Low MPG

[DarkSideJohn]

Hey guys and girls
Just a quick question , noticed lately that my TDI is starting to give abnormally low MPG , and was wondering if anyone had any ideas , obvious ones or otherwise
My car is a totally standard 2.0 GT TDI 140 , just crept over the 90k miles and has had a major service by VW recently , so im a little puzzled as to why im only getting around the 27-30 mpg in the town and barely 38 ish out on the dual carriage ways and motor ways.

My last golf , a mk4 1.9 tdi achieved around 35-40 in the town and pushing 50 on the same dual carriageways and motorways.

So if people could list a few things to check that would be great  , like i say , obvious and less so obvious suggestions are welcome

thanks :)
John


p.s ive not had  my heavy foot on the loud pedal lately either before anyone asks !

[apples12]

if you've got a vagcom check your engine temperature sensor...mine was faulty...
easy way to check for this without vagcom would be to take the car for a run to warm it up (dont trust the temp gauge in the instrument cluster) and when you stop, roll down the window and cut the engine - you SHOULD hear the fan's running and then slow down...

when i bought my golf i used to hear the fans slowing down as i used to turn my car off/get out of my car then all of a sudden it stopped - never thought anything of it... MPG dropped drastically.... some troubleshooting later found that my temp sensor had failed - its easy enough to change out yourself and i'd recommend buying the sensor from Andrew Page - its about £7 for a Febi sensor (eurocarparts charge 20quid, VW around 40 i think)

after i changed the sensor my 'fan' behaviour returned to normal as did my MPG's!

(theres my thread over on tdiclub http://forums.tdiclub.com/showthread.php?t=344094)

i also changed out my thermostat which was knackered ad did a complete coolant change

good luck!

[dmac1969]

Theres deffo something wrong , I'm getting better MPG than that out of my GTI!

[apples12]

just read that you did a thermostat and coolant change - which thermostat did you replace with?...

i did the following and noticed improvements of 8/9mpgs roughly

1) oil change
2) air filter change - remove the air filter housing from the car and give that a good clean too - after 90k it may well be quite dusty in there, i know mine was!
3) fuel filter change - but rather than JUST a fuel filter change, actually remove the entire fuel filter canister out of the car, empty its contents into a bottle/container, dry out the inside of the housing using a lintfree cloth, make sure the inside is spick and span!, swill some diesel back in there and then dry it out again
4) put a NEW fuel filter into the housing after replacing it in the car - fill the filter housing halfway with forte diesel treatment
5) empty the rest of the forte diesel treatment into your fuel tank
6) take the car for a good drive/italian tune!

i'd also highly suspect your temperature sensor is faulty if your thermostat was knackered - it would certainly help explain the low mpg's....apparently VERY common..

[GrayMK5GTI]

Also check the following:

-Heated rear screen / mirrors aren't on
-A/C isn't on
- handbrake sticking etc

[DarkSideJohn]

Hey guys , thanks for your replies so far , been checking them while in work
as a result of your suggestions i tried a few things on the way home this evening after my car had been stood in the works carpark for the previous 8 or so hours

Timed how long it took the gauge to get to its usual 90 Degrees C point = 6 Mins
This equated to a distance being travelled of exactly 3 miles , which was a mixture of small roundabouts , and some 40/50 roads
The rest of my journey was then another 8 ish miles and about 25 mins which comprised of a lot of town driving so a fair few stop starts being rush hour and all.

Once home , i pulled into the driveway , before killing the engine i hopped out and got as close to the front end as then cut the engine as per Apples12 suggestion , could not hear any signs of a fan spinning , be that slower or otherwise.

During the entire journey my average mpg peaked at a stunning 31.4 MPG

Sorry if that all sounds a bit anal and all , you know measuring times and such but i do not have access to a vagcom and when it comes to fault finding i like to be a bit meticulous and go through things step by step

hope this helps someone :)

[apples12]

ignore the temperature gauge in your instrument cluster - i've learnt it doesnt mean squat!lol

if your engine bay fans arent spinning that means that your engine thinks that the coolant is still cool (doesnt need cooling so doesnt bother turning the fans on) - this wouldnt be true if you've just driven some 20+mins including 40mph+!

so whats happening is...the temperature sensor is quite likely faulty, sending a low temperature signal back to the ECU, the engine thinks the car is still 'cold', so injects fuel as it would if the engine was 'cold' - a LOT more if it was warm.... also doesnt turn the fans on...

change the temperature sensor - its just below the tandem fuel pump - you have to remove the airbox...
good luck!

 

[flipmyflipper]

Even if I boot the hell out of my 2005 GT TDi which is once in a blue moon I'll get more that 31mpg out of it. I average 53mpg mixed driving a 60+mpg on a run and it's also been remapped to 175-180bhp! As already mentioned get it a VW specialist asap and get it on the vagcom