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

W

Wheelie

Member
Hi,

Less than year ago I found that on-board computer is telling me higher consumption
than what I get by dividing fuel amount from pump with odometers kilometers.

It was almost always 0.5 l/100km off. Sometimes it changed a little, but 0.5 was usual error.

This month error grew to 0.9 l/100km.

Any ideas what could be causing this? What is going to break next?

The car has been driven for about 290,000km.
 
Hi
I dont know the answer, but have from some time of browsing both A2freunde and lupo3Lclub got the impression that worn injector could be involved in this.
 
Worn injectors would affect both figures surely. What size tyres are you running?

RAB
 
I am not aware of any means of calibrating this, although there might be a way. If you always fill up at the same pump, maybe there is a problem with the pump. This would mean that the DIS figure is the more correct. Also you may have a fuel leak.

RAB
 
Atleast it's not leaking out. Computer said 4.1 l/100km yestarday when I fueled up 9 l after driving 282km.

I guess I'll check odometer on some precalculated distance and then start to think options with injectors if odometer is ok.
 
Why would the cause be injectors? They might need attention but they would affect both figures equally - unless the DIS figure is calculated from what it thinks should be injected. But in the latter case, the DIS value would be the lower of the two.

RAB
 
Have you checked the speedometer against GPS speed? If the former is lower by the ratio of the consumption error, it might indicate a problem with the gearbox. The G22 sensor (next to the gear actuator on top of the gearbox) monitors the output shaft and provides the signal for the speedometer, as well as the tachometer, presumably. It also compares the shaft speed with the engine speed to ensure there is no clutch slip. When that ratio exceeds a certain value, the system gives up and dumps you by the roadside! The loss in reading rate is caused by excessive clearance between the G22 and the output shaft, due to bearing wear. There was an issue with excessive overload on early DSK gearboxes; our silver Lupo 3L has already had its' gearbox replaced for this reason - it luckily failed in our road! I'm not aware of a similar problem with later FLE gearboxes (which is probably what you have). I hope I'm wrong!

RAB
 
My car seems to report too slightly higher than pump/odo calculation, but it is only a tiny bit. Odometer seems to be 1-2% off in my case.
But 0,9l/100km seems like pretty large calculation error on 1.2. I'd suspect injectors too. Probably calculates by injector open time, and if injector does not let fuel through as much as should, calculation would be off.
My A8 was overly optimistic on other hand, and after calculating the error value i corrected it in VCDS to nearest available correction... Was 109% of reported figure and had to correct to 110%, since it had 5% increments. Have not investigated if A2 has correction values somewhere.

Speedo and GPS speed are different anyway. If i set cruise at 90, speedo shows almost 100 while colorMFA shows cruise set at nearly GPS speed. So the car knows real speed, just lies on speedo. If speedo would show much less, i'd suspect uniformed blue men would have informed about that politely...
 
Most car speedos over read slightly, I was thinking about this the other day. This could also give the added benefit, from the manufacturers perspective, of indicating slightly better fuel consumption than if calculated 100% accurately.
 
If the injectors are worn, they let more fuel through, not less. Thus the DIS would be less than actual, not more.

In the UK, the speedo should never show less than the actual speed and never more than 110%. I suspect that it's the same for Europe. The affect of the G22 would be additional, so would be noticeable.

RAB
 
Do injectors only wear (increase in orifice size, therefore, fuel volume higher than ECU thinks), or do they accumulate a deposit (like limescale) that reduces the orifice, and hence fuel volume?
Fuel lines and filters definitely do accumulate residue, so maybe injectors do too.
We talk about getting injectors cleaned, not about getting them bushed back to compensate for wear.
Just a thought ...
Mac.
 
PlasticMac said:
Do injectors only wear (increase in orifice size, therefore, fuel volume higher than ECU thinks), or do they accumulate a deposit (like limescale) that reduces the orifice, and hence fuel volume?
Fuel lines and filters definitely do accumulate residue, so maybe injectors do too.
We talk about getting injectors cleaned, not about getting them bushed back to compensate for wear.
Just a thought ...
Mac.
Click to expand...
So are you suggesting that injector deposits improve fuel efficiency? I don't think so!

RAB
 
If you have VCDS i'd check ECU values at idle with warm engine, there probably is somewhere something about injection times and/or injector timing corrections. But i'm not sure if it even helps somehow or what to think about those numbers.

RAB said:
injector deposits improve fuel efficiency?
Click to expand...
Fuel efficiency has not been changed, it is still "3L", but car somehow reports "4L". If injectors would be clogged, then that could happen if it only looks at spray times to calculate it.
 
If you do a little research you'll find that neither worn injectors or deposits on injectors improve fuel efficiency. Actual consumption is more or less what we would expect. Therefore we can dismiss injectors as the cause, assuming that the distance travelled is correct.

RAB
 
RAB said:
If you do a little research you'll find that neither worn injectors or deposits on injectors improve fuel efficiency. Actual consumption is more or less what we would expect. Therefore we can dismiss injectors as the cause, assuming that the distance travelled is correct.

RAB
Click to expand...
Did you consider @maxufeiss last section? Its more about the engine ECU perception of fuel efficiency than the true one?
 
maxufeiss said:
If injectors would be clogged, then that could happen if it only looks at spray times to calculate it.
Click to expand...
But the actual fuel consumption suggests that the injectors are not clogged (or worn).

RAB
 
RAB said:
So are you suggesting that injector deposits improve fuel efficiency? I don't think so!

RAB
Click to expand...

Any change in the size of the injector orifice will change the volume of fuel that flows through it (in a fixed time).
If metering uses time and pressure to calculate volume, based on assumed orifice size, that measurement will be incorrect if the orifice is not of the size assumed in the calculation.
Mac.
 
PlasticMac said:
Any change in the size of the injector orifice will change the volume of fuel that flows through it (in a fixed time).
If metering uses time and pressure to calculate volume, based on assumed orifice size, that measurement will be incorrect if the orifice is not of the size assumed in the calculation.
Mac.
Click to expand...
Correct, but in that case the DIS figure would always be less than the actual figure, not vice versa.

RAB
 
PlasticMac said:
Any change in the size of the injector orifice will change the volume of fuel that flows through it (in a fixed time).
If metering uses time and pressure to calculate volume, based on assumed orifice size, that measurement will be incorrect if the orifice is not of the size assumed in the calculation.
Mac.
Click to expand...
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.
 
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