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Loss Of Mpg.

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#1 blue flash

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Posted 29 November 2012 - 08:22 PM

i'm hoping someone can help because this is doing my nut.my car is a focus 2005 1.8 diesel.i've just given it a 70,000 service(oil/air filters and europarts engine oil 5w30).was getting anything from 51 to 54 mpg.and now i'm lucky to get 40 mpg.i can acually see the fuel gauge going down slowly when driving.
so if anyone has an idea,that would be great as have a 500 mile trip to do next month.cheers guys.

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#2 ZetecSmitch

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Posted 29 November 2012 - 11:27 PM

try re-setting the ECU? unplug the battery for 30 mins then plug it back in. keep your radio code to hand! thats not good mpg though! im getting more than that from my 1.8 petrol!!!

#3 blue flash

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Posted 29 November 2012 - 11:44 PM

yeah have tried that 4 times,but still the same.

#4 FOCA

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Posted 30 November 2012 - 12:14 AM

Its a "learning" ECU, so it takes a bit of time for it to "learn" (different conditions etc)

the colder weather can have an effect on MPG because a diesel has a better MPG once its warmed up, it can take a long time to warm up in cold weather (if it ever really does) especially for short trips

Winter diesel has anti-freeze in it and there is a small? drop in mpg with this too

Same with winter tyres (more drag)

Check your brakes are not binding and your tyres are at the highest recommended pressure

try resetting your avarage MPG, round town driving can drop the avarage down

are you driving harder, more weight in the car, or shorter runs, round town or in heavy traffic -

seems a big drop in MPG though

#5 artscot79

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Posted 30 November 2012 - 01:27 AM

With the cold weather diesels take far longer to warm up and it effects mpg just as it does in a petrol it may be the cold and nothing more also check the underneath of the fuel tank for any leaks

#6 blue flash

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Posted 30 November 2012 - 10:03 AM

thanks for your reply guys,foca,i have been getting 50+ mpg even in really cold weather.tyres are at 35psi, and brakes are not binding.have done a 70 mile trip this morn and the mpg has gone down to 37.9.i'm driving the same roads in the same way,of the 2 i'm driving MORE carefully.and artscot79,i used to leave it running for 15 mins in cold weather to warm up a bit before going for a trip and still got 50+ mpg.no leaks under the car either.is there a fuel sensor somewhere on it that maybe on it's way out.thanks for your replys guys..

#7 artscot79

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Posted 30 November 2012 - 10:20 AM

very strange indeed then have you checked the air filter is housed correctly and alls tight

#8 blue flash

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Posted 30 November 2012 - 12:28 PM

yep just had a look,and it's all fine.

#9 Pitmonster

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Posted 30 November 2012 - 01:49 PM

Has this MPG been validated by manual calculation (brimming the tank and crunching the numbers) or is it just what the trip computer says ?

I use an app to work out my fuel economy, and I check that figure back to what the trip computer says.
In the summer my trip computer was accurate to 1%
Now it's colder, my trip computer is out by 10% (it says 45, actual is 50).

Maybe this has something to do with it?

And diesel fuel economy does suffer in cold weather, as the engine takes so long to warm up

You say you can actually see the fuel gauge going down. Perhaps is was also doing this before, but you didn't notice it?

#10 blue flash

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Posted 30 November 2012 - 05:41 PM

i must admit pitmonster i'm going what the trip says,havent done the full tank way yet, no the gauge has always been steady when driving,it makes me wonder weather the gauge is faulty.it's only gone wonky since it's had a service.i've put europarts own engine oil in the car,now i'm no fitter but have been told to change the oil to a better make??. thanks for the advice pitmonster.

#11 alkyz89

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Posted 30 November 2012 - 05:51 PM

try re-setting the ECU? unplug the battery for 30 mins then plug it back in. keep your radio code to hand! thats not good mpg though! im getting more than that from my 1.8 petrol!!!


are you sure your not driving a diesel?? lol you get more than 40mpg in a 1.8 petrol?? i struggle to get 26mpg haha

#12 StanH

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Posted 30 November 2012 - 08:11 PM

Such a large drop would posssibly indicate a blocked air filter, so since it is a new filter is there any chance of a partial blockage somewhere before or after?

#13 blue flash

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Posted 01 December 2012 - 09:50 AM

good shout StanH,will have a look later.

#14 blue flash

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Posted 01 December 2012 - 05:37 PM

well have had a good look at the car today,and i'm still nowhere nearer than when i started.so tomorrow going for a drive as pitmonster suggested then after that i dont know..................................

#15 Stoney871

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Posted 01 December 2012 - 05:55 PM

In cold weather the car will pump in more fuel to compensate for the denser colder air.
In the summer I can get 500 miles to a tank but in winter it drops by about 40-50 miles.

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