Mis leading mpg from trip computer

I've just experienced a diesel pump going bad as far as its seals were concerned, in warm weather since May. Filled the tank (the 34 litre one). With a non-perfect car, doing similar commute to yours I get 53 mpg average on fuelly, would get more on long runs but I average 70mph on these (got 68mpg on a 50mph drive to visit friends 60 miles away last year). With the project car, drove a couple of commutes and town trips - total distance elapsed 85 miles or thereabouts. Realised after the last journey and then a short run using the car to start another one from the battery that I was down to <50% and then <30% tank fuel remaining and there was a big pool of diesel on the driveway, then checked it all out and found the big diesel slick down the engine and at least two coolant pipes that were dissolving like a jellybaby in saliva where they were heavily contaminated with liquid diesel in that exact area. New pump/gasket and probably pipes required to make this good completely- did the pump / gasket but still not convinced I've fitted it properly.

70-80mpg indicated to just over 50+ mpg actual doesn't to me suggest a bad fuel leak like the above, rather an electrical calibration issue. If you drive like a saint chauffering Miss Daisy you might get 60-65mpg (the real hypermilers have both a well-serviced car and tyres that are optimised as well as a really trained driving style). 50+ mpg in a 1.4tdi with indeterminate maintenance isn't terrible - a good service with filters all round and injector cleaner might sort some of this out. What are your tyres? These can give 5-8mpg difference, let alone with pressurisation optimised for efficiency.
 
It *is* possible to recalibrate the DIS (more research is behind that statement) though how it's done I do not know.

If you're going to do pump-to-pump: use the same pump every time and fill to the first click only for accuracy purposes.
I'd also consider calculating in l/100km because you can see discrepancies in terms of ratios, rather than a 1-2 mpg difference which tells you absolutely nothing about actual consumption because it's a logarithmic scale.
If your outside temperatures change, the consumption should change too; I'd expect a 0,3l/100 increase with rain / lights / wipers.

This is a listing of 66kW A2s in Germany and their listed consumptions (I expect it's a rather bigger dataset than fuelly, though I haven't looked at that directly).
Cheap tyres will not help. Nor will 17"s. But they do not explain the discrepancy. My DIS reports correctly winter / summer wiht the tyre change what accurately goes through the system.

- Bret
 
16" SE alloys are dirt cheap here and apart from pepperpots pretty light unless you want to start chucking serious money at OZ rims or similar to go the next kilogram step lighter each ; I got a set of 16" 185/50/16 Aoteli 607s for my partner's car for <£30 each as 4 similar budget tyres was better than the 4 mixed wear / brand budget tyres that were on it, and the fuel economy improved 6mpg per tank (around 45 miles (20%) extra in her 1.4i) in that change. Like many I was quite doubtful about the brand and whatnot, but I've driven her car extensively and frankly I'm impressed with how they drive and feel, accelerating, braking and steering in all conditions. Had some feedback from one of the guys in the tyre place I use to swap and he had some excellent feedback on them and Landsails in terms of recent evolution and improvements of batch consistency in their ranges over a decade (I should stress that I supplied the tyres unseen, not him) - he does a lot of taxis and has seen both of these extensively in the past 3 years. Similar to the way that Nexen and Hankook have gone from the margins to mainstream in the past 20 years. I can't argue with the way they feel now and the fuel economy doesn't lie (although I still need to do the injectors to get the next 6-10mpg, hopefully)
 
decent tyre size choices are limited in 16" though. Personally, unless it's about looks, I don't see the need for more than 15. Even then, 195/50R15 looks pretty good on dark rims.
Pepperpots are incredibly light for the price and thus hard to beat if you want economy.

- Bret
 
Just fill your tank each time and use Fuelly. That'll give you a pretty accurate reckoning.

I find your 58mpg far more believable than 82mpg, but your trip computer (assuming you're not leaking fuel) is the least of your worries right now.
 
Hi there,

Here are a couple of thoughts for you regarding your trip computer discrepancy.

The viscosity / temperature of the fuel is important for the computer to calculate the consumption. If the fuel is actually colder than the computer is reading then the MPG will read too low (and vice versa). The inaccuracy caused by this effect can be quite large eg reading 45mpg when its actually doing 65+MPG! (and vice versa). Armed with this knowledge, I think a logical suspect might be the fuel temperature sensor. If this sensor is not working the computer might use the coolant temperature sensor as a substitute or it might just use a standard set temperature. Either way if the temperature the computer is reading for the fuel is hotter than reality then this would explain your over reading trip computer.

Might also be due to the previous owner messing around with the calibration. Its quite easy to check and adjust with VCDS.

I know a thing or two about fuel viscosities and MPG as I have run neat veg oil for the last 10 years.

I see you are in Gloucester. I'm only up the road in Upleadon. I have VCDS and could check both the above if you fancy a trip out.

