1.2 TDI - computer says consumption is higher than it really is

Correct, but in that case the DIS figure would always be less than the actual figure, not vice versa.

If variables in fuel Volume calculation are: Time, temperature and Pressure. Then if they stay open longer, the perceived volume would be higher even if in reality some orifices might be blocked and actual flow is lower. I'd dismiss temperature since it probably cause many other problems, unless fuel temp is really off. Hmm... since fuel temp is separate sensor if i'm not mistaken, worth to check in VCDS if it's anywhere near normal. Pressure i'd dismiss too.
That's why i'd check what corrections ECU is placing on injectors, if they are very off, then they are probably the culprit.

On my A8 injectors are probably pissing into cylinders, computer tries to compensate by lowering open times as i've seen in VCDS. And it shows fuel usage less than actual. Messed with my long trip fueling plans, distance to empty was always off. After correcting fuel usage i have no problems (except fuel usage itself :D).
So if times would be longer, it would show more usage as in this case A2 is showing.

There is another variable that can be off: Distance.
But that can be verified by simply driving a little, and if it would be nearly third off from actual, i'd suspect it would have been noticed already.

Of course i'm no car specialist and might be missing some other obvious thing, and i do not know how it calculates it, just guessing....
 
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Hmm... since fuel temp is separate sensor if i'm not mistaken, worth to check in VCDS if it's anywhere near normal. Pressure i'd dismiss too.

I don't have VCDS, but I have ODBeleven. Is there list of channels and values somewhere? I've seen some temperatures there, but I have no idea which is which.

Tested odometer and it's accurate.
 
I have problems locating that data in elsa atm, but googling finds in german forum:

Block 007: Temperaturgeber
72.0°C Kraftstoff- temperatur (G81)
0.0 % Öltemperatur (G8)
45.0°C Ansaugluft- temperatur (G42)
95.4°C Kühlmittel- temperatur (G62)

so my guess is in ecu block 007 value 1 (G81).

Might slip into car to check, but it's raining...
 
Check the G2 and G62 readouts. They are coolant sensors and are located in the same housing. I am not sure how they are used but if they show different values (they shouldn't), it might lead to a miscalculated consumption figure.

RAB
 
I watched G81 today while driving to work and it looked sane. It started from ambient and got about 20 degrees under coolant tempererature.

btw. coolant only got to 71°C. Is it too cold? Ambient temp was +10°C here today.
 
Hm. 71 is not much. For +10 I would expect 90C; if you're driving through Tampere on the E12 at 80km/h, I would really expect to see 85C and higher. The dash showed you a steady 90C?

- Bret
 
Currently my coolant/engine temps will get around 70 too with this weather, but mainly because trip consist of cruising at 50 and 70km/h without much need to stop/accelerate. Fuel got to 50 something by that time.
At 90km/h it will go to 90-95C at least up to zero ambient.

But those temps should not affect fuel metering. Only fuel usage itself.

Maybe ECU is still on summer time (faster), but it's winter time (slower)... So it'll count seconds faster than they really happen and voila: thinks that it holds injectors open longer than they really are!!! :eek:
(Not a joke)
After reading this very funny and original joke: is trip usage calculated within ECU or is it done in Instrument cluster?
 
The G62 supplies the coolant temperature to the ECU and the G2 supplies the same to the instrument panel. The ECU and instruments are probably making the same calculation, one for the actual fuel injected and one for the calculated value - that's why they usually track one another. However, a well known failure is the G62 and G2 giving different values - they should both be the same because they are located in the same housing. The original black sensors were notorious for such a failing. As far as I am aware, the G2/G62 is the only dual sensor in the system so is the most likely source of a difference between actual and calculated consumption. So check and compare the G2 and G62 outputs. You may also need a new thermostat for better fuel economy.

Also be aware that the temperature gauge has a large tolerance, i.e. it will show 90deg when the actual temperature is in the mid-seventies. In other words don't rely on it - it's a comfort gauge!

RAB
 
I agree with this, and actually I just consulted one of my collegues who is working with diesel engine development and he gave me the same information.
Better late than never! The OP was complaining about calculated fuel consumption, not actual! That's because the G2 was under-reading the coolant temperature and therefore miscalculating the calculated fuel consumption.

RAB
 
Better late than never! The OP was complaining about calculated fuel consumption, not actual! That's because the G2 was under-reading the coolant temperature and therefore miscalculating the calculated fuel consumption.

RAB
RAB, was this a mis-quote?
(I am confused as I dont quite see the connection between your comment and my qouted comment.)
Also puzzled by your usage of exclamation marks, normally in written text this implies strong feelings.
If I in some way have contributed to make you feel uncomfortable it was unintended and regretable.
 
RAB, was this a mis-quote?
(I am confused as I dont quite see the connection between your comment and my qouted comment.)
Also puzzled by your usage of exclamation marks, normally in written text this implies strong feelings.
If I in some way have contributed to make you feel uncomfortable it was unintended and regretable.
The OP was complaining about the calculated consumption, not the actual. Therefore the state of the injectors was not relevant. Even if there had been a problem with injectors, i.e. a requirement to be cleaned, the system would attempt to compensate which wouldn't necessarily lead to a reduction in consumption. Quite possibly the opposite.

I'm not feeling uncomfortable, just trying to clear up an old thread.

RAB
 
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