320d M Sport 2007 sudden drop in MPG. Please help!

Discussion in '3 Series' started by DenProject, Nov 30, 2019.

  1. DenProject
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    DenProject

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    Hi guys,

    I’m new to the forum.

    As the title suggests, my cars mpg has suddenly dropped and I don’t know why. I was getting 49 mpg for the first 3 months or so, 670 miles to a full tank, but now it’s gone down to 37 ish, 530 or so miles to the full tank. I know the weather is a lot colder now but surely that wouldn’t reduce the mpg by 24%?

    Anything spring to mind I could check if you guys think there’s something amiss. Driving style and commute hasn’t changed. I’ve just had it serviced too so the oil and filter are brand new.

    thanks in advance!
     
  2. Wynne71
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    Wynne71 Site Supporter

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    Does the temp of the car seem normal?
    A few possible issues but I’d look at the thermostat to start with. Impact upon the mpg as the ecu reads the car as being cold so over fuels.
     
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  3. DenProject
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    DenProject

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    How would I do that? Sorry, not very clued up
     
  4. Peter
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    Peter WARLORD Site Supporter

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    First place to start, will be to view the engine coolant temperature (ECT) via the hidden menu. Plenty of videos on how to access and use, (Google is your friend for that procedure). Find out how steady the ECT is during the warm up phase and once hot.

    Thermostats are known to be a weak link to efficient mpg. Another possible issue for a sudden change in mpg is EGR rates have changed. EGR valve may have seized or gummed up.

    Check the ECT first. You are wanting to see steady warm up to ~88°C and then pretty constant in the 88 - 92°C range.

    Peter
     
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  5. bishbosh
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    bishbosh WARLORD Site Supporter

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    As above, and if further investigation is required I would also check tyre pressures and if there are any boost leaks.
     
  6. DenProject
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    DenProject

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    Thanks for the speedy replies chaps. Will check temp on way to work
     
  7. DenProject
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    DenProject

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    Well I accessed the hidden menu, put the ect on and it was steady all the way up to around 92 where it sat steadily give or take a degree.

    Tyre pressures all fine.

    What are EGR rates? And can I check this somehow?

    Mark
     
  8. Wynne71
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    When was the last time the air filter was replaced? Worth a look.
     
  9. DenProject
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    DenProject

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    Well I’m not certain but I think it was changed when I got it serviced about 2 months ago but I may have found out the culprit. When I was coasting really slowly I could feel intermittent resistance coming from one of the wheels. I think maybe a brake disc could be slightly warped? Or maybe wheels need balancing?
     
  10. Wynne71
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    Possible dragging handbrake or a seized calliper. Would certainly cause a reduced mpg.
     
  11. bishbosh
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    bishbosh WARLORD Site Supporter

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    Are any of your wheels hot to the touch after a drive? That would indicate a binding brake pad/ shoe.
     
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  12. bmwzoom
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    bmwzoom

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    Check for egr recall my mpg increased once mine was done and yet I had no visual symptom's, all so dpf regens if they are failing etc then you are burning more fuel each time it try's again hence low mpg. This will show in the fault codes & will need to be sorted as it will lead to other more serious issue's.
     
  13. Wynne71
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    Did you sort this?
     
  14. DenProject
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    DenProject

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    Not yet, strangely my MPG has gone back towards normal levels. But the seized shoe or whatever it is hasn’t been sorted, I’m gonna borrow a jack and see what wheel is the culprit and hopefully sort it myself
     
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  15. Alan Gunn
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    Alan Gunn

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    It's only brakes no rush to get them sorted out after all if they fail altogether it would be pointless.
     
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