The Stovebolt Site Forums The Shop Area Engine Shop General oil pressure question

Hi guys,

I've got a general question about oil pressure. In my 37 Chevy 1/2 ton I now have a 235 with new valve job, rings, etc. The oil pressure is great, somewhere above 50 lbs. I notice that after the engine gets warm, the pressure drops 10-20 lbs when I am at idle. In my new 1999 Jeep Cherokee with 35,000 miles, I notice that the pressure runs at about 50 consistently; but again, once the engine is warm it runs at about 30 at idle. On the other hand, my 1989 F**D 3/4 ton pickup has an oil pressure gauge showing L M and H and the needle consistently stays just above the M--even at idle. My question is whether the drop in pressure when hot and at idle witnessed in my 37 and in my 99 Jeep is acceptable and to be expected. If so, why doesn't the F**D do the same at idle--or is simply a matter of more accurate/sensitive gauges in the Chevy and the Jeep? Should I be concerned at all about the drop in pressure when at idle in these vehicles? Any insights/thoughts on this point would be much appreciated.

Thanks in advance,

Jim

AFAIK, your **** doesn't have an oil pressure gauge. It's just an idiot light with a pointer.
FWIW the GMC engines that I have register similar oil pressure to your 235.
jason

Sell the frud!

Those readings are acceptable.

Oil pressure drops when the oil comes up to operating temperature. A multi viscosity oil would avoid that. I bet you are running straight 30w.

The oil just gets thinner as it heats up. The engine also gets looser as it heats up. With everything expanding the thinner oil leaks thru the bearings easyer. Its nothing to worry about. The **** most likely just has a different type of sender and gauge. I have a factory gauge and add on mechanical gauge in my 85 Dodge. The factory gauge hardly ever moves from the green, but like your engines, the mechanical moves from 15 to about 50 once it warms up. Its just normal engine stuff. Thicker oil would help some. Joe

Another option
Someone put a high volume pump in the F***.
More volume & pressure will not drop as noticeably.
others are normal
Brad

OK My two cents: Oil does leak by stuff all the time.Multi visc oil allows you to guess what the pressure drop might be and take action regarding the oil pressure relief valve.....Stiffer spring more pressure when hot.But pressure squeezes the oil and causes more heat and some less pressure.So you go to a high output pump If it pumps too much oil two things result.It overrides the relief valve and your back to the temp vs pressure thing again and/or you pump so much oil up to the head on bypass that it gets not only hotter(because of the head's hot) but also overruns the valve stem seals!Soooooo it just isn't simple.
Look at it this way .... If you have a good oil you need no more than 10-15psi for every thousand rpms the motor is running 3000=30-45 lbs
I look at a hot vs cold drop of more than 30% as a sign to start looking.....Lots a things effect this number....An old or worn pump first...next easy is a worn or sticking relief valve.Rocker shafts and bushings and then cam brgs.This order because I've seen a bunch of new cam brgs and not too many new rocker shafts and bushings AND they are easier to change!!!!
If you still have a problem there maybe an internal leak but they are rare.
Main and rod brgs ARE supposed to leak!!!!As the motor turns the little hole in the crank is spraying oil AT PRESSURE!!The oil has to go somewere this is what people call an oil wedge.If you have enough flow the wedge will insure a constant stream of fresh clean oil . If the oil is clean and the volume correct NOTHING TOUCHES.When you get to an oil temp of say 200+ degrees The oil starts to really breakdown and can't maintain (oil has a psi rating at operating temp) it's psi spec and things touch

So if everythings good and you still have a BIG drop CHANGE the oil weight....otherwise you are just masking future problems.Oils have an add pack be sure you match you trucks engine.If you are working it hard .....please pick a good fleet oil ....Rotella or Delo If you have babbit DON'T use a synthetic or TURBO oil You may have nothing to get you home!!
A word on synthetic oils I just don't use em cause in old style motors,like most of ours,They LEAK
on the ground FROM everywhere and they don't help rope seals much.Or oil style rings

Now I'm kindda tired and way too A.D.D. to check this so I hope I got it in some real order that most of you can understand!

I'm sorry this got kindda 'Chiefed' but sometime I just can't 'splain it in a few words
Now if you want it explained further get one a Smokie's books

Smokey Yunick ,,,that is!!!(just for the ones who do not know)

Thanks for all the good replies. It's pretty much as I figured, but I wanted to make sure--but I also learned some things I didn't know. I appreciate it.

Jim

also be aware that when the engine is idling cold the choke is usually on and the engine is running faster. as the engine rpm's come down when its warm the pressure naturally drops as the pump its self is idling!i have a hopped up 350 in my 37 pickup and it does the same way....keep trucking my friend.

Sorry but REAL Stovebolts don't have automatic chokes!!!!!!!!!!!

J. H. ,,The Hydramatic GMC's came standard from the factory with electric chokes..

Jimmy, Later FoMoCo oil pressure guages do not really measure oil pressure.It acts more like a warning light. As long as the pressure is above a certain point , 10 lbs or so,the guage registers in the normal position.Told to me by a FoMoCo mechanic.Jim, Phat doesn't like the mention of Yunick's name here, now don't do it again.