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

I changed the oil in my '51 and every time I start the truck, the oil gauge gets pegged at
60lbs. After the engine gets nice and warm, it starts operating as normal. I'm using 20-40 oil. It stays pegged until the oil gets real hot. Whats going on? Do I need to go to a 10-40? Cecil........

Does it have an over-pressure relief valve?

I've never heard of a relief valve. I have a 228 GMC engine. Where would that valve be located if I had one? Cecil...........

That valve he is refering to is built in the oil pump & is in the oil pan. You could put a master gauge on it & see how high it acually goes. I would go to a lighter weight oil like 10-40 or 10-30 if your engine is in good shape.

You'll find a lot of opinions on oils if you run a search but most agree to use a single viscosity oil in these old engines.

going to a 10-30 or 10 40 oil should help.you will normally have higher pressure when the engine is cold as the oil will thin after it reaches operating temp.

Straight thirty weight would make the cold oil pressure even higher than the 20W-40 being used, but I think the relief valve should come into play before the pressure pegs, regardless. Too high pressure can erode bearings, Wrenchbender may know what constitutes too much.

Baldy

Now you guys really have me confussed. What if I go to a 10-30 like V6 jimmy says? I thought I understood oil viscosity, but apparently I don't. I thought that if I used 20-40 the oil would be thin when cold. Thats when its 20wt. As it warms up, it would become thicker because the viscosity changes over to 40wt. Cecil.....

Cecil-

If you use 20W-40 oil, it is 40 wt cold and 20 wt hot. The "W" after the first number refers to the viscosity at 120 degrees F. The other number is the viscosity at 32 degrees F. Oil cannot thicken as it warms up. So, that said, straight 30 wt will be thinner on start-up, and thicker at operating temp.

fab51,

Your last sentence is backward. Straight weight oils thin out as they heat and the lower the number the thinner they are. The multigrades would flow similar to a straight weight oil at the first number, ie. 20W, at lower temperatures and flow similar to the second number, ie. 40, at higher temperatures.

Baldy

As I remember, multi-grade oils are measured as follows...in the case of say, 20W-40. The first viscosity is measured at 0 F which is denoted by the "W" for "winter". The next viscosity is measured at 210 F. The oil doesn't become thicker, but because of it's high viscosity index, it measures as a SAE 40 at 210 F.( it doesn't thin out as much as a low VI oil at the same temp). This is a slippery subject.

Stuart.

All those numbers get confusing. I would put a master gauge on it. Your dash gauge may not be accurate. If it goes up beyond 60-70 pounds I think you should go in & check the pressure relief valve & see if is stuck. Otherwise I would go to a lighter weight oil. I like to use as thin as possible as long as the engine sounds good & doesn,t smoke or burn oil. It seems like those old Gimys always did carry a lot of oil pressure.

it really doesnt sound like a big problem 60 psi cold isnt excessive pessure my 250 runs 60 psi hot going down the road and 40 at idle but it has a melling Hi Volume pump

Hey, I'm happy when my stovebolt can manage 10 psi. Maybe we should just average our engines out and we'll both be all set.

Some of the GMC engines of this vintage had 60 psi pumps. I agree, I would connect another guage and check.

BTW, I've never seen a 20W-40 oil. I/ve been running 20W-50 in the summer, 10W-30 in the winter. Oil pressure in my 235 runs just shy of 40 psi.

The pressure relief valve is part of the oil pump. It could be filled with gunk. First, try going to 10W-30 oil and see what happens. I don't recommend 10W-40, it has an extremely high polymer count and can break down easier than 10W-30.

I think I'm going to try some straight 30wt oil. If that doesn't do it, I'll pull the pan and take a look at the pump. I installed the pump about a year ago, so if its screwed up its my fault anyway. Thanks guys, Cecil..........

SAE 30 oil will just make the pressure higher.before you pull the pan you might try an old time mechanic trick of adding about 1 quart of Kerosene to the oil to flush everything out good and then drain and replace with fresh oil.

Baldy-

I meant when compared to the 10W-40 oil. Sorry for the confusion.