Decreased MPG, any suggestions?

This is not another thread about MPG, but instead I am trying to work out why my MPG has changed.
I usually average 26/27mpg out of the my disco 200tdi 110. I track my mpg through fuelly. Since the beginning of September my mpg is down around 23mpg. I have not changed the way that I drive or the fuel that I use (various supermarket brands) so I am assuming something has changed in the vehicle. Every so often when exiting the vehicle after a journey there is a smell of fuel, but so far I have been unable to find any leaks. Usual suspects of the leak-off pipes, and filter pipes etc. are all fine and there has been no change in performance or starting.

If my maths is correct (no guraentee) in 300miles (the point at which I fill up) the "extra" fuel usage is 6.8l, I would have though if this was leaking out it would have been fairly obvious to spot. Previously when I have had things leaking other than the tell tale patch on the ground as a very minimum it gets caught in the vortex and eventually coats the back door.

So with that information does anyone have any suggestions of things to check?
I will give it a more careful and thorough inspection of all the fuel lines again but so far have not found anything obvious.

Air filter and inlet piping ok ?

tottot said:

Air filter and inlet piping ok ?

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Was serviced a couple of weeks ago, so the filter is new and did not make a difference. I will check the inlet piping, but as there is no additional noise or performance change I had discounted a split/collapsing pipe, I have had that before.

might only leak when high rpm when pressures higher.

Sounds like a fuel leak to me. There should not be even the slightest smell of fuel. Get a really bright torch and look at all the joints in the lines - particularly around the leak off pipework. I'd also look from underneath - around the FiP, and check the timing case breather/drain, in case there's a leak from the front of the FiP (unlikely)

UV dye in fuel, Demon Tweeks does it. Makes it easy to find, and works best the more concentrated it is so pour dye when tank is low on fuel.

Besides Fuelly uses US Gallon, so not a true mpg figure as we know it here in the UK. Just sayin

zeaphod said:

Sounds like a fuel leak to me. There should not be even the slightest smell of fuel. Get a really bright torch and look at all the joints in the lines - particularly around the leak off pipework. I'd also look from underneath - around the FiP, and check the timing case breather/drain, in case there's a leak from the front of the FiP (unlikely)

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I have to say a fuel leak was my first thought I am just struggling to track it down! I may replace all the leakoff pipework as a matter of course as I keep a length of that in the garage due to the regularity with which it needs to be replaced anyway. I will have a closer look at the other places you have suggested, annoyingly the smell of fuel is not consistent and not always apparent making it more difficult to trace

I had a similar small leak on the old NA, I found using clean blue roll or similar worked well - even a small amount of fuel will show up on the paper.

Have you checked the fuel tank pipes, fill and breather and if the fuel gauge sender and pickup tubes are sealed ?

windy81 said:

UV dye in fuel, Demon Tweeks does it. Makes it easy to find, and works best the more concentrated it is so pour dye when tank is low on fuel.

Besides Fuelly uses US Gallon, so not a true mpg figure as we know it here in the UK. Just sayin

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You can set it to UK units so you input litres, miles and it displays in UK MPG. I have tracked nearly 135k through it, so I trust the change in average to be something to do with the vehicle rather than other factors.

I will have a look into the UV dye if I cannot find a leak the old fashioned way. Other than leak are there any other suggestions for what could be causing it?

some one syphoning your tank???

dag019 said:

You can set it to UK units so you input litres, miles and it displays in UK MPG. I have tracked nearly 135k through it, so I trust the change in average to be something to do with the vehicle rather than other factors.

I will have a look into the UV dye if I cannot find a leak the old fashioned way. Other than leak are there any other suggestions for what could be causing it?

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I just installed it to see and I cannot see any Imperial Gallons used only the US Gallon. Doesn't matter other than the equivalent US mpg will be lower than imperial mpg I'm familiar with.

windy81 said:

I just installed it to see and I cannot see any Imperial Gallons used only the US Gallon. Doesn't matter other than the equivalent US mpg will be lower than imperial mpg I'm familiar with.

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In settings under units you can choose:

aint got that on my install, I might have downloaded a copy

Found it! I think registering did the trick. Anyway I hope you find the leak!

Only ever owned one diesel in my life and the mpg always went down by a reasonable amount during the cooler months

Tyre pressures? can make quite a difference.