B6 Passat - poor fuel economy

Dial Hard

Hi All

I have a 07 B6 Passat 2.0 TDI PD DSG 170bhp, 203,000km on the clock. It's serviced annually and the last service was three weeks ago.

I've recently noticed (since the service, actually, but that could be a coincidence) that I'm not getting anywhere near the mileage out of it that I used to. Last week I put a quarter of a tank in it (about 17 litres) and the fuel light came on after only 79km. On Thursday I put another quarter tank in, and the fuel light went on last night at 100km.

Should I be going back to the garage or is it just the age of the car? Any ideas/suggestions/advice?

Thanks a mill.

Work out the proper Km per L.
Your just guessing with those numbers.

Atlantic Dawn

Cold weather, battery under pressure from lights, heater, rear demister constantly on fuel economy is going to drop considerably.

Dial Hard

Atlantic Dawn wrote:

heater, rear demister constantly on fuel economy is going to drop considerably.

Rear demister is only on as and when it's needed. It cuts out automatically after a few minutes anyway so you couldn't even forget about it on a regular basis. And it's my understanding that when you've the heat on in the car it's coming from the engine heat anyway, it's only artificially cooling the car through aircon/climate control that significantly impacts fuel economy, no?

Cee-Jay-Cee

Your method if calculation is completely unreliable. Fill the tank to the neck, reset the trip meter and drive it till the light comes on and fill it again. That is the only accurate way you can measure fuel economy.

Dial Hard

Cee-Jay-Cee wrote:

Your method if calculation is completely unreliable. Fill the tank to the neck, reset the trip meter and drive it till the light comes on and fill it again. That is the only accurate way you can measure fuel economy.

Well in fairness, I haven't calculated anything yet. Just observing that the same amount of fuel doesn't seem to be going anywhere near as far as it used to (and yes, I am aware that the price of fuel has gone up, which is why I've been looking at the number of litres I've been putting in as opposed to the amount of money).

Unfortunately I don't have the money to do a full fill at a time.

colm_mcm

Car has a trip computer. Reset it and see what you’re getting.

DaveyDave

What kind of driving are you doing? The cold won't help nor will the Christmas shopping traffic if you're hitting any. Best thing to do is fill the tank to the brim, log the mileage then drive until a little bit after the fuel light comes on and log the mileage again. I use an app for these calculations but I'm sure there's plenty of websites for it too.

jca

DaveyDave wrote: »

What kind of driving are you doing? The cold won't help nor will the Christmas shopping traffic if you're hitting any. Best thing to do is fill the tank to the brim, log the mileage then drive until a little bit after the fuel light comes on and log the mileage again. I use an app for these calculations but I'm sure there's plenty of websites for it too.

App for this app for that. Whatever happened to pen and paper and basic arithmetic? It isn't difficult. @op, where are you buying your fuel? I'm driving a petrol car and find huge differences in mpg using different garages, cheapest isn't always best.

DaveyDave

jca wrote: »

App for this app for that. Whatever happened to pen and paper and basic arithmetic? It isn't difficult. @op, where are you buying your fuel? I'm driving a petrol car and find huge differences in mpg using different garages, cheapest isn't always best.

He could use pen and paper if he wanted but most people have a smartphone to hand and not everyone carries a pen and paper with them. I personally don't know the calculations off hand so it's easier for me to use an app. Whatever suits really it's not a big deal...

Dial Hard

colm_mcm wrote:

Car has a trip computer. Reset it and see what you’re getting.

I reset it every time I refuel. As I said, 17 litres got me 100km this week and 79km last week before the fuel light came on. I used to get easily 180km out of that before she'd light up.

I'll stick 20l in tomorrow and see how far I get before the light comes on then do the same next time and work out the exact fuel consumption.

As for the kind of driving I do, Firhouse to Rathcoole and back every day, via the outer ring road and N7. That's pretty much it. Not ideal for a diesel, I know, but my commute was a lot longer when I bought her. This range issue is only in the last few weeks though.

Usually refuel in Topaz or Maxol.

Indricotherium

Cee-Jay-Cee wrote: »

Your method if calculation is completely unreliable. Fill the tank to the neck, reset the trip meter and drive it till the light comes on and fill it again. That is the only accurate way you can measure fuel economy.

It's at least as reliable as what you are proposing.

