Alright, so my '06 MCS with 87k on the clock is getting horrible mileage and, while I've only had it for about a month, doesn't seem quite as quick as some of the other R53's I test drove when I was shopping around. I average between 15-18 mpg when I should be getting at LEAST 5 more than that. Things I've done so far that would contribute to mileage:

New OEM spark plugs
Oil change with Castrol Edge 5w30
Transmission fluid change with Redline MTL
ensured tires are properly inflated / about 50% tread
Replaced control arm bushings & had car aligned
Verified no brake calipers are dragging
Replaced fuel filter
Installed Alta foam filter into JCW intake air box

Even after all of these maintenance items and improvements checked off the list, the car still gets awful mpg. I also feel that it is running rich, as the exhaust smells like fuel when I start it up and there is a decent amount of suit on the tail pipes after just a day of driving after washing.

I have an ELM OBDII module with Dash Command on my iPhone. I have read that looking at the long term fuel trim is a good way to diagnose a failing o2 sensor but I cannot figure out how to view long term in addition to short term through the app. Are there any other parameters I could be looking at?

The last thing I suppose could be contributing to this is the ignition system. Is there any way I could go about measuring / testing the plug wires and coil before just throwing parts at the problem?

Thanks!

 

15-18 mpg under what conditions? That would not be unusual in San Francisco street traffic. I was getting below 20 mpg even with a non-S R52 manual transmission, in San Francisco traffic.

 

15-18.. I might get this too. But at the moment I'm averaging 21-24 just because I don't use the ac often. When I just got the my mini I had to replace both 02 sensors due to having p2093 code. But that never solved the problem. I also did the maintenance you mentioned above but still no luck. Since then until now I'm running with the light on.. But never had any weird observation.