Why is oil pressure lower when level is low

If you remove enough oil, or add extra oil via an oversized oil pan you will get a different temperature/viscosity/pressure of oil.

Reply to
tnom
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Avoiding the question.

Let's ask it a different way. Are you claiming that no matter how worn the oil pump is the output volume per revolution will always remain the same (assuming no air in the oil, no temp difference, no viscosity difference).

-jim

Reply to
jim

I'm not avoiding anything.

To many variables. Define worn. A proper gear pump will be a positive displacement pump. If not it's time to change the pump.

Reply to
tnom

So then answer the question. Why does your theory fail on engines that aren't worn?

Even a worn pump is a positive displacement pump. Positive displacement doesn't mean it has perfect volumetric efficiency. Just because the action used for pumping is positive displacement doesn't mean there is no leakage. Simply saying that once it starts to wear it no longer exists and therefore can be disregarded is hardly an answer to the question. Its an evasion. Even a new pump doesn't pump 100% of the displaced volume. If you turn the pump slow enough (like one rev per day) it will pump nothing at all as it will all leak back (due to gravity) at a rate greater than the rate of displacement.

-jim

Reply to
jim

I guess now I should mention that I have a brand new oil pump (but not a new pickup tube). Replaced the original 200K pump due to a freak accident that broke it.

I expected the oil pressure to be more consistently maintained with the new pump because of the effect you mentioned of a worn pump draining back, but it behaves the same with respect to oil level as the old pump did, as far as I can tell.

Reply to
runderwo

Your original question was a hypothetical question based on hypothetical set of facts you presented. Now you are asking a real question with no real facts to go with it. My crystal ball says it has something to do with the "freak accident that broke it" part. But sucking air at the pick up seal would be a good guess too. You haven't explained how it gets a quart low on oil. If you fill the engine with fresh oil get it up to operating temperature and then drain a quart out you shouldn't see any sudden change in oil pressure if the pump is good.

-jim

-jim

Reply to
jim

Perfect doesn't exist so I guess you can say there is no such thing as a positive displacement pump. However in the real world a pump that has negligible blow by is a positive displacement pump. A pump that has enough blow by to facilitate a pressure drop is no longer a positive displacement pump. It becomes a worn out pump and should be replaced,

Reply to
tnom

snipped-for-privacy@mucks.net did not author the above quoted text.

Reply to
tnom

No, it has nothing to do with it (it was a physical break on a pulley mount), and you misread my post if you think I'm asking for a solution to the oil burning or leaking.

As far as I can tell from analyzing the facts, I agree with this statement, at least at the instantaneous moment of restarting the engine. If the temperature of the now-lesser amount of oil were allowed to reach its new equilibrium, I believe the pressure would register lower at that point since the new equilibrium temperature would be higher.

Reply to
runderwo

IF IF IF...... you might actually try it instead of speculating. a few degrees temp difference at operating temp doesn't make enough difference in viscosity unless maybe your doing something goofy like running pure STP in the engine. On a good engine with a good oil pump the temp gauge can move quite a lot in the normal range without any movement in the oil pressure. More likely by the time your engine gets to the point where its a quart low the oil has broke down and your filter has filled up with crud and that plus the worn out pick up seal sucking air is the cause of the lower oil pressure. If temperature has anything to do with it then there is something seriously wrong with the cooling capacity of your engine. The only time I've seen anything like what you describe is when an engines cooling system was so full of sediment that the only thing cooling the lower half of the engine was the oil.

-jim

Reply to
jim

I just don't buy it. If a) the engine is that dependent on oil cooling, and b) the oil heat rejection capacity is that sensitive to oil level then its a piss-poor job of engineering by the manufacturer. More likely, its an oil aereation issue- either because of inadequate baffling (does the engine have a windage tray?) or a small leak or leaks on the suction side of the oil pump plumbing.

Reply to
Steve

Well, the rate of the filter filling with crud should not depend on the amount of oil in the crankcase.

So the most concise answer would be simply: The pressure is low because the pump is sucking air somewhere when the level is low, and/or your remaining oil is too contaminated or worn to produce sufficient pressure.

Reply to
runderwo

Nobody said anything about rate of the filter filling with crud depending on anything. Since there is no information of the rate of oil leaving the crankcase or how much crud is in the engine to begin with the reader is simply left to guess what the specific facts involved might be.

If there is some good reason for you to believe the oil is contaminated or worn at the point where it becomes a quart low then it is also likely the filter has become more restricted than it was when it was new.

If you really care to know how the level by itself affects the pressure you need to try it in a way that eliminates the other variables.

-jim

Reply to
jim

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