125tdi woeful fuel economy, help!

[harlie]

Agree with Greg - forget what the MFD is saying. Calculate fuel usage the old fashioned way (or have you already confirmed it?).

BTW - average 1 will reset every time the car is turned off for more than 2hrs. Average 2 keeps going.

Doubt this is your issue but just for fun - What is the engine temp? What is it really not what the gauge says? Modern cars use the temp gauge as an indicator for the driver, if it does not move all is good. However, gone are the days where the temp gauge actually tells us what the engine temp is - seriously they should just replace the gauge with a red light - some cars have.

The ECU receives a signal from the temp sensor which it uses for several purposes. The ECU will display to the driver (temp gauge) that everything is normal if it is happy (with our Skodas the gauge sits on exactly 90) to keep us away from the dealer. However you will find that when the gauge displays 90 the engine temp can be anywhere from 70s to 105 degrees. As engine temp rises to 70ish the ECU is happy that the engine is warming. With a thermostat calibrated to 88-90 degrees it will do a good job at keeping it near that point, but hold the throttle down hard and the temps will rise – come over a big hill and coast down and the coolant temps will drop – the dealers don’t want us coming in every time the gauge moves, so it doesn’t - unless it goes outside the acceptable range. There is no possibility that the engine temp can stay exactly 90 at all times….

Now why my long winded rant. The engine ECU will over fuel if it is cool. I don’t have the doco for the VW fuel management system but this is what I learnt from our other car which is BMW / Bosch diesel.
• According to the documentation, it over fuels to aid engine warming when coolant temps are below 78C.
• According to my diagnostic gear - The gauge sits on the normal spot from 69C.
• With a lazy thermostat (steel spring) I didn’t notice the cool engine until it was running in the mid-60s – that’s how cold it had to be for the gauge to read below 90.
• I was wondering why the fuel consumption had gone to pot – about 20%.
Once a laptop was plugged in I could see the temps sitting in the mid-60s, rising into the low 70s when pushed, and dropping below 60 on the freeway. With a new thermostat it runs between 95 and 102 – gauge says 90 all the time.

There is a member on here that always swears his fuel consumption is better when it is hot (only guy I’ve ever heard say this) – I bet his is running cool and only gets to full temp on a really hot day.

our Octi 125 runs up the highway at 4.5-4.8; high 5s around the city to mid/high 6s when the city stop/start traffic is bad.

[ZokEE]

I'm not sure what part of QLD you are but can you make use of a VW Service Centre that's closer - possibly just for the initial diagnosis?

Is Roma or Emerald closer?

Haha, neither. We're in Mount Isa. Townsville is 900 km (bought our octavia from there, bad experience won't go back), got the superb from cairns which is 1400 km.

[ZokEE]

Agree with Greg - forget what the MFD is saying. Calculate fuel usage the old fashioned way (or have you already confirmed it?).

BTW - average 1 will reset every time the car is turned off for more than 2hrs. Average 2 keeps going.

Doubt this is your issue but just for fun - What is the engine temp? What is it really not what the gauge says? Modern cars use the temp gauge as an indicator for the driver, if it does not move all is good. However, gone are the days where the temp gauge actually tells us what the engine temp is - seriously they should just replace the gauge with a red light - some cars have.

The ECU receives a signal from the temp sensor which it uses for several purposes. The ECU will display to the driver (temp gauge) that everything is normal if it is happy (with our Skodas the gauge sits on exactly 90) to keep us away from the dealer. However you will find that when the gauge displays 90 the engine temp can be anywhere from 70s to 105 degrees. As engine temp rises to 70ish the ECU is happy that the engine is warming. With a thermostat calibrated to 88-90 degrees it will do a good job at keeping it near that point, but hold the throttle down hard and the temps will rise – come over a big hill and coast down and the coolant temps will drop – the dealers don’t want us coming in every time the gauge moves, so it doesn’t - unless it goes outside the acceptable range. There is no possibility that the engine temp can stay exactly 90 at all times….

Now why my long winded rant. The engine ECU will over fuel if it is cool. I don’t have the doco for the VW fuel management system but this is what I learnt from our other car which is BMW / Bosch diesel.
• According to the documentation, it over fuels to aid engine warming when coolant temps are below 78C.
• According to my diagnostic gear - The gauge sits on the normal spot from 69C.
• With a lazy thermostat (steel spring) I didn’t notice the cool engine until it was running in the mid-60s – that’s how cold it had to be for the gauge to read below 90.
• I was wondering why the fuel consumption had gone to pot – about 20%.
Once a laptop was plugged in I could see the temps sitting in the mid-60s, rising into the low 70s when pushed, and dropping below 60 on the freeway. With a new thermostat it runs between 95 and 102 – gauge says 90 all the time.

