Check engine light comes on...BMW clears the light and says the O2 sensor was running the fuel lean...they clear the light...I pick the car up and from that day forward I cannot manage more than 23.5 mpg when the car has averaged 27.8 for 85,000 miles. No other codes have tripped the check engine light.

Any hints here? The tail pipes are black as coal. This is a 2005 JCW and feels a wee bit sluggish...as lots of extra fuel will do.

 

If you figure this out, please let me know; I've been fighting this for months. The exhaust fumes on startup make my eyes water.

 

will do

 

Perhaps they flashed your ECU with new software?

 

Black, sooty exhaust tips and lousy fuel economy would indicate an overly rich mixture, not a lean mixture.
My '05 MCS was getting lousy mpg, check engine light was on. Replacement of O2 sensor fixed it.

 

85000 miles is a lot on an O2 sensor. Running rich kills them quicker. They degrade gradually over time. As they get older, the reading changes so the ECU thinks it's running lean, and starts richening the mixture, accelerating the sensor death. It'll get to a point where it can't richen the mixture any more and you'll start getting codes-but the codes will be lean because the reading is off.. It's not that the mixture is lean, just the O2 sensor reading lean because it's old.

So another vote for O2 sensors.

 

It seems to me that meb's MINI's chronology is as follows:
1. Car throws lean CEL
2. Dealership clears code
3. meb immediately notices sluggishness, lower fuel economy, and soot in the exhaust

Could be that the O2 sensor was replaced by the dealer and the ECU is in the process of adjusting... But seeing as how the car is at 85K miles, it's unlikely that it was covered by the factory warranty (so seeing the bill would have immediately ruled out that possibility).

 

I know I have always had bad mileage and plenty of soot on the pipes. Of course, it is still within tolerance so "nothing can be done" but I would sure love to be able to get over 27 mpg on a long highway trip for one or maybe break 22 in the city.

 

Since it's out-of-warranty, when he went in for the code, he would have had a pretty good bill to replace the O2 sensor and not just a code reset. I'd be more willing to guess that at 85K, and unless he says he recently had the O2 sensor replaced, that it has got to be bad and replacing that would be a start.

 

I replaced all the O2 sensors in every car I owned after 100,000, even if there were no problems.

So here's something interesting. All the sudden - today, the mileage is up to where it should be. Temps are also around 88 deg F.

A little more history...I installed a cooler thermostat a year ago. 8 weeks ago I installed a new stock thermostat and the engine light went off after about 300 miles of driving...it went on about a month after installing the cooler thermostat. When the check engine light came back on four weeks after reinstalling a stock thermostat I was suprised and was told it was running lean...or the O2 sensor indicated a lean condition.

Perhaps the engine simply needed a month or so to understand the changes...6,000 miles of driving in all. Back up to 27.8 and a spunkier throttle too!

 

Could be from fuel... Try running some Chevron Techron Fuel System Cleaner (big bottle) through for your next two fill ups of PREMIUM fuel. If it doesnt improve, you've not wasted anything, and could be a lot cheaper than a new 02 sensor.

 

I use this every other oil change, and just before the oil is changed...it's time!

 

when they replace o2 sensors at the dealer they usually clear all the DME adaptations, so it takes a couple of drive cycles for the computer to really learn in all the parameters and bring your MPG back to normal.

 

I don't think he said they'd actually been replaced

 

No, the O2 sensors were not replced. But I did find the problem...it's sort of a bone head thing because I did not catch it twice.

The mpg improved dramatically today. I thought it might have been the high temps etc. Well, over the weekend, I re-built the entire back end of the car; removed and cleaned all 8 rod ends, removed, cleaned the Webb bar, installed new H&R Sport Coilovers, AND, removed and cleaned the Tyrol Sport pistons that the rear brakes slide on...they were in horrible shape! I just had the rear brakes replaced at BMW about 2 months ago and asked them to please remove these, clean them and re-lube - at an extra cost, no problem.

First bone head part? I never checked to see if they cleaned and lubed these.

Second bone part was I did not look at the rotor wear...until tonight while making final ride height adjustment to the H&Rs. The rear pads are almost down to metal after 6,000 miles of driving...and this explains why the rear wheels are always so black.

Dumb all around for sure. I'll do the brake job this time!

 

good call, I was reading too fast. Either way they probably reset the adaptations after that type of a problem.

 

SO, driving around with your brakes on all the time is bad for mileage?

 

who knew I'm actually getting 5.3 more miles per gallon. That equates to about 120 more miles every week for me.

 

I had a car that I kept forgetting to undo the emergency brake (wasn't that good). Doh! It's amazing how that works! Glad you found it!

 

I hit 29.2 mpg today in my typical combined city/highway...moving along, but not hammering. I gave thought to the E brake cable...but the sticking calipers cured all other problems. Thanks!!!

 

are you referring to the tyrol brass caliper bushings? I guess im confused.

 

Yes. I cleaned and lubed these three times with 9 months. But the later part of the winter here was brutal; lots of salt and sand. So when BMW were asked to install new rear rotors and pads, I asked them specifically to clean and lube these parts...obviously not done.