I Have an 08 R56 and i normally get about 29-34 mpg and i am now getting 24-27. I had this problom a few months ago but it fixed itself. What could this be, i think it may be the o2 sensor but i am not sure.
Cheers, Badge from SC

 

O2 sensor would be my first check , after a while you will get a CEL if the 2 sensor fails.

Then I would look at carbon build up on the intake. Do you have a Cooper S ?

 

Yes it is a cooper s, i have been thinking about the carbon build up, i have not cleaned it since i got it.

 

Carbon Blasting DIY here: 81292208038 81292208032

https://www.northamericanmotoring.co...-cleaning.html

 

thanks M8

 

Welcome.

 

O2 sensor would be far from the first check done.

When were the plugs changed last?

Carbon build up is a slight possibility.

Is the cooling system in tip top order? A coolant leak, or faulty thermostat electronics can hurt mileage.

Air filter quality?

A VANOS solenoid full of sludge can affect the function of the variable timing advance and cause power and mileage drops.

How does the alignment look? Poor alignment can cause poor mileage.

These are all things I would look at before installing a $290 Lambda sensor, especially since they aren't very failure prone in these cars.

 

Check your tire pressures. Low pressure will cause more drag. Changing tires, e.g. from summer tires to winter tires, can change the rolling resistance as well. I don't remember which tends to have more and which less, though.

What about ambient temperatures? They've started dropping where I live, and lower temps hurt your fuel economy in something like a dozen small ways.

Alignment can also cause drag, plugs and carbon build-up can affect engine operation.

Lots of stuff to test before you start throwing parts at the problem.

 

i should probably check the spark plugs and i didn't know a coolant leak could hurt mpg, because i have one. air filter is almost never changed, i always forget to. thanks for the help. ill be doing a water pump change in a little, it should fix the leak. the spark plugs have not been changed science i bought the car at 56,000 miles now it has 98,000. i do not know if the previous owner changed them.

 

Plugs and air filter are the first to check but when it was fine before and drops off quick its the O2 sensor or some bad carbon build up at some point.


If you need anything else let me know.

Here is the DIY for the water pump and the thermostat with the parts linked.

Doing them both at the same time will get two problems done at once.

https://www.northamericanmotoring.co...hermostat.html

And

https://www.northamericanmotoring.co...rmostat-6.html




Also a great link here: Thermostat , waterpipe , and waterpump DIY.

https://www.northamericanmotoring.co...8-no-heat.html




Waterpump: 11518604888


http://www.ecstuning.com/Search/SiteSearch/11518604888/



Thermostat: 11537534521 KT


http://www.ecstuning.com/Search/SiteSearch/11537534521KT3/





Waterpipe: 11537589713

http://www.ecstuning.com/Search/SiteSearch/11537589713/



Waterpipe Clip : 11537548652

http://www.ecstuning.com/Search/SiteSearch/11537548652/




Thanks

 

Just in the maintenance work you listed I can see a loss in MPG.

The thermostat is constantly fluctuating to stay thermally efficient, and provide the best MPG, any discrepancy in coolant temp can affect MPG due to changing fuel tables, timing tables, etc.

MINI states plugs are "due" every 36k in the JCW package cars.

 

thanks for the help guys, when i get my next paycheck ill be right on it. Cheers