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hi ive got a bmw 520d m sport berfore xmas my mpg was doing around 38-40 mpg around town but since this cold weather my car is now struggling to do 30 mpg any ideas what this could be

someone please help is cracking me up slowly
 

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Hi Dean

There has been a lot of talk recently about faulty thermostats preventing the engine heating properly and resulting in worse fuel economy

this would be a good thing to check using the "hidden" menu in the on board computer
 

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My E61 520d does 38-39 mpg normally and now is doing 33-34 I just put it down to defrosting the car on tick over and the lights and air-con being used all the time, plus I do shorter trips in winter and do not venture as far as in the better months. I think that 5mpg down on summer or so is a reasonable thing to see.
If you have the correct tyre pressures and reset the computer and drive on the w-way for a sensible journey and back you will soon see if the car is poor. Maybe do it good aswell IE clearing out the DPF after slower driving in all this snow and stuff we have had for a while.
 

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Weather i'm afraid we've had so many customers complaining of poor mpg since frosty weather, car takes longer to warm up using more fuel and ther things. Get fault memory read if any faults in DDE stored put your mind at rest.
 

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Weather i'm afraid we've had so many customers complaining of poor mpg since frosty weather, car takes longer to warm up using more fuel and ther things. Get fault memory read if any faults in DDE stored put your mind at rest.
A properly working thermostat (which many aren't on diesel engines) should be able to warm the engine up to the correct operating temperature (88 - 93C) within 3 or 4 miles. Mine was taking about 10 to 15 miles and only achieving mid 70's. New thermostats (both main and EGR) and all has changed, including the MPG.

To the OP, 82 is a little cool but is that stationary or on a run? You don't need an OBD reader to determine the coolant temp. It's available on a hidden menu in the OBC (on board computer)
 

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My 320d's mpg is suffering by prob around 5mpg, I put this down to the winter months :frown May be worth getting the thermostat changed tho.
 

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If 82 C is the max temperature you saw then something is in need of replacement.
Difficult to say which thermostat is the bad one but I am sure that once you get the engine temperature back to where it should be then your mpg will also improve.
 

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a new thermostat is marked 88C so it should regulate at that or marginally above, say 88-93C.

My own warm up times improved dramatically after fitting new main and EGR thermostats. Values below were taken on an identical route.

Distance----Temp ( C ) before change-------Temp ( C ) after change

1 mile-----------------43------------------------------48
2 miles----------------48------------------------------55
3 miles----------------56------------------------------70
4 miles----------------61------------------------------74
5 miles----------------62------------------------------80
6 miles----------------66------------------------------86
7 miles----------------61------------------------------86
8 miles----------------62------------------------------86
9 miles----------------64------------------------------89
10 miles---------------67------------------------------90
 

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a new thermostat is marked 88C so it should regulate at that or marginally above, say 88-93C.

My own warm up times improved dramatically after fitting new main and EGR thermostats. Values below were taken on an identical route.

Distance----Temp ( C ) before change-------Temp ( C ) after change

1 mile-----------------43------------------------------48
2 miles----------------48------------------------------55
3 miles----------------56------------------------------70
4 miles----------------61------------------------------74
5 miles----------------62------------------------------80
6 miles----------------66------------------------------86
7 miles----------------61------------------------------86
8 miles----------------62------------------------------86
9 miles----------------64------------------------------89
10 miles---------------67------------------------------90
 

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i'm getting something similar to you Vista before you changed both stats - what kind of cost am i looking at? - will ask a few indies to quote me a price before i decide who gets my business ;-)
 

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i'm getting something similar to you Vista before you changed both stats - what kind of cost am i looking at? - will ask a few indies to quote me a price before i decide who gets my business ;-)
I'm sorry, I can't tell you exactly how much it was as I had both of mine changed under a BMW warranty (after a bit of a disagreement with the first dealer I went to who claimed there was nothing wrong with running at 65C :jaw-dropping )

The parts weren't that much (about £35 for the main & £28 for the EGR) and it took them about 2 hours to change both thermostats................though they'll probably try and bill you for 4 hours!
 

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hi mate that reading was on a 20 min run and that was the highest it went to 82 do you think its a faulty thermostat
Change EGR stat. Its an easy DIY and costs less than £30.

Both my stats needed changing as car was running 60 oC, however when I changes only the EGR it went up to 75 oC.
 

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One thing to note, that in England the Diesel changes grade as there is a winter and summer grade to stop waxing due to freezing temperatures. The winter grade is notorious for getting less mileage per litre. Winter grade is normally supplied between October and march.
 

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One thing to note, that in England the Diesel changes grade as there is a winter and summer grade to stop waxing due to freezing temperatures. The winter grade is notorious for getting less mileage per litre. Winter grade is normally supplied between October and march.
That's interesting, I've never heard that before. Where did you learn that?
 
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