The Stovebolt Site Forums The Parking Lot '60-'66 Trucks oil pressure

I had a guy in town re-build a 350 for me it is a 1976. Everything new finally have it back, took it for a ride and after it warmed up when I come to a stop the oil pressure dropped to about 5-10 pounds. Could that be correct?

I would do two things:

1) Make sure your gauge is accurate, then

2) Call the mechanic.

Its not crazy to see a SBC running around 10 psi at 700 RPM, but on a fresh motor with tight clearances it sounds a little low to me. Hopefully you are not running a super low viscosity oil.

You have a problem. I wouldn’t drive it until you figure out what’s wrong. Could be a bad oil pressure sensor or something much worse. Does the dash low oil pressure light turn on?

Did the pressure come back up again when idling? I had a problem of loosing pressure on my 235 when coming to a firm stop. It came back up in a bit, maybe 10-20 seconds at idle. Problem was someone took the baffle out of the pan. Got another pan. No problems since.

Dennis

How much RPM does it take to raise the pressure, and what pressure does it have at 1500 RPM or so? Also, how extensive was the "rebuild"? If the engine was not removed from the car, and the cam bearings changed, they can be the source of low oil pressure at idle. In the early 1960's when the 55-56 265's started needing to be rebuilt, we discovered that an "in the frame" rebuild without changing cam bearings was not a good option. Those engines would make the idiot light flicker at hot idle, which meant the oil pressure was under 7 PSI.
Jerry

I will go see the mechanic today I haven't been driving it. When the pressure was down it still sounded good. Its not a mechanical gauge it is electric.

I don't have a tac but when I push the peddle down to go through the intersection it comes right back up to about 30 pounds. It was a full rebuild

There's some wide clearances somewhere. Good luck, but when I gaze into my crystal ball from nearly 60 years of building engines, I see a short-life engine in your future. I'd be looking for a leaky oil pump pressure relief valve.
Jerry

Thanks Hotrod

I ordered the correct pressure censer for my gauges they are the omega gauge. If it is still the same pressure I will go back the the mechanic he doesn't seem to think anything is wrong don't know what he will do. What would you have done in this case. Can you change the pump with out taking the motor out?

The first thing to do is to plumb a direct-reading gauge (not electric) into the port beside the distributor. If the pressure of that gauge matches the electric, I'd recommend pulling the oil pan and inspecting the pump. Depending on the vehicle the engine is installed in, the oil pan may or may not come off with the engine in the frame. So far, you haven't told us what vehicle we're dealing with. Once the oil pan is off, the pump removes from the rear main bearing cap with one bolt. Melling makes a high volume pump that will maintain pressure at idle even with a race-clearanced engine.
Jerry

The 350 is in a 1966 C10 stepside. with a powerglide.

I believe it's possible to drop the pan on that one with the engine in the frame. It might be necessary to loosen the front motor mounts and jack the engine up a little to do it. If you do, be sure to remove the distributor cap to keep from breaking it against the firewall as you raise the engine. (Guess how I learned that trick?)

If your "mechanic" isn't convinced there's a problem, it might be difficult to get him to do that. Good luck!
Jerry

Thanks Jerry

One more question Jerry. I have driven it about 30 miles mostly on the highway but some on main street were you do some stopping. If my gauge is correct have I hurt the motor or should I change the pump and keep the motor. I am not going to drive it for now and I am going to stop at the auto parts and get a mechanical gauge that way I will know for sure.

Any oil pressure at all means the engine is getting more oil than it needs. Pressure builds up because the pump is supplying more oil than can be leaked away from the bearings, etc. while the engine is running. The only problem is that the pressure port on a small block Chevy is closer to the pump than to the end of the oil circuit. The front two connecting rods, #1 on the driver's side, and #2 on the passenger's side are the last items in the pressurized oil system to get lubricated. If you're only getting 5-10 PSI back at the pressure port, there's no way of telling how much oil pressure, if any, those front rods are getting. When you've got the pan off, drop those front two rod caps and inspect the bearings for scoring or scuff wear. Chances are you're OK, but it only takes a few minutes to check. It would be pretty easy to use Plastigauge on all the bearings, rods and mains, while you're in there. An engine with that kind of oil pressure after a rebuild would never have left my shop until the cause of the problem was discovered!

BTW, the high volume oil pump part number is a Melling M55HV. It has slightly longer gears and pumps more volume than their OEM equivalent M55 pump. It's usually used on race engines with bigger than standard bearing clearances to reduce rotating friction. That causes low pressure at idle and low speed due to more leakdown at each bearing.
Jerry

climberman:

Before you drive yourself crazy, perform your due diligence (accurately measuring oil pressure, making sure you have a proper oil) and then give the results to your mechanic. Hopefully if he is a stand-up pro, he will sort the issue out. Digging into your motor will just shift the blame to something you did. A good moral to the story is always have your motor started and tested by the shop before you take it home. Even better, spend the money and put it on a dyno.

Ok I installed the mechanical pressure gauge took it for a long drive 3 or 4 miles after it was already warmed up. When I come to a stop the pressure goes from about 48 to 21 pounds. So I probably didn't hurt anything but is that enough pressure for the long haul

Thanks
Rocky

Rocky:

21 pounds at idle is normal. I am assuming that when you say the oil pressure drops from 48 to 21 pounds, you are going from a higher RPM to the stopped idling RPM. If that is the case, this is normal. Your oil pressure will build with RPM. Something like 10 pounds per 1000 RPM is typical.

As an additional word of caution, oils matter for your new engine. Follow your mechanics advice. If this is a flat tappet cam engine, you should be using a specialty oil like Brad Penn with higher levels of zinc.

Ditto- - - -that's a normal amount of pressure drop at hot idle. If you're using one of the water-thin oils like 5W-30, etc., an even greater pressure drop might be considered "normal". The whole situation hinges on how much clearance the bearings have- - - -the more clearance, the bigger the hot idle pressure loss will be. With the numbers you're reporting now, you're good to go.
Jerry

Well it looks like I have a motor again I was pretty nerves about it. but to be expected. It has been almost a year since the 283 went out and I decided to put a 350 in my truck. I would explain all the bad things that happened to me but it would take a long time too right and too long for anyone to read. I pulled a motor out of my truck and put another one back on 4 times. That's not counting the time the mechanic took it out and put it back in with my help. It has been a crazy year. You absolutely have to know who you are dealing with when getting a new motor.

Thanks for all your time

Rocky