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Discussion Starter · #1 ·
Hi guys over the last 6 months I have noticed that every morning I start the car I drive about a mile down the road
And with in that mile my fuel left display drops by 10 to 15 miles
Is this normal ?
I drive a 2.0 tfsi 2008 petrol
 

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Hi

I have a 2007 tfsi and not noticed this . However I dont normally keep a very close eye on it .

I will try and make a note next time I'm out. Its only really an estimate of range so don't worry about it unless it's causing any issues

Regard

Sent from my SM-A526B using Tapatalk
 

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The dash fuel and mpg Average displays work in a strange way. I've tracked the average mpg many times on long journeys. When you start, the average sets very low mpg which you might expect. Then after 10 miles or so at constant speed mpg starts going up - and keeps going up slowly over quite a few miles. If you reset to zero whilst maintaining a constant speed it comes back to start at a reasonable average. But when you get to down gradients when not much fuel is being used, it is extremely slow to change but very fast going down in mpg when you go up hills. At the end of a 50+ mile cruise the Average end results seems to be about right. The instantaneous mpg seems to respond as I would expect.

I think your answer depends on how the V.W average (over time) is worked out and the further and longer the driving period, the more accurate it will be? I only look at the fuel needle and once it gets close to 1/4 I gas up to the neck. But with a Tdi that's lots of miles. :)
 

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I'd expect this sort of thing to happen with range, because it's calculated based on recent driving (the last few hours, maybe?)

Adding another cold start, combined with the low MPG you get when setting off, into the history it's using to calculate range could cause a drop like that. I often see a drop of 10km or so when setting off, but with a light right foot I usually get those 10km back again :)
 

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Yes I was going to add that. The fuel computer gets the present tank capacity from the fuel gauge sensor. It also gets fuel consumed per second, and distance covered. All these are 'actuals' although the fuel gauge sensor isn't highly accurate. The value of 'range remaining' is a calculation based on those measurements of engine fuel consumption at the time. It is a changing value that usually goes down, but can also go up.

Here's a hypothetical example: The fuel tank has 10 gallons and the car is started from the same cold temperature and run each time for 1 mile. The fuel computer will recalculate the range remaining from a previous value e.g starting at 500 miles. On that 1 mile journey it calculates your range left as 490 miles. You repeat the same cold start 1 mile journey each day and range drops 10 miles per journey. The tank is empty when you have driven 50 actual miles, used 10 gallons and averaged 5 miles per gallon!! This tells you that your cold engine is very inefficient for each first mile.

Change the driving parameters and assume the engine is fully hot and efficient after the first mile (never!). You drive the first mile and the range drops by 10 miles, but in the next 10 miles at freeway speeds the engine runs with less fuel and is more efficient. That first 10 miles drop changes to say 12 miles and 488 miles range remaining for BOTH segments of the journey. You have now covered 20 miles and your average mpg has gone up. Therefore range is only a useful value for constant driving e.g long journey cruising. I was never good at sums, but you'll get the idea.

However, range remaining does give some clues about the efficiency of your engine according to your driving conditions e.g cold weather starting, local stop start driving. My example shows why comparing your gas consumption with others doesn't work unless both are driving the same conditions. Even then, the best result for comparison should be actual odo miles driven full tank to full tank.

Mr Cadbury isn't missing fuel, he is actually using it for the type of journey he is driving, getting low mpg and should have bought an electric vehicle. :)
 

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Discussion Starter · #6 ·
Right iam convinced something isn't right
I will be changing pcv valve and diverter valve
As I have read this could be the case of the missing fuel
As for the electric car the Tesla model x is a bit out of my price range 😊
 

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If you fill up the gas tank, there's no leakage and fuel is used, it's not 'missing', but just got used up (inefficiently?) or some perp. has been siphoning the tank? The real time mpg display is more real than the other readouts because it's based on injector stroke timing and the odo miles. It's fairly instantaneous reading and changes quickly, but with practice you can do an eyeball average on fluctuating readings.
 
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