A bit down on MPG

[The Aardvark]

Hi All

Just had my E39 530d serviced (Inspection 2) and water pump replaced. Seem to be getting approx 15% less MPG according to the onboard read out (not saying that the 2 are connected) - anything I should perhaps be looking out for / checking?
Appreciate that economy is down during colder weather, but even during this recent milder spell I'm still not returning what I used to achieve.

Any thoughts?

[Herbs]

Why is MPG down during colder weather?

I always assumed MPG would be marginally better seeing that the air is denser and the chances are you would have A/C switched off.

[mike325112]

Herbs said:

Why is MPG down during colder weather?

I always assumed MPG would be marginally better seeing that the air is denser and the chances are you would have A/C switched off.

Heating, lights, demisters/heated mirrors etc. Also the engine is colder meaning it runs richer at start up for longer.

[soda]

My consumption drops a similar amount over the winter months, affects turbodiesels more than other engines.

Cold air is denser, means more air and more oxygen going in the engine - more fuel put in as a result.

As mentioned above combined with longer warmup times and heaters/demisters on full blast the economy takes quite a dent.

[sparkythecat]

OP is your engine taking a lot longer to warm up than it was before the work was undertaken?

If so, have they changed the thermostat for a cooler one, or left it out completely ?

[Silver993tt]

I would have thought that the ECU would lower turbo boost pressure when the air is colder in order to provide the same power as when it's warmer, when the boost pressure would be increased. All done to maintain constant power.

[The Aardvark]

Not aware of the thermostat being replaced (although could be faulty I guess). Took approx 12 miles last night on the journey home to get the temp needle up to fully vertical, ie where I consider the engine to be fully up to temp. This is all 'A' roads and country lanes so no stopping in traffic either.

[R26Andy]

Had your previous mpg been averaged over a long long time? When the techs done the work they would have disconnected the battery and reset the mpg so its now only an average of winter driving since you got the car back hence lower mpg. Might be wrong but just a thought.

Andy