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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / January 2007Why is oil pressure lower when level is low
Assuming that: - the oil pump moves a fixed volume of oil per rotation; - the oil level never drops so low that the pickup is even partially starved; - oil is not compressible; why is it that the system oil pressure drops as the crankcase oil level drops? I have two hypotheses. One is that it is related to the amount of air trapped in the oil, and the other is that it is related to crankcase pressure. Maybe I should have taken fluid dynamics in school... > Assuming that: > - the oil pump moves a fixed volume of oil per rotation; [quoted text clipped - 8 lines] > trapped in the oil, and the other is that it is related to crankcase > pressure. 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. > Assuming that: > - the oil pump moves a fixed volume of oil per rotation; [quoted text clipped - 4 lines] > why is it that the system oil pressure drops as the crankcase oil level > drops? 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 > I have two hypotheses. One is that it is related to the amount of air > trapped in the oil, and the other is that it is related to crankcase > pressure. > > Maybe I should have taken fluid dynamics in school... > why is it that the system oil pressure drops as the crankcase oil level > drops? ========== 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. > Assuming that: > - the oil pump moves a fixed volume of oil per rotation; [quoted text clipped - 4 lines] > why is it that the system oil pressure drops as the crankcase oil level > drops? Mine never has, over a few cars. And sometimes I let that oil level get quite a bit too low... > Assuming that: > - the oil pump moves a fixed volume of oil per rotation; [quoted text clipped - 4 lines] > why is it that the system oil pressure drops as the crankcase oil level > drops? 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). > Assuming that: > - the oil pump moves a fixed volume of oil per rotation; [quoted text clipped - 10 lines] > > Maybe I should have taken fluid dynamics in school... 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. >Assuming that: >- the oil pump moves a fixed volume of oil per rotation; >- the oil level never drops so low that the pickup is even partially >starved; >- oil is not compressible; >why is it that the system oil pressure drops as the crankcase oil level >drops? >I have two hypotheses. One is that it is related to the amount of air >trapped in the oil, and the other is that it is related to crankcase >pressure. >Maybe I should have taken fluid dynamics in school... 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 in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > Assuming that: > - the oil pump moves a fixed volume of oil per rotation; [quoted text clipped - 10 lines] > > Maybe I should have taken fluid dynamics in school... It's the same reason that you'll show a higher pressure with a cold engine. > 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. >> 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 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) >>> It's the same reason that you'll show a higher >>> pressure with a cold engine. [quoted text clipped - 3 lines] > > Isn't is supposed to be the other way around? No. > 10W30 is 10 weight when > cold and 30 weight when hot? Sorta-kinda. But a hot 30 weight is still thinner than a cold 10 weight. > >> It's the same reason that you'll show a higher > >> pressure with a cold engine. [quoted text clipped - 4 lines] > Isn't is supposed to be the other way around? 10W30 is 10 weight when > cold and 30 weight when hot? 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 > 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 http://www.imagestation.com/album/pictures.html?id=2115147590 > (More Off Road album links at bottom of the view page) >>>> It's the same reason that you'll show a higher >>>> pressure with a cold engine. [quoted text clipped - 17 lines] > > -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 >> Mike >> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 [quoted text clipped - 6 lines] > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups > ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- > >>>> It's the same reason that you'll show a higher > >>>> pressure with a cold engine. [quoted text clipped - 3 lines] > >> Isn't is supposed to be the other way around? 10W30 is 10 weight when > >> cold and 30 weight when hot? Something resembling that...The following link has a pretty good explanation. http://www.howstuffworks.com/framed.htm?parent=question164.htm&url=http://www.mi capeak.com/info/oiled.html Note this link is wrapped, so you may have to copy and paste it into the browser url window. You see how much is dependent upon the polymer function in multiweight oils. One thing we know about polymers is that they ALL shear degrade. That is, at temperature and with mechanical shear, the polymer chains break, and you begin to lose the effects of the polymer. Lubricating oils have other additives to improve lubricity, decrease spalling, etc, but these polymers are there for viscosity maintenance. "Mike Romain" <romainm@sympatico.ca> wrote in message news:45b657e2$0$1987 > 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. > "Mike Romain" <romainm@sympatico.ca> wrote in message news:45b657e2$0$1987 > [quoted text clipped - 9 lines] > obvious > but maybe it is. 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. > Also you might have slightly more tendency to aerate the oil when it is a > quart low. > > Interesting premise, at any rate. 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. > "Mike Romain" <romainm@sympatico.ca> wrote in message news:45b657e2$0$1987 >> [quoted text clipped - 15 lines] > > Interesting premise, at any rate. 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! > 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?............................... 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 > 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 [quoted text clipped - 5 lines] > *This word was well-adapted in another reply, so I obliged my self of its > use! >>"Mike Romain" <romainm@sympatico.ca> wrote in message news:45b657e2$0$1987 >> [quoted text clipped - 22 lines] > when the oil level is 12 inches deep or when it is 2 inches > deep?................................. 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. >> "Mike Romain" <romainm@sympatico.ca> wrote in message news:45b657e2$0$1987 >>> Yup, like I mentioned also, I can see it in my gauge when hot and at [quoted text clipped - 29 lines] > *This word was well-adapted in another reply, so I obliged my self of its > use! 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 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) >Pumps only suck harder as the rpm go up which is evident in the gauge also. Pumps don't suck. They can only partially remove a portion of the atmospheric pressure. >>> It's the same reason that you'll show a higher >>> pressure with a cold engine. [quoted text clipped - 4 lines] >Isn't is supposed to be the other way around? 10W30 is 10 weight when >cold and 30 weight when hot? 