Poor economy Passat

selfbuilder1

I currently have a 2004 1.9TDI 130bhp passat. When I first got it I used to get between 47-50mpg. Recently I am only getting 40-44mpg. The car is fully serviced and only passed the NCT last night. I haven't changed my driving style and no matter how easy I drive it I cannot get the mpg to improve. There is no loss in power either. Can anyone suggest what the problem could be?

EPM

The colder weather and wet roads affect my economy noticeable every year.

166man

Could dodgy fuel be the problem? Could that reduce mpg?

deadlast

Tyre pressure will affect mpg...

MidlandsM

In the summer, engines run "hotter" and are slightly more fuel efficent alright.

paulmclaughlin

If you keep the tank between half and a quarter full you get better mpg.

George Dalton

Are you going by the on board computer or are you measuring it yourself?

MugMugs

Check your Tyre's.

Also remember you're running in the dark more. Lights are on more and as it's colder (kind of) you're going to be running your heater and A/C more. (I am anyway)

Any heavy stuff in the boot ?

Also try run some Dipetane through her.

nogoodnamesleft

What items were changed during the service?

I find that when I replace the airfilter (every 20k) and fuel filter (40k) I get better mpg.

-Corkie-

paulmclaughlin wrote: »

If you keep the tank between half and a quarter full you get better mpg.

How did you come to that conclusion????

paulmclaughlin

-Corkie- wrote: »

How did you come to that conclusion????

Less strain on the fuel pump. Try it, it actually works.

chinwag

paulmclaughlin wrote: »

Less strain on the fuel pump. Try it, it actually works.

Would that be for diesel only or petrol as well?

paulmclaughlin

Not sure about diesel but it works in my petrol anyway.

THe fuel pump is oblivious to how much fuel is in the tank, unless its empty.

MidlandsM

paulmclaughlin wrote: »

Less strain on the fuel pump. Try it, it actually works.

total bollox:rolleyes:

-Corkie-

MidlandsM wrote: »

total bollox:rolleyes:

I was going to say bar stool talk but your post sums it up a lot better...

unkel

The logic is wrong, but the conclusion was right

The less fuel you have in the tank, the lighter the car is and the less fuel you consume...

Also in wintertime - dipped beams increase fuel consumption by 3-4%, that alone would bring the consumption from 47 to nearly 45MPG

-Corkie-

unkel wrote: »

The logic is wrong, but the conclusion was right

The less fuel you have in the tank, the lighter the car is and the less fuel you consume...

Also in wintertime - dipped beams increase fuel consumption by 3-4%, that alone would bring the consumption from 47 to nearly 45MPG

Yes and rear window defogers and fans on would also affect MPG..

MugMugs

paulmclaughlin wrote: »

-Corkie- wrote: »

How did you come to that conclusion????

Less strain on the fuel pump. Try it, it actually works.

If you go uphill all the way you've got more fuel too! Crazy!

MidlandsM

unkel wrote: »

The logic is wrong, but the conclusion was right

The less fuel you have in the tank, the lighter the car is and the less fuel you consume...

Also in wintertime - dipped beams increase fuel consumption by 3-4%, that alone would bring the consumption from 47 to nearly 45MPG

sure, but my "total bollox" expression of ghast was in responce to the assertion that leaving the tank qtr to half creates full Less strain on the fuel pump, thus saving fuel. Which is rubbish, or errrrmmmm, bollox:p

re; The less fuel you have in the tank, the lighter the car is and the less fuel you consume... .......i've tried to argue this to my partner when i come back with her car and the fuel light on......she does'nt get it at all, at all.......:D

darragh o meara

I'm having a similar problem. Was getting 50-55 mpg but now only getting 45 ish. But as said earlier, all them electrics working at the one time that usually would be turned off in better weather cause a big drain on the power output of the engine. My car for example at any one time this week had: dip beams on, heated seats on, radio on and the heater on to keep the windscreen from fogging up. Most of which if dry would be off.

selfbuilder1

I have done a bit of Googleing and have come up with the following possible causes;
faulty MAF
faulty EGR valve
faulty temp sensor.

