Fuel Consumption during the cold snap

From March 2010, once I got my coolant bleed, my fuel consumption was a steady 9.0 to 9.4 l/100km. However, during December (artic air and all that) my consumption shot up to 10.5 to 10.8 average.

Now that the thaw has set in, all is fine with a spin today averaging 9.1l/100km.

Any reason for this? Surely the ECU does not take into account the ambient temp, and the auto box was always in Economy mode.

Could the auto box oil cooler be an issue, not allowing the auto box into the highest gear possible?

A full tank fill of €90 has to last longer than 400 miles these days.

My 520 has been the same. It takes alot longer to warm up during this cold weather.

stephen520i wrote: Also when it's colder the air will be dneser so you will need more fuel to maintain the correct ratio. This should give more power but could also make the car less economical. I would have taught that colder air (winter time) was better for an engine than warmer air (summer) and thus an engine would not need to be feed as much fuel as normal. Thus I should not have seen my MPG go thru the roof.

The density of a gas is dependant on temperature. The lower the temperature the greater the density. So the mass of 1 litre of air is greater at 10 degC than it is at 30 deg C. It follows then that 1 litre of air at 10 degC contains more oxygen than air at 30 degC. Combustion requires oxygen and the more oxygen there is present the more fuel can be burned.

Your car takes in a fixed volume of air, dictated by the engine dimensions but as the temperature changes so does the mass of air, hence the requirement for a MAF. The greater the mass of air the more fuel is required to give the correct air/fuel ratio.

There are too many variables to consider  all the previous comments are correct  you can only compare like for like driving conditions.
the recent cold spell has caused lower average speeds due to traffic congestion, lower gears being used and more stops mean lower mpg or in your case kpl

UweM3 wrote: In the good old days we used to cover half of the rad up in the winter when temperatures went below 0 degress.

Us youngster with todays new fangled modern computer powered cars have adjustable air rams.  Behind my kidney grills are slats which are controlled by the ECU to either let plenty of cold air in to keep it cool or shut to keep the cold out.  With the engine off I can push the slats open quite easily and then see into the rad, but on a cold morning on idling just after start up the slats are locked shut and I can't push them open speeding up the warm up.   

At the start of November in minus 3 weather with the heater on full, after driving for 14 miles on a mixture of B Roads, M8 and Glasgow city centre traffic, my thermostat had not opened and the rad was stone cold when I parked up.  Granted the engine hadn't worked hard i.e. not above 55 mph or even 2,000 rpm it just didn't get a chance to heat up with the heaters sucking any warmth out the engine to heat the cabin.

Andrew

Dergside wrote: Peter Fenwick wrote: The density of a gas is dependant on temperature. After the end of the recent cold snap I checked my tyre pressures that had previously been set in the depth of the cold.  The tyres were about 4-5psi over inflated as a result of the rise in temperatures.

Indeed. It's worth keeping an eye on stuff like this when there is a significant change in ambient temperature.

Edited by Peter Fenwick