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Original owner, 2004 X3: mileage is lousy

Discussion in 'E83 X3 (2004-2010)' started by rkcjn, Apr 26, 2016.

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    rkcjn

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    We've owned our X3 since around 2/2004. Great car and now has some 97,000 miles. I've always had it maintained at local 90266 BMW independent who has always done great. This is my wife's car and her driving style and habits haven't changed, and her daily runs around town are the same as always.
    But some years ago, the mileage started to get really bad. Earlier in its life the X3 got a combined 17 - 18 mpg. Since she doesn't put that many miles on it, I sort of ignored it. Also some years ago, we kept getting a check engine light, and correlated that to my wife's increasing inability to tighten the original gas cap. So we replaced the cap and haven't gotten that fault since then.
    Now its our only car and I've not been able to have the cause of poor mileage diagnosed. Lots of really clever ideas, but none of them pan out.
    I might say that I know a LOT about cars in general, but I can't figure it out for sure. I've checked the mileage manually ala' miles driven versus gas used, and the computer is pretty much right on.
    I've been doing some testing. If I'm driving around at 30-40 mph and reset the mileage tracker, I get a great 25+ mpg. On the freeway it's great as well. But after I've done a driving speed reset, if I come to a stop light, you can visually see the mileage literally plummeting at idle. Using a ton of gas at idle!!
    I'm told that the fuel pump puts out constant volume. At idle, there is supposed to be some kind of "return" line that sends "unneeded" gas back to the tank. If that's true, then it doesn't appear to be working. And there's never any sign of or smell from gas leaking.
    So why is our old 2004 X3 "using" so much gas at idle?
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    charlson89

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    Anything replaced so far to try and repair this?
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    rkcjn

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    Not yet. I'm thinking oxygen sensor and/or mass flow sensor. But most likely the oxygen sensor. I hope it's easy to replace?
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    rkcjn

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    How many oxy sensors are there and is it/are they easy to replace? I'm pretty good around engines....
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    charlson89

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    The oxygen sensors are on the passenger side right on the both the manifolds. The after cat sensors are down below the cats. Do you have any smoke from the tailpipe at idle or rough idle? At idle a normal engine will consume the most fuel at idle. Has the MPG gone down then?
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    MGarrison

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    Wouldn't a bad 02 sensor typically throw a check engine light? You can use realoem.com to lookup parts and general locations. Presumably, your engine is the M54, you should be able to google up tons of info, try a search for BMW M54 mileage. There are so many things that can impact mpg, unless something obvious is evidencing itself, not sure I'd throw parts/money at it without more specifics. A code readout of any errors might be helpful as a starter.

    If you do your own work, one of these could be very helpful:

    http://www.bentleypublishers.com/bmw/repair-information/bmw-x3-2004-2010-repair-manual.html
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    rkcjn

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    No smoke or anything weird. If I reset mileage computer at say 45 mph, and then come to a stop sign, you can literally see the mileage dropping. On the road, get 24+ mpg. Around town, lots of stop and go, it's 12 mpg or so...
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    rkcjn

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    I've done a lot of web searching, and there are many things that can throw the check engine light. I've got a reasonable mechanic looking at it and I know a lot about cars. He just tested some O2 sensor reading that is supposed to be in the 1.0 ± 0.01 or so, and mine reads 1.46. The ratio, assuming it's linear, is just the drop in mileage I've been experiencing. I'm getting 12 mpg around town, should be 18 mpg as it was years ago, but dropped several years back. 18 mpg/1.46 = 12 mpg. But there are some other little things that are somewhat contradictory, so I'm leaving it alone. The cost to replace the 2 each O2 front sensors is like $800. That's a lot of gas, and the payback, if that's the actual answer, is like 1.5 years....

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