In fact his way is probably more accurate. Both ways have one bogey measurement.

Yours, when the pump clicks off, his when the light comes on. At least his way it's the same sensor every time.

With random fuel pumps and random "briming" a tank there's 2 variables that are bogey for the fuel measurement.

Putting a known amount of fuel in and waiting for the light is at least as accurate.

jca

Indricotherium wrote: »

It's at least as reliable as what you are proposing.

In fact his way is probably more accurate. Both ways have one bogey measurement.

Yours, when the pump clicks off, his when the light comes on. At least his way it's the same sensor every time.

With random fuel pumps and random "briming" a tank there's 2 variables that are bogey for the fuel measurement.

Putting a known amount of fuel in and waiting for the light is at least as accurate.

We're not looking for mpg figures accurate to the nearest tenth of a mile here. Filling and arithmetic is just as accurate as most on board computer systems on the average car.

vandriver

Indricotherium wrote: »

It's at least as reliable as what you are proposing.

In fact his way is probably more accurate. Both ways have one bogey measurement.

Yours, when the pump clicks off, his when the light comes on. At least his way it's the same sensor every time.

With random fuel pumps and random "briming" a tank there's 2 variables that are bogey for the fuel measurement.

Putting a known amount of fuel in and waiting for the light is at least as accurate.

Which is why the helpful person said fill it to the neck(ie to the brim),not to the first click.
It always helps to actually read what people post,not what you think they mean.

Indricotherium

jca wrote: »

We're not looking for mpg figures accurate to the nearest tenth of a mile here. Filling and arithmetic is just as accurate as most on board computer systems on the average car.

The op is putting a known amount of fuel in and driving until the low fuel light comes on, then using arithmetic.

My point is that this is as, or more accurate than briming a tank because it's giving you a much more accurate variable for the fuel. It certainly isn't "completely unreliable" as was suggested.

I'm assuming that the low fuel light is coming on at the same point every time of course, which it probably isn't.

vandriver

I know in my car,the light comes on at 115 km left.However,if I've been motorway cruising,that could be 7 litres.If I've been stop starting and crawling round town,that could be 12 litres.
As you can see,the 'wait till the light comes on ' method can be horribly unreliable .

Toyotafanboi

vandriver wrote: »

I know in my car,the light comes on at 115 km left.However,if I've been motorway cruising,that could be 7 litres.If I've been stop starting and crawling round town,that could be 12 litres.
As you can see,the 'wait till the light comes on ' method can be horribly unreliable .

Exactly.

You need 2x accurate fixed points to measure from to properly check your economy and the fuel light isn't an accurate point.

The only reading you can be certain of is wheel the tank is 100% full. Drive "x" distance and fill the tank to 100% full again, then you know how much you've used.

Saying you can't afford to fill the tank is silly too. You only have to 'take a hit' to fill the tank once. Other than that you are only replenishing what you used, so if you're only doing €20 worth of driving you're still just putting €20 in. You're just bringing the car from 3/4 to full instead of from the fuel light to 1/4. The only difference is you will get accurate usage figures.

The fuel light does not always come on at exactly the same time, particularly in more modern cars and the fuel gauge itself is only a ballpark reading so you cannot expect that €20 to fuel light will get you the same useage time after time as you are basing it off of one unknown and one moveable feast.

9935452

Indricotherium wrote: »

The op is putting a known amount of fuel in and driving until the low fuel light comes on, then using arithmetic.

My point is that this is as, or more accurate than briming a tank because it's giving you a much more accurate variable for the fuel. It certainly isn't "completely unreliable" as was suggested.

I'm assuming that the low fuel light is coming on at the same point every time of course, which it probably isn't.

Id disagree with that .
Statistically it would be more accurate to fill the tank and empty it and fill again .
Your figures are accross a larger range ie 55 litres over 17 litres giving more accuracy.
I know myself the low level loght can come on at different times .
Go up or down a slope can trigger it early

vectra

Does that model have a dpf?
If so it could be stuck in a Regen?

Nuw

vectra wrote: »

Does that model have a dpf?
If so it could be stuck in a Regen?

That actually is something worth looking at. I don't think it matters so much how is the behaviour noticed, what matters is that under the same (or as close as makes no difference) circumstances, OP's car is not acting the expected way.

