Sudden decrease in fuel economy

[rayspurgeon]

Any ideas what may be causing a decrease in economy from an overall 40.4mpg to 38mpg. No change in journey mix or driving style. However, last diesel fill was from a garage not used before. Could that possibly be cause? I understand with petrol different octane ratings can effect mpg, did not know if this could be same with diesel. This is my first diesel and have had for 4 months/6,000 miles.

[Viper_7]

You say no change in your journey/driving style, but does your typical journey consist of?

My MPG has dropped about 2 from 44 to...funnily enough 42 ( based on my current journey)
Reason it's dropped is that my journey at the moment is only 11 miles. The car takes about 7 miles before the temp gauge has normalized.
Naturally cold engines are not efficient compared to warm and it's getting a tad cold old. Has done the same in previous years and it increases again when things warm up.
If your journey is longer then the longer cold running time won't cause as big a drop.

vipes

[rayspurgeon]

Thanks Vipes for helpful and possible reassuring response. Roll on the warmer days. Oh for the days of radiator blinds to give the engine a quick warm up!

[Clan]

how many miles does it take to warm up ? 7 is too many half that would be normal ...! try changing the thermostat first ...

[RoyMacDonald]

how many miles does it take to warm up ? 7 is too many half that would be normal ...! try changing the thermostat first ...

I thought it was more like 10 to 20 miles to get fully warmed up. I know the temperature gauge shows the water temp up to full long before but that, means very little dosn't it? Just that the coolant temperature at the sensor has reached full temperature, but there is still all that metal and oil get warmed through.

Outside air temperature has a major impact on fuel consumption. If you take regular readings you should see the mpg rise and fall in tune with the temperature.

Roy

[biglads]

Don't they put more additives into the Diesel in the Winter to stop it sludging up in the cold?

If so, perhaps that may make a difference...?

[Clan]

I thought it was more like 10 to 20 miles to get fully warmed up. I know the temperature gauge shows the water temp up to full long before but that, means very little dosn't it? Just that the coolant temperature at the sensor has reached full temperature, but there is still all that metal and oil get warmed through.

Roy

you are right to some extent , once the gauge has reached normal the oil and crankcase wont be fully up to temperature , but the cylinder head will be , it is the position of the coolant temp needle which matters , if the thermostat is faulty then the nedle will hang around the lower region expecialy when cruising at low speed with little throttle , it will be worse when the weather is colder . So the needle should quickly rise to the central position and stay there and not move what ever the temperature outside .
The "problem" you mention is not so true these days as Volvo engines have a heat exchanger which heats the oil from the coolant quickly up to the coolant temperature in winter and cools the oil to the coolant temperature in summer , so pretty irrelavent now ... especialy as the cylinder block coolant volume is comparitivly low . Thats partly why the D5 is so efficient .

[RoyMacDonald]

Don't they put more additives into the Diesel in the Winter to stop it sludging up in the cold?

If so, perhaps that may make a difference...?

No it's the reduction in thermal efficiency that causes the mpg to fall I believe. Petrol engines are just as badly affected. The diesel additive is just to keep it flowing freely.

Roy

[RoyMacDonald]

you are right to some extent , once the gauge has reached normal the oil and crankcase wont be fully up to temperature , but the cylinder head will be , it is the position of the coolant temp needle which matters , if the thermostat is faulty then the nedle will hang around the lower region expecialy when cruising at low speed with little throttle , it will be worse when the weather is colder . So the needle should quickly rise to the central position and stay there and not move what ever the temperature outside .
The "problem" you mention is not so true these days as Volvo engines have a heat exchanger which heats the oil from the coolant quickly up to the coolant temperature in winter and cools the oil to the coolant temperature in summer , so pretty irrelavent now ... especialy as the cylinder block coolant volume is comparitivly low . Thats partly why the D5 is so efficient .

Thanks Clan. I must say I had noticed it seemed to heat up more quickly than any other car I've owned. I had been saying to my wife this car warms up much quicker than our VW Golf despite being much bigger.

Roy