Fuel Economy in Cold Weather?

[Cambslad]

Hi all,

I have a 2.2 tdi. W203 sports coupe.

Have noticed my fuel economy has got very poor recently, originally I thought it was because my car was coming up for an A Service. But I have now had my car serviced about 500 miles ago and am still getting rubbish mpg.

Usually I would expect mpg of at least 40, and have achieved 46-47 mpg on motorway runs. Now on an hours drive at 80mph on the motorway I only get 34mpg.

Any thoughts on what could be causing this?

thanks

Lee

 

[silver_star]

What temperature is your engine getting to? After a good half hour even in this cold weather it should be reaching over 80 and nearer 90 ideally.

 

[This is Pete]

You might find

THIS

interesting. Someone ran a comparison of mpg against seasonal temperature variances.

 

[Deleted member 11979]

I find my V70 D5 is a lot less economical in this very cold weather, theoretically air is denser so takes more energy to push the car through, no idea if that is a factor, lower tyre pressures if they've not been adjusted might make a difference in low temps, also engines running generally cooler for longer may make some difference. Wet roads always use more fuel and also tend to have more electrics on in the car. All combined might go some way to explaining it.

Mind you, that is one big drop you've got there!

 

[kusanku]

All the diesel cars I have owned have given less mpg in the winter. On two occasions when I got drastically lower mpg and the engine did not seem to maintain proper running temperature, a faulty thermostat turned out to be the cause and replacement cured the problem.

 

[st4]

A diesel will return at lot less mpg in cold weather. More energy is lost as heat loss from the engine and the fuel contains mitre additives to stop it waxing.

 

[Beno]

Diesels will have worse MPG in the colder weather as they will take longer to warm up, meaning they will be running cold fuel maps for longer you will also have more battery drain (Heated seats, rear screen etc) so the alternator will be working harder, add in people tend to drive slower with more stopping/pull away.

 

[John]

I used to notice with my old Scab that it would drop from a reported 29mpg to early/mid 20s in the colder months.

Similarly, my reported 20mpg but actual 18mpg in the Merc drops to... well a lot lower than that let's say.

 

[steve333]

Diesels will have worse MPG in the colder weather as they will take longer to warm up, meaning they will be running cold fuel maps for longer you will also have more battery drain (Heated seats, rear screen etc) so the alternator will be working harder, add in people tend to drive slower with more stopping/pull away.

^ The really cold weather affects the mpg on petrol engines too,i do the same run early morning,26 miles(exactly an hour round trip dropping the mrs to work),usually the obd reads 22mpg however when it's below or around freezing it drops to 20mpg.(Same driving style/traffic conditions).

 

[Peter DLM]

My economy suffers by at least 15% during cold weather over short journeys. Long ones aren't quite so bad, 10% lower.

 

[jay1983]

Must say my car has been burning the fuel recently. I leave for work at 6am so the car never gets up to temp cos I live 5 miles from work.
I replaced the thermostat as that was faulty but this mornin I left for work with a full tank, when I got there it had gone down 1/8!! I had done 20 miles with the needle on the full line and to see a drop like that was concerning.

 

[wemorgan]

On long journeys I have found that fuel economy only falls by ~10% with lower temperatures (15C vs 0C).

I imagine that short journeys are a significantly worse.

 

[Dieselman]

Diesels will have worse MPG in the colder weather as they will take longer to warm up, meaning they will be running cold fuel maps for longer

Diesels don't run fuel mixture maps and only increase fuelling to overcome the additional load placed on the engine.
There is no mixture richening on diesels, there can't be as the air volume is unregulated.

 

[Beno]

Diesels don't run fuel mixture maps and only increase fuelling to overcome the additional load placed on the engine.
There is no mixture richening on diesels, there can't be as the air volume is unregulated.

Sorry DM all modern diesels run fuel maps in the ecu, what map is governed by temp, load, pedal demand, egr, gear, etc, etc. I know because I'm a diesel calibrator/tester. We run fuel emission cycles from -15 unto +30 and have to calibrate the engines for these temps. The vehicles will use far more fuel at lower temps because they run rich maps.

 

[Dieselman]

I know they fun more fuel, but how can the mixture be calibrated off boost if the air volume is unrestricted?
You can only regulate the on boost mixture.

 

[Beno]

Sorry Off Boost? do mean closed throttle or non turbo? If you mean closed throttle the injectors are switched off so there's no fuel delivery.
I have no experience of non-turbo diesels. I'm afraid we don’t make so of them to be able to comment.

 

[Deleted member 37751]

Since the cold weather has eased, I'm getting at least 5mpg more.

 

[Dieselman]

Sorry Off Boost? do mean closed throttle or non turbo? If you mean closed throttle the injectors are switched off so there's no fuel delivery.
[/COLOR]

There's no throttle, which is why the air is unregulated.
A diesel engine varies the torque output by varying the volume of fuel, but the volume of air is constant per induction, apart from the boost pressure giving greater cylinder filling.

Surely all you can do is adjust the volume of fuel delivered to cause the boost to alter and restrict maximum torque output, you can't set a mixture strength as such, it will still vary between 100:1 and ~20:1 depending on torque required.

 

[Beno]

There's no throttle, which is why the air is unregulated.
A diesel engine varies the torque output by varying the volume of fuel, but the volume of air is constant per induction, apart from the boost pressure giving greater cylinder filling.

Surely all you can do is adjust the volume of fuel delivered to cause the boost to alter and restrict maximum torque output, you can't set a mixture strength as such, it will still vary between 100:1 and ~20:1 depending on torque required.

Yes sorry should have said off pedal = "closed throttle" its force of habit been with engines to long. The turbo vanes are altered to give greater or lesser air volume/boost pressure and its metered by the MAF which is giving you the open loop control. But as you say the fuel delivery is the way to best increase torque

 

[Cambslad]

Ok thanks, at least it is not just me!

Engine temperature is getting up to 80-90 every time, and front tyres were changed last month so assume they are ok and correct pressures.

Did get my first 39mpg for sometime last night so hopefully things are on the up!