when hot (in my opinion).

I had the same issue with my ‘59 235 when I first bought my truck in 2019. The engine was rebuilt by a supposedly quality machine shop before I bought it. On cold start, it would peg the needle on my 0-30 lb. gauge. Once warm it would drop to nearly zero. After messing around for over a year trying to determine what the issue was, I pulled the engine, only to find a few of the journals were over ground, necessitating a complete rebuild with larger oversized bearings.

You stated you didn’t check all of the rod bearings and didn’t mention anything about the cam bearings nor the rocker arms. Somewhere in your oil system, something is bigger than it should be.

Pull the oil pan and start checking every thing with the oil pump. Does the oil pump have a bypass valve and is it stuck open? What kind of assembly lube did you use? I've seen it where the assembly lube plugs the oil pump suction screen and starves the oil pump. Then move onto rods and mains. Hopefully the low oil pressure hasn't caused any damage.

I had a problem similar to this. I had great pressure then nothing. I found my distributor wasn’t sitting in the oil pump correctly (oil pump moved due to a crack where you bolt it in).

Good luck.

Drop the oil pan and rig up a pump in a 5 gallon bucket of oil. Plumb a flex line to the oil pressure port instead of the gauge. Drive the auxiliary pump with a heavy duty 1/2" drill motor, and put a BIG drip pan under the engine. Look for excessive oil flow from rod, main, and camshaft bearings. That's how Dad discovered the cause of low hot idle oil pressure on the first few small block Chevy 265s he rebuilt in the late 1950s- - - - -worn cam bearings. They stopped doing "in the frame" piston ring and rod bearing rebuilds after that. Full engine overhauls or nothing at all from his shop!

" I plasti gauge all the mains and half of the rods. All within specs." "Within specs" sometimes isn't good enough. I shoot for the low end of the specification when possible. For instance a ".001-.003" spec bearing actually needs to be as close to the minimum if possible. One other thing- - - - -RUN- - - -do not walk away from anyone who recommends oil weights like 5W-20, 10W-30, etc. These engines were never intended to run water-thin oils. I buy 15W-40 oil by the 5 gallon pail and run it in everything from lawn mowers to Diesel engines in my tractors- - - -even my Harley Davidson EVO engine. Haven't had an oil related engine problem in the past 30-something years. "That's my story and I'm stickin' to it!"
Jerry

I used a 5/30 wt oil for break in oil with a zinc additive. I plan on using a 15/40 wt oil for running from now on. Maybe it will help. I have ordered a NOS pump for it maybe the new pump and heavier oil will help it just enough. Thank you for your advise. Dave

Check the oil pump relief valve spring. It might be getting weak as it heats up, and/or the relief valve could be sticking. Polish it, and the bore it runs in with jeweler's rouge and a hard felt buffing wheel.
Jerry

GMC Maintenance Manual says this about oil minimum pressure with the engine hot:

Used engine at idle 2 PSI, 2750 RPM 25 psi
New or rebuilt engine at idle 5 PSI, 2750 RPM 40 psi

I wouldn't put up with that low idle pressure. That said, my '52 GMC 228 (same engine design including matching oil pump, main bearings, cam and cam bearings) with 40,000+ miles I have driven it, original pistons and possibly original bottom end runs down the road at 60 MPH (2310 RPM) warmed up and shows around 55 PSI. This reading was taken with a 4" diameter oil pressure gauge that is rated for +- 1%. A hot idle (500 RPM) shows 14 PSI.

I did an autopsy on a 270 that had the problem Chris mentions above where the distributor wasn't sitting in the oil pump far enough. Once the distributor tang had come out of the oil pump's slot it never went back. Totally wrecked the engine.

GMCJammer51 suggested checking the pressure relief valve in the oil pump for debris. Just FYI, the oil pump in the 270 that I did the autopsy on regulated to 57 PSI when cold and being spun by a 1/2" drill motor.

Have you looked at the amount of oil coming out of the valve gear overflow tube (8.4550A below) attached to the rearmost rocker arm shaft stand? It usually has a flow at idle about the rate of an anemic drinking water fountain.

I thought about checking the relief spray in the oil pump but I can’t find any specs on what it should be. A friend of mine says the caterpillar actually has a spec for their springs. like for checking valve springs but I found no such specifications for GMC trucks oil relief spring. Thank you all I’m gonna be trying very many different ideas you fellows have given me. Dave

I think the 5W30 breakin oil was most of your problem. I've always used straight 30w to break in an older engine, then went to 15w40.

The tension of the spring isn't terribly important- - - -it's just a "fail safe" in case pressure might go very high with cold, thick oil on a winter startup. Free travel of the relief plunger ahead of the spring is vitally important- - - - - -if it sticks in even a slightly open position, either from burrs on the piston or a tight fit in the bore, the oil pressure will drop dramatically. Be absolutely sure the relief plunger travels freely.
Jerry

Dave: Spring specs from GMC Maintenance Manual can be found on the bottom left of this page.

Jerry and Bill thank you both. I have some work to do. I am going to change the oil and see what happens. David

You could be right about the oil. Local people I have talked to have said the same thing. I will try it today. Thank you.

Even a slightly dirty assembly job can introduce debris into the oil pump relief system, resulting in a sticky pump relief valve. I do my best to keep everything associated with the oil system surgically clean as I put an engine together.
Jerry

Yes I did my best. Covered it with a garbage bag during assembly when done for the day. Thank you. Dave