Fuel Economy - Dodgy fuel?

[bbrown1664]

This week I have to drive from Sussex to Cumbria and back. Heading up I started with 3/4 of a tank of diesel from Applegreen (a local cheap station) and managed 45mpg over the 330 mile trip. Heading home I filled up at Asda and managed 41mpg. Both journeys at the same time of day, same weather/temperature, very similar light traffic conditions. Cruise set at 74mph most of the way and aircon on auto. The only real difference between the two trips is the extra 20kg or so of fuel being carried on the way back and the different fuel providers.

Car is a 2012 2ltr TDI MultiTronic S-Line with 105,000 miles on the clock and 19" wheels and air suspension. I believe that I have an issue with the thermostat as it stayed below 90C pretty much for the whole journey both ways. Thats being looked at next week.

Any thoughts on what else could be causing the poor economy?

[dean warren]

Was the one way up hill and the other down?


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[johnsimcox]

Couple of thoughts. Firstly what was ambient temperature at the start of your journey in each location? I suspect it was colder in the Lakes and that would affect the initial fuel consumption whilst the engine was warming up. I used to see anything up to 5mpg difference on my commute to work depending on how cold it was. Realise the impact over a long journey will be mitigated but it will still have an effect.

Also I suspect the start of your journey in the Lakes, when the car was cold and before you got to the motorway, involved a lot more stops and starts and acceleration, especially up hills, than in Sussex. This would also have an effect


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[Elansear]

You mention the temp showing below 90. It has been mentioned several times that the temp shown is not a true reflection of the actual temp. My understanding is that it showing 90 is an indication that it is within a range either side of that, likely quite a wide range. So if you are showing below 90 and it’s not the sender but actually a low temp then that would have a dramatic effect on fuel usage.

[beemerboy9]

One journey in either direction is not enough to draw a conclusion. Take an average of at least 3 journeys to see if there is a difference.

[bbrown1664]

Couple of thoughts. Firstly what was ambient temperature at the start of your journey in each location? I suspect it was colder in the Lakes and that would affect the initial fuel consumption whilst the engine was warming up. I used to see anything up to 5mpg difference on my commute to work depending on how cold it was. Realise the impact over a long journey will be mitigated but it will still have an effect.

Also I suspect the start of your journey in the Lakes, when the car was cold and before you got to the motorway, involved a lot more stops and starts and acceleration, especially up hills, than in Sussex. This would also have an effect


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Ambient air temperature was very similar throughout the journeys.

Uphill would be the first (more economical) journey heading north :)

I am 90% certain the thermostat is stuck open, or at least partially open, causing the lower running temperature and it is booked to be replaced next week along with the overdue (time not mileage) service so hopefulluy I will see an increase in the mpg after that.

After 5 years, almost the longest I have owned any car in about 35 years of driving, I am in no hurry to change it. IT is a great car to drive.

[Whippy53]

I experienced something similar not so long ago, filled up in a Sainsbury's and immediately thought the car less responsive, got few miles out of the tank as well. Only time it's ever happened.

[beemerboy9]

After 5 years, almost the longest I have owned any car in about 35 years of driving, I am in no hurry to change it. IT is a great car to drive.

That's wonderful - it means that you have bought the right car for you. How unusual to have air suspension on a 2-litre car.

The A6 is a bit of a barge and if you accept it for what it is, namely a big cruiser, and don't expect it to corner like a Mini, one can extract a great deal of driving pleasure.

I drove mine through the Cairngorms last summer and I found it responded well to being driven on the steering wheel paddles. Slow entry into the corner with the gear locked in on the paddle, then power through when the corner opens up. Very satisfying when one gets it right but requiring much more concentration than "Auto".

[philipharmes]

Diesels with the auto gearbox usually have two thermostats.

It's usually the gearbox fluid cooler thermostat that fails stuck open.

This causes the cooling system to run much cooler & leads to poor fuel economy.

Get that thermostat checked first, it's much easier & cheaper to replace than the main coolant stat.