Given the info provided, another guess would be to confirm the oil used for the service is the correct grade, or at least the same as the one used last time it was serviced.

Cee-Jay-Cee

Indricotherium wrote: »

The op is putting a known amount of fuel in and driving until the low fuel light comes on, then using arithmetic.

My point is that this is as, or more accurate than briming a tank because it's giving you a much more accurate variable for the fuel. It certainly isn't "completely unreliable" as was suggested.

I'm assuming that the low fuel light is coming on at the same point every time of course, which it probably isn't.

I'm going to assume that you don't actually have a car and never had.

The only accurate way is to fill the tank to the very top, not the first click or second click but till you can physically get no more fuel in. Reset the trip meter and drive till the tank is empty or the light comes on, it doesn't really matter. Fill the tank again and like the last time fill till you can get no more fuel in. Take a note of how much it took to fill it, take a note of the distance you drove since the last fill and you will have a 100% accurate figure.

Putting in x amount of fuel and driving till the light comes on is completely inaccurate and to suggest that it is actually more accurate than the method above really does show your ignorance on the subject. The fuel sensor in many cars can be activated if you were driving up/down a long or steep hill. Fuel also expands in heat so the figure you get in winter will be different to the figure in summer. Your method also relies on being able to fill the instant the fuel light comes on and does not take into account that you may have to drive x KMs to a filling station. And finally fuel gauges and low fuel lights are only indicators and in no way accurate, the hand book for the car will tell you as much and they should not be relied on exclusively.

zilog_jones

Dial Hard wrote: »

Unfortunately I don't have the money to do a full fill at a time.

I don't get this - think about how much of your time and fuel you're wasting by going to a petrol station every week, when you have a car with a tank big enough to only have to go about once a month.

vectra

zilog_jones wrote: »

I don't get this - think about how much of your time and fuel you're wasting by going to a petrol station every week, when you have a car with a tank big enough to only have to go about once a month.

Thats all very fine for some people.
But maybe the OP might not be in a position to fork out for a full tank in one go .

tossy

vectra wrote: »

Thats all very fine for some people.
But maybe the OP might not be in a position to fork out for a full tank in one go .

Exactly.

Also

zilog_jones wrote: »

think about how much of your time and fuel you're wasting by going to a petrol station every week.

Come on now.....

OP for what its worth even by some back of the envelope style calculations your MPG seems way off what it should be.

zilog_jones

vectra wrote: »

Thats all very fine for some people.
But maybe the OP might not be in a position to fork out for a full tank in one go .

Maybe I'm a terrible narrow-minded person, but I just don't understand this at all. If my cashflow was so critical that I couldn't afford €70-80 at the pumps in one go, I would seriously reconsider car ownership. Cars are a damn money pit, and you can face huge bills any day with various mechanical failures, accident damage, etc. Not to mention the insane cost of tax and insurance in this country. How can someone afford any of that if they can't afford to fill the tank?

I'm not a wealthy man either - there is no bank in the country that would give me a mortgage, for example...

vectra

zilog_jones wrote: »

Maybe I'm a terrible narrow-minded person, but I just don't understand this at all. If my cashflow was so critical that I couldn't afford €70-80 at the pumps in one go, I would seriously reconsider car ownership. Cars are a damn money pit, and you can face huge bills any day with various mechanical failures, accident damage, etc. Not to mention the insane cost of tax and insurance in this country. How can someone afford any of that if they can't afford to fill the tank?

I'm not a wealthy man either - there is no bank in the country that would give me a mortgage, for example...

Ans possibly not.
Maybe it just might not have dawned on you at the time of comment.
And if the OP is that Badly off. Maybe...Just maybe He/she enjoys having something out of working hard for it... OH..There is a thought... Maybe the OP needs it to go to work?
Who knows and who cares?
it is not part of the initial issue.

Dial Hard

vectra wrote:

Thats all very fine for some people. But maybe the OP might not be in a position to fork out for a full tank in one go .

This in a nutshell. I moved from Jobseekers to a minimum wage job last week and won't get paid til the end of the month so I simply just don't have the 90-odd quid it would take for me to fill the tank, sorry.

9935452

Op who did the service on your car ?
Is it possible they left off the engine cover .
A friend is complaining the cover fell off his car under the engine and the car is suffering from poor fuel economy since