too much oil= low MPG?
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This is a discussion on too much oil= low MPG? within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; So this has happened to me twice and i wanted to see if anyone else had this problem. I have ...

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    too much oil= low MPG?

    So this has happened to me twice and i wanted to see if anyone else had this problem. I have a 09 wrx and the first time i changed the oil i put in a little too much. The car instantly started getting 1-2 less mpg. I checked the dip stick and it was a little high, I drained a little and the car went right back up to where it normally cruises at. Ive done another oil change and was carefull to make sure to get the right amount and everything is great, even with stage one i average 25.7 mpg. I do baby it most of the time. It was time for another oil change and i didnt feel like doing it so i found a local place that would install my oil and my filter for 16 bucks.. seemed good to me, no jacking up my car, getting oil on me and making a mess.. So after they are done i notice there is a little less oil left over than what should be. ON the ride home my mpg drops right away down to between upper 23's to 24.3. mpg from 25.7 So tonight i spent a half hr draining some oil out... I am assuming ill be back at 25.7 mpg tomorrow on my way in. Has anyone else had this issue? It cant be a coincidence. Since having installed the accessport over the course of 1000 miles i managed to average 25.7 and babying my car driving home and less than 20 miles that average dropped more than 1 mpg... Seems to me like our cars are supper sensitive to too much oil, and im not talking a lot extra, instead of 4.2 maybe 4.5. It doesnt take much, atleast for my car to get pissy.... Just curious if im the only one or is everyone putting in too much and just getting crappy mpg??

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    He simply abides. SD_GR's Avatar
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    I routinely add a bit over, it just happens that way, and I've not noticed this. However, your map is almost certainly leaner than mine, since I run the stock mapping. Maybe that has something to do with it, IDK.
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    Good news, everyone! xxxxxxxAnub1s's Avatar
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    I suppose it's plausible, but I don't see it causing that much of a drop. What method are you using to verify this? With too much oil comes oil sloshing. The crank can actually dip into the oil which makes it churn. Either the effect of it going into the oil could cause this or the decreased protection caused by the frothy oil not being picked up as well by the oil pump (mighty dangerous that).
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    Registered User teflon_jones's Avatar
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    In general I always overfill the oil a little. For instance, on my STi I put in a 5 quart jug though the amount is 4.8 qts per the manual. On my 911 (and my previous one) I always put in 10 though the level is technically 9.8 or something like that. I've never seen any MPG difference in any of the 3 cars, or on any of my more "regular" cars.

    Stay away from places that charge $16 for an oil change. The guys doing it generally have less car knowledge than a dead horse. I've had more than a few friends that have had major damage done by those places through sheer stupidity. Either take the time to do it yourself, or spend the extra $15-20 to take it to a decent independent mechanic. Well worth it.
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    Registered User teflon_jones's Avatar
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    Quote Originally Posted by Anub1s View Post
    I suppose it's plausible, but I don't see it causing that much of a drop. What method are you using to verify this? With too much oil comes oil sloshing. The crank can actually dip into the oil which makes it churn. Either the effect of it going into the oil could cause this or the decreased protection caused by the frothy oil not being picked up as well by the oil pump (mighty dangerous that).
    That's why you need dry sump lubrication.
    Scott
    Past Subies - '11 STI Limited stage 1+ and '04 STi stage 2+
    '13 Wrangler Rubicon - custom front bumper w/12k lb winch & off-road lights, skid plates, diff guards, etc.
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    '77 F-150 heavily modded for off-roading

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    Good news, everyone! xxxxxxxAnub1s's Avatar
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    Quote Originally Posted by teflon_jones View Post
    That's why you need dry sump lubrication.
    Would that we could all have that. Much too expensive and overkill for what is a performance based econobox though.
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    Thanks for your replies. Anub1s what are you referring to when asking what i use to verify this? If you mean MPG i use the on board computer and the accessport to monitor the mpg. The first time this happened i didnt have the accessport yet so i dont think the map has anything to do with it. I just think that atleast my car it is very easy to put a little too much in and have the oil start churning. I just figure if it can make my car get 1 mpg worse instantly it must not be good. Stupid question but could their be something wrong with my oil pan, that would make it extra sensitive to too much oil? no accidents or anything.

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    Good news, everyone! xxxxxxxAnub1s's Avatar
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    I see, that's a calculated number, not always 100% accurate, but it's probably consistant enough to get applicable data from. I can't imagine that the oil pan is SO sensitive as to have problems with .3qt extra oil. That just sounds silly to be honest, though it's not exactly outside the realm of possibility.

    I would be more worried about your oil pressure and temp than your MPG, to be honest. It's not a good thing to have too much oil in the pan, but I can't imagine a car company not allowing some leighway there just in case it is overfilled by a bit. In general I keep my levels just a very small distance from the max line so as to avoid sloshing altogether, but again, it just doesn't seem reasonable to think that a major car company wouldn't account for a bit of overfill every now and again.
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    He simply abides. SD_GR's Avatar
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    If your car has no accidents and no parts altered the oil pan is the same as any other.

    It's interesting though. Consider filling up the tank and then doing the division when it's empty vs. the miles covered to get MPG calculations, then repeat for, say a few months and see what happens.
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    I always put the whole 5 guart jug in, even though it only calls for 4.8 but my thought is whats 0.2 qt for an engine and I have consistantly get about 23-26 mpg, depending on how much I am in the turbo (of course).

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    I would love to get 23-26mpg. My STi drinks more gas than a redneck drinks cheap beer!

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