C200 CDI 2003 reg Poor Fuel Economy After MOT

[vsmuthu]

Hi
My MB C200 CDI 2003 reg has started giving Poor Fuel Economy after MOT along the same route I used to be proud of getting 55 MPG constantly without any fuel additives. Now the economy has dropped to 43-47 MPG. Incidently the temperature has dropped to sub-zero (0 to -6). I am not sure whether it is because of the cold temperature or because the car was thrashed during the MOT smoke test which blew some part Kindly advise. BTW the garage used Fuel CleanXXX before MOT to reduce black smoke during the test.

 

[walsi]

Hi ya, rest easy, diesels do less mpg in cold weather when compared to warmer conditions. This is partly due to them not running as hot in cold weather but there are other things that are effected with them aswell

Cheers Martin

 

[wemorgan]

How long is the journey that you were getting 55mpg?
Is it city or motorway driving?

My Audi A2 seems to change very little, since I do mostly long motorway journeys, so always gets to fully operating temperature. But my wife's Smart car goes from 55mpg to 45mpg in the winter, since she does short town journeys and it seldom gets to full temperature.

 

[mykulmerc]

There was a thread on this not long ago - we all agree that MPG drop off a lot in cold weather for a number of reasons. How long was your jouney? My C220CDi is taking at least 8 miles to get up to temp at the moment - so a short journey is a diesel will be particularly bad.

 

[vsmuthu]

Hi thanks for your replies, I do 40 miles of which it takes 15 mins to cover initial 3 miles (sat at the traffic lights) and 30 miles on Motorways and rest on A roads. My trip computer was showing 53-57 MPG constantly until Mid-December and then I went on Christmas break and MOT was done during this break and I have started doing the journey on 04th Jan and noticed this difference in MPG. Is there a need to check anything ? or take it to a specialist for inspection?

 

[Stratman]

It's the cold weather. My usual 50-55 mpg has dropped to 45 ish. Juat like it did last winter.

 

[HumberMart]

What I find strange is that (being a former motor boater) it is generally accepted that diesel boats go faster when it's cold, due to the inlet air being more dense and creating better combustion. This should also translate to more economy at equal speed.

Of course the warm up period would be negligible in a boat trip, but why doesn't the same logic apply to cars? Also (certainly with petrol engines) better cooling means more power.

 

[lynall]

My mpg drops at least 10% this time of year.



Lynall

 

[paul_f]

Diesel cars don't get warm enough to heat the cabin up to a reasonable temperature, so within the cooling circuit heater boosters are added (like glowplugs). These use a significant amount of current so the alternator load is increased, therefore MPG goes down.
This added to the fact it takes the engine longer to get to temperature are the main reasons

 

[yachtman]

What I find strange is that (being a former motor boater) it is generally accepted that diesel boats go faster when it's cold, due to the inlet air being more dense and creating better combustion. This should also translate to more economy at equal speed.

Of course the warm up period would be negligible in a boat trip, but why doesn't the same logic apply to cars? Also (certainly with petrol engines) better cooling means more power.

but of course the air the car travels through is denser a well,so no overall eco gain but a loss, where as the boat benifits from the denser air, , but the main resistance , the water , does not change, so a benefit.