Which of these is the most probably cause of poor MPG? What is the easiest way to diagnose these faults? Can they be diagnosed without OBD?

Thanks for any help.

Tea drinker

Temp sensors a known weakpoint on passats....

darragh o meara

selfbuilder1 wrote: »

I have done a bit of Googleing and have come up with the following possible causes;
faulty MAF
faulty EGR valve
faulty temp sensor.

Which of these is the most probably cause of poor MPG? What is the easiest way to diagnose these faults? Can they be diagnosed without OBD?

Thanks for any help.

Faulty maf : disconnect while the engine is running. If the car cuts out you can cross that one off. A car with a faulty maf will keep running without the maf but will be down on power and run like s. hit. I'm pretty sure it doesn't bring on a light.

Faulty egr : car will be smokey if faulty. Would be worth cleaning anyway. They can be removed without much hassle. I know when I disconnected mine it brought on the engine light.

Faulty coolant temp sensor: this controls what ecu map the car uses when starting. A faulty sensor will cause the car to be very hard to start. A cold engine needs a more aggressive mapping to start it than a hot engine. These are a common failure point on vw's.

Diagnostics depending on the system should be able to give live readings from each sensor so spotting a dodgy one should be easy if you know what your doing.

shedweller

I would have thought a faulty MAF etc would throw a code? In any case, the cold affects mpg by lowering tyre pressure (boyles law i think) , increasing aero drag due to denser air, longer warm up times, more accessories on. While nothing much can be done about it, it's not really much of a financial hit, in the bigger scheme of things. Dipethane helps but not much. I've tested it.

ShadowHearth

That is very small decrease and for such cold weather it's very reasonable.

Back in Lithuania when you sell car, you ask how much petrol car uses on winter and summer cycles.

It's just simple logic:

1. You need more time to warm up car and so it would be in proper working temperature.
2. Driving in bad conditions can effect your mpg as you will be driving lower speeds and stuck in traffic. ( in short: not on optimal speed for fuel consumption)
3.You will generally use more equipment in your car, which can effect that too ( not much, before you jump on me )

Henry Ford III

unkel wrote: »

...Also in wintertime - dipped beams increase fuel consumption by 3-4%, that alone would bring the consumption from 47 to nearly 45MPG

This old nugget confuses me unkel.

I'm not entirely sure it makes a difference what electrical load a car has, once the engine is running with the battery fully charged. Doesn't the alternator's gubbins spin regardless, and the "work" the engine has to do to keep it turning remain the same whether charging the battery or otherwise?

Maybe I'm all wrong here?

At idle the revs rise a tad when lights or air con etc are turned on so presumably a little more fuel is used, now cruising along I don't see there being any difference though as wind resistance etc etc will make any electrical draw insignificant.

Henry Ford III

RoverJames wrote: »

At idle the revs rise a tad when lights or air con etc are turned on so presumably a little more fuel is used, now cruising along I don't see there being any difference though as wind resistance etc etc will make any electrical draw insignificant.

Or heated front/rear screens too. Yep.

A/c is a bit different though I reckon. Turning that on means the compressor has to start work mechanically. That will hurt fuel consumption a bit.

True about the air con, a different load than a purely electrical one

pajo1981

paulmclaughlin wrote: »

Less strain on the fuel pump. Try it, it actually works.

If anything the fuel pump will have an easier time with a full tank.

lau1247

anyone considered possibly rusted exhaust??

they cause issue too and more often that not you don't really know the difference until it get so bad that a hole has opened up and make it hum louder and louder..

of course MPG drop a bit too

Mar4ix

my friend has 08 1.9 .l.diesel passat, it some times didnt start right away when warm(no any warning lights on dash) ... like "on button ", he went to garage, did diagnostic,and it said crankshaft sensor working intermittent, after they changed that sensor, warm start went perfect, and noticeable fuel economy. .... Just on that example, suggest you do diagnostic ... even VAGCOM will show the problems.