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post #31 of 39 (permalink) Old 07-07-2005, 10:19 AM
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I drained mine and changed the filter last week, and added exactly 3.5 quarts of MobileOne 5w-30 which brought the level to within a mm of the top dot. You have to remember that no matter how hot the engine is and how long you wait that you will never drain every last drop of the old oil.
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post #32 of 39 (permalink) Old 07-07-2005, 01:28 PM
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Not my point. The fact is you end up with higher pressures if you overfill. If you overfill enough you will eventually find the weakest seal in your crankcase (by blowing it).

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Originally Posted by hyperion
The engine oil pump has no idea of the quantity available, or cares. Hopefully it just keeps on pumping and pumping.
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post #33 of 39 (permalink) Old 07-07-2005, 04:14 PM
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Originally Posted by stevewa
Not my point. The fact is you end up with higher pressures if you overfill. If you overfill enough you will eventually find the weakest seal in your crankcase (by blowing it).

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Originally Posted by hyperion
The engine oil pump has no idea of the quantity available, or cares. Hopefully it just keeps on pumping and pumping.
The key term is overfilling "enough." The amount of oil pressure being pumped up toward the top of the engine through the oil channels is going to be a function of the pump, not the oil level in the case. In a true overfill situation the crank ends up splashing oil all around in the sump causing the bottom of cylinders and pistons to be over lubricated. I can also see where stevewa would see there being an issue with blowing a seal in the crankcase, but this just isn't possible on a 4 qt fill on the Prius. That isn't an overfill situation like what you are thinking about that would cause engine failure.

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post #34 of 39 (permalink) Old 07-07-2005, 06:03 PM
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Overfilling may cause two problems: foaming due to splash of connecting rods in the oil bath, or forcing of oil into sensors that will then cause a DTC code, as the factory notice warns. Except at the extremes of absolutely filling the crankcase with oil, or running it to pickup starvation, the pressure is independent of oil quantity.

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post #35 of 39 (permalink) Old 07-07-2005, 06:10 PM
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You might also consider that the more space in the crankcase is taken up by incompressible oil, the less space is taken up by compressible air. Crankcase pressures rise with oil fill level.
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post #36 of 39 (permalink) Old 07-07-2005, 11:30 PM
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Originally Posted by stevewa
You might also consider that the more space in the crankcase is taken up by incompressible oil, the less space is taken up by compressible air. Crankcase pressures rise with oil fill level.
Doesn't the PCV valve set the crankcase pressure independant of its volume? Or don't I understand the PCV valve.

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post #37 of 39 (permalink) Old 07-08-2005, 11:53 AM
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Yes, the PCV system opens the crankcase to the intake system (earlier cars to the atmosphere) so there is no overpressure.

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post #38 of 39 (permalink) Old 07-08-2005, 02:09 PM
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Please explain then why the more oil in the crankcase the lower the MPG. I'm asking an honest question here...I have observed this to be the case but if you say it can't be due to oil pressures or crankcase pressures I don't have an explanation...
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post #39 of 39 (permalink) Old 03-23-2018, 05:15 PM
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Quote:
Originally Posted by stevewa View Post
Please explain then why the more oil in the crankcase the lower the MPG. I'm asking an honest question here...I have observed this to be the case but if you say it can't be due to oil pressures or crankcase pressures I don't have an explanation...
The PCV system is designed to maintain a negative crankcase pressure in almost all cases during engine operation. There are two avenues for the crankcase pressures to be vented to the intake system. The vacuum operated PCV valve and a hose (line) from the crankcase (usually a valve cover) to the upper part of the intake system (above the throttle plate) for those times when the lower intake system vacuum drops (hard acceleration). That portion operates on pressure differential.
The possibility of lower MPG resulting from higher oil levels is a combination of excess oiling splashed through the lower portion of the crankcase and crank rotation interference. Except in the case of a serious overfill, I believe the MPG difference is minimal. Every ICE has a preferred level of oil quantity. Once that is determined, the ICE will operate @ its optimum level of performance with minimal oil consumption. Hope this helps. Peace to all.
If you need additional info on oil pressures in a wet sump system, post here. It is a combination of ICE RPM, Pickup head pressure, oil viscosity and ICE internal resistance, but the maximum pressure is usually controlled by an oil pressure relief within the oil delivery system.
Dry sump system are another cat all together because the storage is divorced from crankcase area. Again hope this helps.
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