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. > 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. Umm.... what? ;-) >> 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 [quoted text clipped - 3 lines] > >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. >>> 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 [quoted text clipped - 3 lines] >> >>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. Yes, but irrelevent. A 10W30 will behave like a 10 weight oil when cold and behave like a 30 weight oil when hot. >>>> 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 [quoted text clipped - 10 lines] >Yes, but irrelevent. A 10W30 will behave like a 10 weight oil when cold and behave >like a 30 weight oil when hot. And that's exactly what I said. A 10 weight oil is only 10 weight at room temperature. >And that's exactly what I said. A 10 weight oil is only 10 weight at >room temperature. Correction: the temperature for viscosity numbers is taken at 212 degrees. >>>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 [quoted text clipped - 7 lines] > a higher viscosity at 10 degrees than a 30 weights actual viscosity at > 200 degrees. 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. >>>>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 [quoted text clipped - 11 lines] >they say EXACTLY the opposite of each other. The second one is wrong. >Probably just a typo, but rather humorous nonetheless. 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. >>> It's the same reason that you'll show a higher >>> pressure with a cold engine. [quoted text clipped - 4 lines] >Isn't is supposed to be the other way around? 10W30 is 10 weight when >cold and 30 weight when hot? 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 www.donsautomotive.com >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 http://www.imagestation.com/album/pictures.html?id=2115147590 >(More Off Road album links at bottom of the view page) >> 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. Which is the same reason. On Jan 23, 2:34 pm, t...@mucks.net wrote: > On 23 Jan 2007 08:23:43 -0800, "runde...@mail.win.org" > [quoted text clipped - 8 lines] > > Which is the same reason. 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? > On Jan 23, 2:34 pm, t...@mucks.net wrote: > > On 23 Jan 2007 08:23:43 -0800, "runde...@mail.win.org" [quoted text clipped - 16 lines] > In other words, could you help me wrap my head around what you are > saying? 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 > 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. 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. >> 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 [quoted text clipped - 8 lines] >arrived at the conclusion that changing the oil level changes the >temperature or viscosity of the oil. 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. > 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. 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 > 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. > t...@mucks.net wrote: > > The OP states that with a lower oil level he sees lower pressure. If [quoted text clipped - 14 lines] > > because the oil itself has less surface area to dissipate the heat.----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----http://www.newsfeeds.comThe #1 Newsgroup Service in the World! 120,000+ Newsgroups > ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- One could posit the beaters have bearing clearences large enough to allow the oil to escape fast enough that the difference is noticeable. Dave >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? If you remove enough oil, or add extra oil via an oversized oil pan you will get a different temperature/viscosity/pressure of oil. > >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? > > If you remove enough oil, or add extra oil via an oversized oil pan > you will get a different temperature/viscosity/pressure of oil. 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 >> >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? [quoted text clipped - 4 lines] > >Avoiding the question. I'm not avoiding anything. > 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). To many variables. Define worn. A proper gear pump will be a positive displacement pump. If not it's time to change the pump. > >> >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? [quoted text clipped - 6 lines] > > I'm not avoiding anything. So then answer the question. Why does your theory fail on engines that aren't worn? > > 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 [quoted text clipped - 3 lines] > To many variables. Define worn. A proper gear pump will be a positive > displacement pump. If not it's time to change the pump. 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 >Even a worn pump is a positive displacement pump. Positive displacement >doesn't mean it has perfect volumetric efficiency. Just because the [quoted text clipped - 6 lines] >as it will all leak back (due to gravity) at a rate greater than the >rate of displacement. 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, > On Jan 23, 2:34 pm, t...@mucks.net wrote: > [quoted text clipped - 18 lines] > In other words, could you help me wrap my head around what you are > saying? 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. >> >> It's the same reason that you'll show a higher >> >> pressure with a cold engine. [quoted text clipped - 10 lines] >In other words, could you help me wrap my head around what you are >saying? 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. > I never said anything about oil level. Then start your own thread if you want to change the subject. The question is 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. 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 > t...@mucks.net wrote: >.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. 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. > > t...@mucks.net wrote: > >.Pretty much true. But there is some leakage so the volume is not always [quoted text clipped - 10 lines] > back, but it behaves the same with respect to oil level as the old pump > did, as far as I can tell. 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 > 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. 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. > 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. 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. > > Your original question was a hypothetical question based on hypothetical > > set of facts you presented. Now you are asking a real question with no [quoted text clipped - 15 lines] > register lower at that point since the new equilibrium temperature > would be higher. 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 > IF IF IF...... you might actually try it instead of speculating. a few > degrees temp difference at operating temp doesn't make enough difference [quoted text clipped - 6 lines] > and that plus the worn out pick up seal sucking air is the cause of the > lower oil pressure. 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. > > IF IF IF...... you might actually try it instead of speculating. a few > > degrees temp difference at operating temp doesn't make enough difference [quoted text clipped - 9 lines] > Well, the rate of the filter filling with crud should not depend on > the amount of oil in the crankcase. 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. > 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. 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 >>Your original question was a hypothetical question based on hypothetical >>set of facts you presented. Now you are asking a real question with no [quoted text clipped - 15 lines] > register lower at that point since the new equilibrium temperature > would be higher. 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. >> t...@mucks.net wrote: >>.Pretty much true. But there is some leakage so the volume is not always [quoted text clipped - 10 lines] >back, but it behaves the same with respect to oil level as the old pump >did, as far as I can tell. tnom@mucks.net did not author the above quoted text. |
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