The Stovebolt Site Forums The Shop Area Engine Shop low oil pressure.

Hello. I have a problem that has got me stumped once again on my 235.
When i start the engine completely cold, I get about 30-35 pounds of pressure. Rockers are oiling great.
Now as the engine warms up and runs for awhile at idle, I start to loose pressure. It eventually drops to about 10 pounds but im still getting oil to the rockers but not as much. Here is some info that im using.

58 block
54 head
Brand new oil pump
external oil filter mounted on intake.
Running 10-30 oil.

Any ideas or help would be great.

There may be other reason but I'd look at main bearings first. Some of the guys here are more knowledgable than I so keep checking this posting. And hey...WELCOME! to the best site on the net for old Chevy trucks. Nice ot have you aboard.

Drop the oil pan and check the condition of the rod and main bearings. It's not uncommon for a cold engine to have good oil pressure, and then it drops a lot once the oil and the parts warm up. If the crankshaft isn't worn excessively or out of round, changing the worn bearings will bring the pressure back up to normal. Anything over about 2 1/2 to 3 thousandths of clearance on any bearing is excessive. 1 1/2 thousandths is better. 10W-30 oil stays at the 10 weight thickness all the time- - - -it just lubricates like a 30 weight when it's hot. Switching to a physically thicker oil like a 20W-50 or a 15W40, or even straight 30 weight will help keep the pressure from dropping to dangerous levels until you get a more permanent fix done.
Jerry

Thanks 53moneypit and Hotrod Lincoln for the quick replies. This engine was just put together. All new bearings. I was wondering about the oil. I may try the straight 30 weight. Any other ideas?

Was the crankshaft reground before you put in the all new bearings?

How did you determine what size bearings to put in?

New bearings should hold pressure better than that- - - - - -I'd suggest making sure they're the right size. If you've got standard-diameter bearings on a .010" undersize shaft, for instance, the pressure will drop radically as the engine warms up. Have you put any road miles on it yet? A mismatch on rod or main bearing sizes will produce some nasty knocks as soon as road load is into the mix.
Jerry

I thought pressure drop hot to 10-15 was normal on a 235.

Not on an insert-bearing 235. The splash-oil engines ran fine on pressure that low, but that's on the dangerously low side for a fresh full-pressure engine.
Jerry

I am wondering about the oil filter. If it is a bypass setup, I would suggest removing the input feed, inserting a plug and testing the engine again.

What truckernix said.

If the gauge is between the pressure port on the block and the filter you are only measuring how dirty your filter is. The return line from the filter to the block just dumps into the crank case so the only resistance in the lines to cause a pressure reading on the gauge is the filter itself, the dirtier it is the more pressure you will read.

Cap or plug any of the lines between the point where the gauge line comes off and where the filter return line reattaches to the block. Now your gauge is on a dead-end line and will show the pressure that your bearings are seeing.

Kip41, I have a T on the pressure port with one side for the gauge and the other feeds the oil filter.

My concern is that maybe you have an oil filter rig that had an external restriction which is missing. My oil filter has a built in restriction in the inner pipe.

Just like truckernix points out, if there is no restriction at the filter or any of the lines between the "T" that leads to your gauge and the opening to the crankcase you will show low pressure.

I suppose there could be a restriction in one of the fittings like the one in the pipe leading to the rocker arm & shaft area.

truckernix where is the one in your filter and what does it look like?

The restriction in my filter assembly is a very small hole in the inner tube about an inch from the top. The return is at the bottom of the tube. The hole I would guess is smaller than 1/16".

I just measured mine, comes out to approximatly .060".