There is a member on here that always swears his fuel consumption is better when it is hot (only guy I’ve ever heard say this) – I bet his is running cool and only gets to full temp on a really hot day.

our Octi 125 runs up the highway at 4.5-4.8; high 5s around the city to mid/high 6s when the city stop/start traffic is bad.

interesting to know, I'll make sure the dealer checks that when I get it there.
I've read nothing but praise for how good these engines are on fuel, no matter which vehicle they're in. I'm just surprised and a bit disturbed how **** ours is, especially given its literally brand new.

[Ryeman]

It's more to do with your habit of running with AC on, or windows down at speed, (not many do that on a cold day) wind direction and speed usually never the same ....load/ pack rack etc.
possible loss of fuel due expansion in the sun after refuelling (doubt if that's possible with emission requirements) better to fuel first thing in morning, colder fuel has more (free) energy.
In outback Oz aero drag is the big killer at speed and hot air is thinner than cold dense air but your vehicle needs remarkably little power for 100kmph cruising on the flat with nill wind (AC?) and because the hot air is less dense you need slightly less power/fuel flow.
Consider EVERY factor before chasing expensive solutions to a problem that may not actually exist.....it's no more than the application of year ten physics and a critical eye.
Once you've done that THEN look for the problem.
I don't claim to get better economy in high temps because I want to be comfortable too.

[Guest001]

Well I have a Passat wagon with the same engine and on my last 1000km trip I got just over 6l per 100k. Wasnt hanging around and I never turn the air off Its set at 22 and stays there. Best I have got on the same trip going a bit slower was 5.6L per 100km

[kaanage]

Consider EVERY factor before chasing expensive solutions to a problem that may not actually exist.....it's no more than the application of year ten physics and a critical eye.
Once you've done that THEN look for the problem.

Have you looked at the OP's consumption in the early posts? It's so atrocious that it's almost certain the car has a fault.

For comparison, when I did a 75 minute stint in the Winton 6 hour relay last year, my car's consumption was only 0.5l/100km worse than his 1st fill average after his drive home.

[brad]

Have you looked at the OP's consumption in the early posts? It's so atrocious that it's almost certain the car has a fault.

For comparison, when I did a 75 minute stint in the Winton 6 hour relay last year, my car's consumption was only 0.5l/100km worse than his 1st fill average after his drive home.

+1

In my 1.8TSI I average 6.6L/100. The absolute worst single tank consumption I've ever had is 7.9L/100 (+20%) if I'm getting stuck into it. To be getting consumption 100%+ more than expected there must be something wrong.

13.5L/100km is what my wife's V6 Craptiva averages and it's a fuel guzzling brick with the moost stupid power & torque curves I've ever come across in my life.

[Ryeman]

^^^actually I'm simply addressing the belief that higher ambient temps equate to higher fuel consumption as opposed to the original OPs obvious problem.
sorry about the confusion (

[gldgti]

I would tend to agree with Greg roles postulation about dpf pressure sensors on this one.
Correct me if I'm wrong, but surely there's no need to go to the specific dealer the car was purchased from?

[ZokEE]

Just a quick update, tried disconnecting the battery overnight, didn't seem to make any immediate difference to mfd consumption readings, still showing silly numbers under even light acceleration and average numbers while cruising (between 7 to 10L/100km). Will wait and see what I get at the next tank fill.

I would tend to agree with Greg roles postulation about dpf pressure sensors on this one.
Correct me if I'm wrong, but surely there's no need to go to the specific dealer the car was purchased from?

no don't need to go to specific dealer. Townsville is closer, but I refuse to take a car back to them. We have a good relationship with the dealer in cairns so I would like to take it back to them.

[Ryeman]

I've got a 'box' on mine and it makes the maxidot useless as a guide to consumption.
Fill to fill and take off 5% for distance travelled.
I don't know why flow sensors (transducers) for cars are so inaccurate for cars but so accurate for aircraft.

[wai]

When checking fuel consumption you need to check it using the same bowser as if you use a different one, you will find that it may cut out at a different point. Also, you need to stop filling when the pump first cuts out.

You can get a wide variation from using different bowsers at the same service station.

[asa572]

When checking fuel consumption you need to check it using the same bowser as if you use a different one, you will find that it may cut out at a different point. Also, you need to stop filling when the pump first cuts out.

You can get a wide variation from using different bowsers at the same service station.

Foam can cut a bowser off.
I can usually get another litre or two in after bowsers first cut out.
I fill to the brim.

[Ryeman]

Yep!
gotta watch the foam or you'll end up not knowing where the 'solid' level is......even the angle of the nozzle might have an effect if you don't actually see it in the neck.

[gldgti]

I've got a 'box' on mine and it makes the maxidot useless as a guide to consumption.
Fill to fill and take off 5% for distance travelled.
I don't know why flow sensors (transducers) for cars are so inaccurate for cars but so accurate for aircraft.

There are no flow sensors in the car. The ECU works out consumption based on injector duty cycle.

I'm pretty sure that considering the OP's problem, its definitely NOT his technique at measuring fuel economy that is the issue here. Everyone please stop cluttering up the thread with advice about how to measure fuel economy. Cheers.