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  1. #1
    Nate in Phoenix
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    Extremely Poor Fuel Economy (very long, sorry)

    Sorry for the long post in advance, but I want to make sure I have all the symptoms covered. I have a 1987 535i with about 184k on the clock. Car was running great and getting around 20 mpg around town up until about 6 months ago when it refused to start. Diagnosed that problem down to an aftermarket alarm a from a previous owner that was preventing the fuel pump relay from turning on. Unplugged the alarm box and car started up but started running very rich. The car sat for about 6 months and I've just recently begun using it as my daily driver again, and the mileage has now dropped to 7-8mpg with around 80% non-aggressive freeway driving.

    Ok, i know that was long, but here are the more recent details:

    O2 sensor replaced 4 days ago, bought from BMP design. New one has a longer cable by about 1.5 feet, but socket matches up.

    OBC and dash economy meter read 20-22 mpg on average, yet the car is getting around 7-8mpg.

    Averaging about 90-100 miles before the gas gauge hits the 1/4 mark from a full tank.

    Car runs terrifically, has plenty of power, and were it not for the fuel economy I wouldn't think a thing was wrong with it.

    Car has (as far as I know) stock injectors, with no performance modifications at all.

    Any thoughts on what else I could replace and/or adjust to get me back up to a reasonable amount of fuel consumption? Any ideas would be appreciated, this car is costing me about $100 dollars a week in gas alone, and that's just driving roughly 25 miles every day. Thanks in advance all.

    -Nate in Phoenix
    1987 535i


  2. #2
    TJ
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    Re: Extremely Poor Fuel Economy (very long, sorry)

    If the gauge shows 20mpg but you're getting only 8 in reality, that would point to a stuck open injector, too much fuel pressure, or a fuel leak. If there are no driveability problems from running rich, a leak seems all the more likely.

  3. #3
    Nate in Phoenix
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    Re: Extremely Poor Fuel Economy (very long, sorry)

    There is no fuel leak I can detect, visually or by smell, but the car is running VERY rich, to the point where I'm getting worried about ruining my cat if I drive it too much longer. Will look into the fuel pressure tomorrow when I can get a hold of a flow gauge.

  4. #4
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    What about a bad O2 Sensor Relay?


  5. #5
    Nate in Phoenix
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    Solved

    Ok, I suppose this is partially my inattentiveness, but I've found out that the car is not really sucking that much gas, but in fact the odometer (trip and main) is turning over at about 3/8 the speed it should be. Since that was what I was using at my gauge for mileage, I was grossly underestimating the mileage. Thanks to Steve and TJ for the possible causes, now I have to check into the fixing the odometer, thanks again guys.
    -Nate
    1987 535i

  6. #6
    Steve: 85 735i, 88 735i, 88 735iL
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    Couple of questions-

    Is it still running VERY rich as you stated in a previous post?

    Is it still going through the fuel as fast as you thought. You say your driving 25 miles a day. Is that how many miles it really is? Or was that measured from the slow moving odometer? If your driving a known distance and you're still going through a lot of fuel, do you still having a problem?

  7. #7
    Nate in Phoenix
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    Re: Couple of questions-

    Hey steve, thanks for the response.
    According to the OBC, the fuel economy is around 20-21 MPG, not bad for rush hour traffic and around town driving from what I can figure. I did just replace the fuel filter and the O2 sensor, and it does smell like it's running a tad rich, but I don't think rich enough to chalk it up to anything more than having 185k miles on the car. On my next day off I'm going to do a bit of fairly minor maintenance (oil change and such) and I'll check the voltage on the O2 sensor. Any other ideas on how to read the mixture other than the voltage at the sensor? I did adjust the AFM 1 tooth leaner when I had it pulled apart the other night, and it seemed to idle a bit better. Thanks again for the response.
    -Nate in Phoenix
    1987 535i 185k

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