Why is oil pressure lower when level is low

Yes, but irrelevent. A 10W30 will behave like a 10 weight oil when cold and behave like a 30 weight oil when hot.

Reply to
AZ Nomad
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And that's exactly what I said. A 10 weight oil is only 10 weight at room temperature.

Reply to
tnom

When cold it only thickens as much as straight 10 will. When hot it only thins as much as straight 30 will. It still gets thinner with increasing temperature but not as much as straight weight oil.

Don

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Reply to
Don

OK, guys, I've never thought "why" before either, so here is one I'm tossing into the ring. Assume you have a 24-inch 'straw' reaching to just above the bottom of a 14-inch-deep square pan. Will it be easier to suck (Gosh, I hated to use that word!) a tablespoonful of oil into your mouth when the oil level is 12 inches deep or when it is 2 inches deep?.................................Next, given that you are pulling a STEADY vacuum of x-amount, which is what an oil pump "kinda"* does, will the pressure of the 'first' oil exiting the straw be greater with 2 inches of oil in the pan or with 12 inches of oil in it?...................... I think we all can answer both questions, and I believe the 2nd answer will solve the mystery of the original poster's question. At least, until someone shoots me down hard, it reveals the mystery to me! And, now I'm going to sleep. s

*This word was well-adapted in another reply, so I obliged my self of its use!
Reply to
sdlomi2

Yes, it is more work to draw the oil from a lower level, but that ignores the OP's assumptions:

- the oil pump moves a fixed volume of oil per rotation;

- oil is not compressible;

If those assumptions are correct and the extra work of lifting the oil from a lower level doesn't slow down the engine rpm's and the oil temperature and viscosity stay the same then the pressure will not drop. The question was about what effect the level of the oil has. Suppose you have an engine with an adjustable oil pan where you can move the oil pan up or down an inch while the engine is running - will changing the oil level change the oil pressure? If the OP's assumptions are correct it will not.

But the first assumption is not 100% true. There is some leakage at the oil pump. The volume of the output from the oil pump will decrease by some amount as the level in the oil pan is lowered. How much the output is reduced depends entirely on the clearances in the oil pump.

-jim

..Next, given that you are pulling a

Reply to
jim

That is true... but if you read the two sentences that I quoted above, they say EXACTLY the opposite of each other. The second one is wrong. Probably just a typo, but rather humorous nonetheless.

Reply to
Steve

Yes, but the difference between "full" and 1 quart low" is about 1/2 inch on most vehicles. Heck, the difference between "full" and "bone dry" is only about 4-6 inches! That's just not enough to make a detectable difference in the pump's ability to lift the oil out of the pan, in my opinion. If you were talking about a pump lifting water out of a well when the water level is normal versus when its 10 feet below normal, I'd say "absolutely!" but this case is just a trivial difference in lift height.

Reply to
Steve

I guess I don't see how the principle that causes oil viscosity to be a function of oil temperature somehow also causes the oil pressure to be a function of the oil level.

In other words, could you help me wrap my head around what you are saying?

Reply to
runderwo

You are not going to wrap your head around it, because it's nonsense. Changing the level of oil doesn't affect temperature or viscosity. It is only through some very convoluted and faulty logic that some have arrived at the conclusion that changing the oil level changes the temperature or viscosity of the oil.

-jim

-Jim

Reply to
jim

I like that answer.

The oil pumps are just metal on metal 'seals' so a change in the suction on so relatively large a pipe as the oil pickup might just be what we are seeing. It wouldn't need much.

The pump can't suck harder when it has to raise the oil farther before it pushes it because it is fixed to the rpm and it's metal gap seals so there has to be some effect somewhere of the lower volume.

Pumps only suck harder as the rpm go up which is evident in the gauge also.

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
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Reply to
Mike Romain

Temp should be the same as the primary control is the cooling system. The oil should reach the same steady-state temp if it's full or a quart low.

We are holding viscosity constant. Not breaking down due too seeing more wear-tear from there being less of it. All parts are assumed to be in good working order and of proper design.

This really only leaves some sort of odd return system where oil comes back in below the oil level so the pump sees the head height pressure as a resistance.

Practically speaking though, since people are seeing this towards oil-change time, holding viscosity and other physical properties of the oil constant may not be valid. The new oil when it's up to level might be X but the used oil thousands of miles later might be Y. The fact some it was burned or leaked out inbetween not a factor at all in the change in oil pressure, but rather the different physical properties between the new oil and the well used oil.

Reply to
Brent P

Thinner hot oil excapes through the bearing clearances, lifters, etc. faster than thicker cooler oil, so for a given oil pump speed, the pressure that can be achieved is lower. Or in the case of many engines, for a given pressure relief setpoint at the input to the engine's oiling system plumbing, the lower the observed/measured pressure somewhere downstream at the oil pressure sensor.

But, as I said earlier, I don't think being low on oil should raise the temperature nearly enough to cause this effect.

Reply to
Steve

Pumps don't suck. They can only partially remove a portion of the atmospheric pressure.

Reply to
tnom

If you are referring to........

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

The above is accurate but different than the below quote which is also accurate............

"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."

The two quotes do not say the opposite. They are just different.

Reply to
tnom

I never said anything about oil level. The oil level should have no effect until the pump starts to cavitate The pump is a positive displacement gear pump and will only move a set volume of oil per revolution.

The Viscosity of the oil will have a direct bearing on the pressure gage reading. There is no system relief valve that I know of that will ever be reached so the pressure is dictated by one thing. Resistance to flow. A higher viscosity oil will resist movement in a passage more so than a lower viscosity oil.

Reply to
tnom

Then start your own thread if you want to change the subject. The question is about oil level.

Pretty much true. But there is some leakage so the volume is not always exactly/perfectly the same. With a badly worn oil pump lowereing the oil level will show a pressure drop. With a good oil pump the drop is too small to be able to detect.

{other irrelavent stuff snipped}

-jim

Reply to
jim

The OP states that with a lower oil level he sees lower pressure. If we assume that this is true and the pump is not starved or drawing air then the only way a pumps pressure sensor can show a lower reading is because the viscosity of the oil went down when the oil level went down. Heat will change an oils viscosity.

The smaller volume of oil that circulates will get hotter quicker, and the oils ultimate temperature with a reduced volume will be hotter in a fully warmed up vehicle. The oil runs over surfaces of an engine that are hotter than the temperature of the oil in the oil pan. If you circulate a lesser volume of oil its ultimate temperature will rise because the oil itself has less surface area to dissipate the heat.

Reply to
tnom

Correction: the temperature for viscosity numbers is taken at 212 degrees.

Reply to
tnom

So then why is it that when the oil pump is in good shape, you will not observe the pressure drop after you remove a quart of oil from the oilpan? Why is it your theory only works on old beaters?

-Jim

Reply to
jim

pump is in good shape, you will not

Reply to
spamTHISbrp

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