Fuel Computer MkV 2.0TDI innacurate ?

[jackafrica]

Hi All,
Have 15,000k on the odometer, so my 2007 TDI 2.0 Comfortline should be performing okay in the fuel usage stakes.
The onboard computer reading average fuel readings says that I'm consuming 5.0 litres per 100k or less.
Reality, when measuring at the fuel pump says that I'm actually using 6.6 litres per 100ks.
The all time fuel reading suggests my average over the distance travelled is 5.4 l/100k. The reality is 6.5.
I get approximately 700 to 750ks for 50 to 52 litres of fuel. That is 14ks per litre = 7.14 l/100ks to 6.9l/100ks. This is my " real world " average. Car readout suggests as above, approx. 5.4l/100ks.


My questions are
1. Does anyone else keep records which record a similar disparity between what the readout suggests and the reality ?
2. My VW dealer believes there is nothing wrong with the car and that all is normal with the readings. How can a 20%+ optimistic reading be considered normal ?
3. If I accelerate quickly, with my foot flat to the floor, plumes of dark grey to black smoke are visible form the back of the car. Surely this indicates an overly rich mixture ? Surely this isn't normal ?

Thanks I look forward to your replies.
cheers
richard

[STV4SYT]

Its always a bit optimistic i reckon.

I did a trip of 940km and comp said i did 5.4 and in reality i did 6.6

it obviously pays for them to make it like this rather than the other way.... same as speedo accuracy

[drbass]

Hi,

Generally I'm looking at about 10% optimism from the computer. I always get my hopes up but the bowser tells the truth (always fill right up).

Mind you, 6.9/100 (with 80/20 mix of commuting/highway over nearly 12,000 km) is something I'm definitely very happy with considering the size of the car, and the performance (if requested).

Seeya,
Michael

[neil]

Rich hi.
Has been a few post in the past asking the same question.

I have found my MFD spot on.
My average todate after 9000km is 4.8(MFD) and manual calculations is the same.
I mainly do Freeway driving so get good figures.

How does VW calibrate the MFD?? Do they somehow figure in a centan rating of certain diesel ??

I have used only BP diesel which it the preferd fuel for VW this has the highest centan of all fuel companies.
For the first time recently I was forced to use woolies diesel and got the following results, I drive conservatively.
BP fuel. Central coast to sydney and return both ways 4.2L/100km
Woolies fuel. central coast to sydney and return both ways 4.5L/100km - yet to work this figure out manually.

Im just clutching at straws and maybe someone with some knowledge about how a MFD works can fill us in.

[gregozedobe]

The MFD fuel consumption figure on my 2.5 TDI has always been optimistic, but by inconsistent amounts - usually around 10%, but has been as high as 22% and as low as 5%. It isn't due to different driving conditions or fuel suppliers either, as almost all my driving is on the highway, and I tend to buy my diesel at the same service station (doing over 900Km on a tankfull helps there, as I can drive to Sydey and back without needing to refuel quite easily).

I believe this error is a deliberate marketing ploy designed to fool the gullible into thinking they are getting absolutely fantastic fuel econmy when they are merely getting very good economy.

VW speedos also read fast for similar reasons (but is justified as helping us to not get booked for speeding).

I've read somewhere that it is possible to adjust the speedo and the MFD fuel consumption readouts by up to +- 15% using vag-com, but I haven't heard of anyone actually doing it.

Also it is difficult to completely fill the tank to the exact same level every time, as the varying amount of froth seems to stop you unless you are (very) patient, so there may be slight variations from actual usage to manually calculated usage(but if the MFD was accurate our calculated consumption would sometimes be less, and sometimes more - not ALWAYS more)..


Lots of black smoke is quite normal for a hard-accelerating TDI engine. It is created by an overly rich mixture (for maximum power) not being completely burnt, thus creating carbon particles. If your engine had a DPF it would trap these particles for later burning.

[cetane]

Expansion and contraction

Don't forget that fuel volume is affected by temperature. Whilst not so marked as it is with regard to petrol (more dense than petrol so lower coefficient of expansion ?) diesel volume will vary depending on the ambient temperature. So even though the fuel is coming out of underground tanks most of which will be deep enough to be around the same temperature, the fuel that is left in your tank will be subject to temperature changes and this will affect how much you can get in while filling. I suppose the same laws of physics will apply to the way in which the car's computer calculates fuel consumption. I'm guessing but it must measure the amount of fuel being drawn from the tank, the car's speed or distance covered, and calculate it from there (?)

