Oil pump prime 101. What are the basics?

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658
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EU
In exchanges about changing/draining oil, when someone states that the pan is left to drain overnight or for a similar length of time under other circumstances, there is very often the rejoinder that this might be too long because the oil pump might lose its prime. What exactly happens in such a case? Why is overnight too long, but an hour or two is not? What's the worse thing that could happen? Aren't the pumps self-priming? Is the principal concern, then, the time lag for the oil pick up the prime while the engine is running at first startup? How long might that lag time be? Other considerations? Please help clear the air over this issue. Thanks all.
 
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1,874
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Ocala, Florida
Yz, a positive displacement oil pump such as used in engines will not lose it's prime unless it's worn out. If you look at how it's built, the "gears" that move the oil though the pump will retain oil in the output side of the pump and only will the input side of the oil pump drain back. So, oil should be in the output side up to the filter and when cranked, it will supply enough pull or prime to introduce the new oil into the input side and continue the flow. This topic on oil pumps might help show how the pump works. Oil Pumps and how they work
 
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714
Location
Pennsylvania
quote:
Originally posted by YZF150: In exchanges about changing/draining oil, when someone states that the pan is left to drain overnight or for a similar length of time under other circumstances, there is very often the rejoinder that this might be too long because the oil pump might lose its prime. What exactly happens in such a case? Why is overnight too long, but an hour or two is not? What's the worse thing that could happen? Aren't the pumps self-priming? Is the principal concern, then, the time lag for the oil pick up the prime while the engine is running at first startup? How long might that lag time be? Other considerations? Please help clear the air over this issue. Thanks all.
'priming' an oil pump is sort of a misnomer - its not like a well pump with actually needs a liquid prime to form the needed vaccum to lift the liquid. the oil pump is so close to the surface of the oil , only a slight negative pressure is needed, AND, you are not draining the pump cavity unless you take it apart. (even a rebuilt motor does not need filled with oil or grease or vasoline to start sucking oil - just use a drill and a proper adaptor.)
 

YZF150

Thread starter
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658
Location
EU
Thank you. Very helpful. Follow up: What is an approximate volume of oil that is retained in a pump, oil that will become incorporated into the fresh fill? --I realize that there are many variations on the basic design; just a ballpark figure, if you could.
 
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714
Location
Pennsylvania
quote:
Originally posted by YZF150: Thank you. Very helpful. Follow up: What is an approximate volume of oil that is retained in a pump, oil that will become incorporated into the fresh fill? --I realize that there are many variations on the basic design; just a ballpark figure, if you could.
well I happen to have a well worn chevy sb pump in the garage, Id say the room around the gears, and up to where it meets the rear main cap is probably on the order of 2-3 oz.....a shot glass is an ounce right? so ya, about 2-3...mebbe a little more will sit in the passages up to the filter boss.
 
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43,676
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'Stralia
I believe in priming oil pumps on a rebuild depending on the location of the pump. Holdens have a pump located well above the oil level, so that there's a lift of 6-8" when the pump is only pumping air for the first start. After that, there's always a small amount of oil left to seal the clearance between the gears and housing, so it's not as much an issue. An oil pump sitting in the sump should be primed by the time you get to the wheel and turn the key.
 
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