650i drop in MPG?

[OnTheSauce]

Hi all,

2006 650i has gone from a combined 22.5 to about 17/18! could barely break 20 on a run....

I note (with sadness) that the car is also blowing a bit of smoke after idling for a bit, so valve stems could be on the way out.

Could the 2 be linked? Or any other reason why the economy would drop like that? the car performs fine otherwise.

Typically, no garages can fit me in quick and I'm due to take the car to the Isle of Man on Thursday..

Not looking for an internet diagnosis, but any help appreciated!

Cheers

Edited by OnTheSauce on Monday 14th May 15:20

[Collectingbrass]

I don't know whether the two are linked but a fuel economy drop like that is worth investigating. Have you lent it to a teenager who's been hoofing it? Changed fuel brands recently? I know I get about a gallon further if I fill with Shell or BP v Superstar, and have done in a Mondeo, an E36 328, an E39 525 and two SAAB 900s. Finally you don't say how long youve had the car, have you recently put the air con on for the first time?

To be honest I'd chuck some Redex in and run it on V power to the Isle of Man and back then put it in for a service including plugs & investigate the stem seals when you get back

I used to think Redex was snake oil till I saw the 5th gear dyno test: https://www.youtube.com/watch?v=03nL8Z0dRkI

[RichardM5]

A failing thermostat can do that, car runs cool and is continually running rich to try and warm it up quicker. There is a 'secret' menu that will display the running temperature, do a google search for how to do it on your car.

[OnTheSauce]

Thanks both. Will investigate.

Ordered some Redex for the journey. Can't hurt.

Cheers

[Locknut]

What about the loss of the under-tray that you wrote about in your other thread? That is sure to cause turbulance under the car which interferes with the aerodynamics and causes an increase in fuel consumption. If the loss of the under-tray coincides with the increase in fuel consumption then you have your answer.

[budgie smuggler]

Locknut said:

What about the loss of the under-tray that you wrote about in your other thread? That is sure to cause turbulance under the car which interferes with the aerodynamics and causes an increase in fuel consumption. If the loss of the under-tray coincides with the increase in fuel consumption then you have your answer.

Nearly 20% drop though?

[OnTheSauce]

Not that simple, unfortunately. The undertrays have been off for 2 months or so, but the MPG drop has only been the past couple of weeks.

It’s bizarre, because the car starts and runs as normal.

Have got it booked in with BMW to diagnose the smoking (BMW warranty have already told me they will likely cover it if it is indeed the stem seals), and will get the spark plugs replaced at the same time, will also mention the MPG issue. Thinking about it, the car’s on its original plugs, so they’re due now anyway (58k). Hopefully that makes a difference.