[Cupra]

my 2.0tdi gets 5.4l/100 average after 45000k from my lease records & we normally see between 4.5-5.1l/100 around town from the trip computor

[Logzy]

I've read somewhere that it is possible to adjust the speedo and the MFD fuel consumption readouts by up to +- 15% using vag-com, but I haven't heard of anyone actually doing it.

The speedo, no you cant adjust it but you can adjust the consumption in the mfd.

Im in the process of adjusting mine at the moment. So far ive made one adjustment but it requires more.

[Spoddy]

We have done about 22,000km and the average to-date is 6.57l/100km. The MFD always shows about 0.3 to 0.5 better than the actual.

The sort of driving the girl gets is mainly short stuff with 1 longer drive each week, though now my son (the one who lost the keys last year) is back in town the golf is getting to stretch its legs a bit.... strangely enough with similar economy.

We had issues with smoke when using Caltex but way less when using BP.

Has anyone used the Caltex special vortex noted on their web site available only at a few selected outlets in Sydney?

[jackafrica]

Okay, so then the dealer should be able to adjust the readout to be closer to what the consumption is in reality ?
Or do they also have to own a VAG-Com ?

Logzy are you adjusting yours to compensate for the ST3 ?

The thing that gets to me is the real consumption being 6.5 l/100k, if I can reduce that by 10% then great.

Sad thing is I rely on that readout, to keep my sometimes poor driving habits ( sinking the slipper into it ! ) curbed.

I'll phone the dealer and see what they have to say. Thanks for the replies to date, it would appear that I do have an issue which can be attended to under warranty by the dealer.

[cetane]

Caltex

Has anyone used the Caltex special vortex noted on their web site available only at a few selected outlets in Sydney?

"Special vortex" is not available here....

I use "ordinary" Caltex diesel in Mk1 Golf, Mk5 Golf TDI, Nissan Patrol SD33, S1 Landrover Discovery Tdi, Deutz 90hp tractor, Inter 65hp tractor, MF 70hp tractor and NONE of them smoke. The reason that they don't is they're all tuned properly (except the Golf TDI which I can't touch until the warranty expires...but then it doesn't smoke anyway even if I floor it..... with 30,000km on the clock it's not run in yet).

Oh, and according to the MFD I get 4.7L/100km on hilly, windy country roads to and from town...50km, 5.0L/100km on a 300km round trip to the nearest big city on highway running, and 5.2L/100km with mixed highway/city running. Not that I go to the city if I can avoid it. I guess with the error you guys are talking about these figures will look a lot less good

[gregozedobe]

Sad thing is I rely on that readout, to keep my sometimes poor driving habits ( sinking the slipper into it ! ) curbed.

If you want to use the MFD to help you save fuel, display the instantaneous fuel consumption, not the average. It tells you when you are really slurping fuel at that moment, which encourages you to back off a bit.

The average will tell you whether you have been sinking the slipper over the total journey.

[BeigeJet]

Hi All,
Have 15,000k on the odometer, so my 2007 TDI 2.0 Comfortline should be performing okay in the fuel usage stakes.
The onboard computer reading average fuel readings says that I'm consuming 5.0 litres per 100k or less.
Reality, when measuring at the fuel pump says that I'm actually using 6.6 litres per 100ks.
The all time fuel reading suggests my average over the distance travelled is 5.4 l/100k. The reality is 6.5.
I get approximately 700 to 750ks for 50 to 52 litres of fuel. That is 14ks per litre = 7.14 l/100ks to 6.9l/100ks. This is my " real world " average. Car readout suggests as above, approx. 5.4l/100ks.


My questions are
1. Does anyone else keep records which record a similar disparity between what the readout suggests and the reality ?
Thanks I look forward to your replies.
cheers
richard

Hi Richard,

I'm fairly anal and have kept detailed records for almost 2 years on my Jetta TDI manual.

I have found that my MFD is optimistic by about 0.5 lt /100. So when it shows 5.6/100 on the computer its more lik 6.1/100.

Life of car is 6.13 lt /100 km.

Below are totals for life of car and FBT year ending March Mar 2009 so far.

total km total lt total $ cpl km/lt mpg lt/100km $ p km
2009 6,080 376.0 642.59 170.9 16.17 45.7 6.18 $0.106
since new 44,644 2,736.7 3,713.12 135.7 16.31 46.1 6.13 $0.083


I hope it makes sense.

Cheers

Neil.

[cetane]

Optimism/pessimism??

Typographical errors (that's more than 10 characters )

[cetane]

Optimism/pessimism??

We all know that the speedo is optimistic by ~10%. So if the speedo is optimistic then won't the odo also be optimistic? Which means that you're actually covering less kms than you think (by 10%?) and so your fuel consumption will look less by ~10%.

Check out my logic, guys, it could be flawed.