Why is oil pressure lower when level is low

I am guessing it is something design specific. How does the oil re-enter the pan? If there is a return passage that is below the level of the oil, that will cause resistance that will lesson with lower levels of oil or if the oil level falls below it, be eliminated.

I don't think this is something you are normally going to observe with a good oil pump. Your first assumption you made is not quite correct. The volume output will depend on how worn the pump is. In all cases the volume output depends on the pressure differential on either side of the pump due to the fact that some oil will leak back, but on a new pump it will be only a tiny loss and therefore not noticeable. On a worn pump it will be a noticeable reduction in output when it has to draw the oil from lower down. Of course, you will likely only find a good oil pump in an engine that doesn't get low on oil so it's quite likely you have never had the opportunity to observe what happens when a good oil pump gets low on oil.

-jim

========== Maybe if you're running one or two quarts low without an oil cooler, it's enough of a difference to cause the oil temperature to rise which would cause it to thin more and reduce pressure especially if the bearings have some wear.

It doesn't. At least not on any of my vehicles!

If it does, then you have a pinhole air leak in the oil pickup line that is getting uncovered, and that's a Bad Thing (tm).

Borrowing a line from the Limbo song, how low can you go... (how low is low) But my guess is that it is temperature related. The same amount of heat is dispersed into a smaller volume of oil so it stays hotter and thins out.

For the most part, if the oil pump isn't sucking air then it'll generate full pressure. If it's sucking air 5% of the time, then perhaps that'll make the system pressure 5% lower?

I have also noticed that on every mechanical gauge equipped vehicle I have owned. I spot it most at hot idle so I think it might have something to do with the heat of the oil, not sure. You would think a pump is a pump, but....

When my gauge shows low at speed, I normally am in need an oil change.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail > Assuming that:

It's the same reason that you'll show a higher pressure with a cold engine.

I thought a higher pressure with a cold engine is because the oil has not heated up and thinned out yet.

Isn't is supposed to be the other way around? 10W30 is 10 weight when cold and 30 weight when hot?

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)

No.

Sorta-kinda. But a hot 30 weight is still thinner than a cold 10 weight.

10 weight when cold is thicker than 30 weight is when hot. But how thick the oil is (due to different weight or different temperature or how old and worn out the oil is) has little to do with the question asked.

He asked about the level of oil in the oil sump and how that affects the oil pressure reading. One can assume that he meant with all other factors being excluded. That is the oil is not worn out and not cold, otherwise the question makes no sense. For instance when the oil is cold and thick then it will take a long time to run back down to the oil pan so the level in the oil pan will be actually lower than it will be when the engine is warm, yet the oil pressure will still be higher.

-jim

Yup, like I mentioned also, I can see it in my gauge when hot and at idle. The pressure will be lower when I am down a quart. Figure it must be heat related.

Mike

I guess having less oil would cause that which remains to have to carry a higher heat load. I never really thought that this would be a phenomenon which was very obvious but maybe it is.

Also you might have slightly more tendency to aerate the oil when it is a quart low.

Interesting premise, at any rate.

Not exactly. 10W30 oil has the same viscosity of a straight 10 weight oil when cold and has the viscosity of a straight 30 weight oil when hot.

It does not have the viscosity of 10 when cold and 30 when hot.

Which is the same reason.

It SHOULDN'T have a detectable effect, at least not in a healthy water-cooled engine with a healthy oiling system. Relatively little heat is rejected by the walls of the oil pan, and that is the SOLE cooling mechanism for the oil in such an engine. Oil plays a very small role in cooling- just the undersides of the pistons, and most of the piston heat is actually transferred to the cylinder walls and then to the water jackets.

That may be more likely, since the oil spends less time sitting in the pan deaereating after its trip through the engine. That might be more obvious with some brands of oil, since some oils have better deareation agents than others. I still tend to suspect that the oil pickup plumbing has a pinhole leak that gets uncovered. Probably where the pickup "shoe" is welded onto the end of the pickup pipe. Or maybe the velocity of the oil entering the pickup makes a little whirlpool that allows aereation at lower oil levels.

Umm.... what? ;-)

All oil changes viscosity with temperature. A 10 weight oil will have a higher viscosity at 10 degrees than a 30 weights actual viscosity at

200 degrees.