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3.7

Just got back from Jiffy lube. Some good news. I've been busy, so I let it go 6,000 miles, although I've checked it. They showed me the old oil and filter and it was just past brand new...hardly dirty at all.

I debated with myself as I drove there. Should I follow the advice of

Hyperion (4 quarts)
TFM (3.9)
DanMan (3.5).

In the end, I instructed the very competent manager to give me 3.7 quarts, and I checked the dipstick and saw that it was just under full.
 

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What gets me, if the 1/16 of an inch over the full dot is so bad, why is every Prius being delievered just exactly like that? Things that make you go hmm. Things that make me put in 4 quarts, just like the factory.
 

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More oil == more oil pressure == engine working harder to pump oil == LOWER MPG.

I have observed this myself.

DanMan has it right. 3.5 is the magic number. Anywhere between the two dots on the dipstick is considered "proper" oil level...above is too full, under is too low.
 

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It sure could make a lot of differance if your car starts consuming oil and you aren't checking every day.
I check it monthly and after 13 times haven't yet had any noticeable change in level. If oil disappeared at a rate that called for daily checking, that would be a symptom of something badly wrong. Why pour too much oil into it in anticipation of such a problem? I mean, golly: this isn't some old broken-down sh!t European or American car that one *expects* to burn oil, it's a Prius!
 

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If I checked daily, I probably wouldn't notice any more than I would monthly, anyway, since the increments would be so small. In fact, a monthly decrease I'd probably notice sooner, unless one morning I went out to find it quarts lower than it was the day before. And then there's a larger underlying problem than whether I'm checking daily or monthly! :D
 

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My guess as to why the factory overfills is because they expect slightly higher oil consumption during the break in period. My car came seriously overfilled, about half an inch over the top mark. I drew out eight to ten ounces to get it about a sixteenth of an inch below full.
 

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If you checked the oil daily, you would actually be slowly decreasing the oil level in the system each time you checked it (that film of oil on the dipstick that you wipe off each time would eventually add up to a noticable amount)
 

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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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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).

hyperion said:
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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stevewa said:
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).

hyperion said:
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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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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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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stevewa said:
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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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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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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