TD MPG in cold weather?

IanD · Post by **IanD** » 21 Oct 2004, 15:59

Just wondering if it's normal to lose approx 4mpg when the weather turns cold? This is my first diesel (Volcane TD) & have had it since June & always averaged around 50mpg but now it's dropped to around 46mpg suddenly without a change in driving style. I have done a full service using Total Quarts Semi-Syn & all filters changed which improved it from 44 to 46 but where has the rest gone suddenly? Is it purely down to the cold(er) weather of late?
Cheers,
Ian
PS. Also considering putting some injector cleaner through it but it's not smoking more than normal so not sure if this is needed?

drpau · Post by **drpau** » 21 Oct 2004, 16:32

With my last car my MPG always dropped in colder weather, suppose its cos it takes longer to get up to running temperature? You would think it would be better though, colder denser air giving a better burn of the fuel thats injected.

Kowalski · Post by **Kowalski** » 21 Oct 2004, 16:53

I keep pretty accurate records of my cars fuel consumption and have graphed the results. I can't see any seasonal trend on the graph over 3 years of TD Xantia driving (50k miles). My driving tends to be long journeys (35 miles to work) at dual carriageway speed.
Have you checked your tyre pressures?
How smooth running / smoky is your engine?
Does it use oil?
Does it use water?
Do you have a binding brake?
Do you have aircon?
Have you recently installed a roofrack been towing a trailer?
Has your route changed?
There are lots of things that can alter your fuel consumption, just some of them are listed above.

madasafish · Post by **madasafish** » 21 Oct 2004, 17:27

My experience using aircon all the time is fuel cosnumption improves on long runs in winter (no aircon cooling) offset on short runs by worse consumption due to cold engine/more viscous oil ).

Kowalski · Post by **Kowalski** » 21 Oct 2004, 19:01

In theory...
Engine efficiency should be better with colder air.
Aerodynamic drag should be less with warmer air.
So all in all, not a lot of difference winter to summer.
A modern multigrade engine oils viscosity shouldn't change too much since it has all of those viscosity modifying additives in it.

AndersDK · Post by **AndersDK** » 21 Oct 2004, 20:10

Ian -
I suspect you may have a problem with engine temp.
Try read this for any clues : Diesel coldstart device

davek-uk · Post by **davek-uk** » 21 Oct 2004, 20:56

I'd agree with Anders comment. My coldstart device is duff and I have the vacuum pump directly on the pump diaphragm - when this disconnects (due to old pipes) and I don't notice for a while my mpg drops by 4mpg. It has happened a couple of months since summer.

Kowalski · Post by **Kowalski** » 21 Oct 2004, 22:28

I had a duff electrovalve on my Xantia and it was running in 'cold start' mode for some time.
Replacing the electrovalve had no effect on fuel economy.

nick · Post by **nick** » 21 Oct 2004, 23:51

Apparently the switch from summer to winter diesel can cause the mpg to drop slightly. This happens in mid October in the UK so this could be part of it, but 4 mpg perhaps sounds a bit much to be caused solely by the change to winter diesel.

IanD · Post by **IanD** » 22 Oct 2004, 15:25

Thanks lads [:D]
I'd already checked & discounted everything on this thread apart from the tyre pressures (which I don't think should be the problem as I only checked them a few weeks ago & don't seem to be down visually) and the Coldstart Device which sounds most likely! [8D]
Cheers,
Ian

Kowalski · Post by **Kowalski** » 22 Oct 2004, 16:25

How about the thermostat, or dare I mention it, the head gasket?

IanD · Post by **IanD** » 22 Oct 2004, 20:36

Kowalski,
The car doesn't use ANY oil or water & the temp gauge readings haven't changed at all. [:)]