Trevor
 
The fuel economy reported by DIS can be calibrated, though only by +/-5%, if I remember correctly. In my experience, DIS does a very good job of calculating fuel economy, hence why the calibration range is fairly narrow. The inaccuracy reported in this post is massive, meaning something is amiss somewhere. I'm with @TAABVW in thinking that this is probably caused by a temperature sensor fault, but there is also a remote possibility that the cluster's software has been corrupted, meaning it is calculating nonsense based on correct data input.
I've now seen three or four A2 instrument clusters that have managed to corrupt their own software. Although none of them have presented this particular symptom, it can't yet be ruled out as a possible cause. The solution is to bench flash the cluster with fresh software; I've got all the gear and knowhow to do this if needed. However, as mentioned, I'm inclined to point the finger at a sensor. The 2 wires to the fuel temperature sensor on the TDI are easily chafed by a corrugated conduit only 4 incher or so back from the sensor's electrical connector. I've seen/fixed this on a good handful of TDIs. It shows as a fault code but doesn't illuminate the EML. If there is no fault code, look at live data using VCDS to see whether the sensor is reporting something sensible.

marcdavidellis said:
...the live which gets up to 200mpg at points if you can believe that
Click to expand...
I can believe it! When going down a hill in top gear with the wind behind you, using only light throttle to maintain speed, the 1.4 TDI will do 200mpg ...it just can't maintain such high figures once the gradient evens out.

Cheers,

Tom
 
Is it possible that your DIS is reporting km per gallon and not mpg? (or some bizzare equivalent). As a rough guess that looks like the kind of error margin to me.
 
Perhaps some error in the cluster where it is not receiving the correct road speed or mileage driven. Is the odometer working correctly? What wheels and tyres are fitted as this could well give the wrong information to the computer if not a standard size? Or the signal from the injectors is faulty perhaps not registering one cylinder as that would be roughly the error. Simple solution use the DIS in a different mode . Have a full VCDS scan done to see if it picks up any faults.
 
AndyP said:
Is it possible that your DIS is reporting km per gallon and not mpg? (or some bizzare equivalent). As a rough guess that looks like the kind of error margin to me.
Click to expand...
It's either l/100km or mpg. No other options AFAIK. Swedes used to use l/10km. L/100km is standard in Europe and is useful for seeing linear changes.

Road speed is reported via gearbox sensor in early cars and afterwards by the ESP / wheel sensors. This is a 2004, so if the tacho works and the speeds shown are correct, it's fine. ASR, ESP, and EDS use this information and compare rotation across all four wheels.

- Bret
 
Timmus installed a Colour DIS in my 1.2TDi some 9 months ago. I then had the opportunity to easily calibrate the display. It was way off. Showing around 20% too high consumption. After calibration the display was very accurate but after 6 months i noticed the display was off again. This time showing around 5% too high consumption. New calibration and the display is accurate again.
 
Ok minor update. Mechanic gave the a2 a medical and here's the list of resolutions. 1. Oil level sensor replaced. 2. Temperature sensor replaced. 3. Temp cable in engine bay sheared and replaced. 4. Rear brake drum cylinders seized and replaced. 5. Handbrake adjusted switch repaired light on dash out now. 6. Front discs and pads replaced. 7. Changed alloys from 17" 9 spoke to 15" pepper pots and new 195/55 kleber Quadraxer2 (which I 've used before and are great and bought cheap). ... I'll update in the coming days as to how this all impacts on the issues I have had since initial ownership of the a2. Next week going to have the timing belt done (last dine 50000 miles 8 years ago this sounds a lot to me) oil change for Castrol edge and transmission oil with mt90. She better be worth it!!?
 
What is the recommended interval on the timing belt presuming tensioners might need attention too? Ps checked the engine mounts "little" bit of play but minor so don't think this is the cause of the judder or knocking under load , forgot to mention aswell they found some bolts on the drive shaft loose, these were tightened I'll report on if any change.
 
marcdavidellis said:
What is the recommended interval on the timing belt presuming tensioners might need attention too? Ps checked the engine mounts "little" bit of play but minor so don't think this is the cause of the judder or knocking under load , forgot to mention aswell they found some bolts on the drive shaft loose, these were tightened I'll report on if any change.
Click to expand...
I think it is every 5 years or 60000 miles, yours needs doing.- Andy
 
Why MT90? I personally use only the OEM stuff, and we've had some try others but the price differential is minimal and little to zero benefit from not using OEM.

Timing belt has no recommended interval from the manufacturers. Replace the tensioners and water pump at the same time. Mine is 10 years old and I consider that it really ought to be done before stressing the engine significantly; it's a cheaper prophylactic fix than the repair. 8 should be ok, but I understand wanting to replace at that age. Most agree that 8-10 years is acceptable or 90k miles / 150k kms.

- Bret

EDIT corrected the statement there's no change timing. There is and it's listed in the replies.
 
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