I've had a wide-band AFR (air/fuel ratio) gauge on my R53 for several months now.....and I've noticed that sometimes the AFR's get kind of screwy.....is this caused by a bad O2 sensor on the car?

1. I've noticed that a few times a week if I'm sitting at a stop light....as it's idling right around 14.7-15.0.....all of a sudden the AFR's will start to slowly go rich. It will get down to around 13.5 or so...and then after 10 seconds or so it will creep back up to 14.7 (what it's supposed to idle at). Sometimes it goes the other way.....it will start to lean out and creep all the way up to 18 or so (where is starts idling rough), then after a few seconds it will go back down to 14.7. This is all without touching the gas. It might do this once or twice a week.

2. Sometimes as I'm cruising down a flat and straight section of road at a constant speed (around 40mph) the AFR's will jump from 14.7 up to around 16 and then back to 14...it will do this several time really fast and you can actually feel the motor surging a little. After a few seconds of this it just goes away...and everything goes back to normal.

I've never gotten any check engine lights, and other than these two quirky things it does a few times a week it runs like a top. I'm running a canned "quick tune" from WMW......not sure if it acted up like this before the tune or not...never really noticed it until I started watching the AFR gauge.

Could this be a failing oem MINI short band O2 sensor? I would think that I would get a check engine light if that was the problem.

Ideas?

 

Nobody else has an AFR gauge on their Mini?

 

I had this issue on my car (non-MINI). What gauges do you have? I have the AEM AFR gauge and my tuner doesn't necessarily like it. I did notice though after a track day that my gauge would be a bit weird. At idle it would go from 14.7 and dip into the lean bracket towards 16-17ish and then correct itself back up to 14.7. If I sat there and blipped the throttle a little and right when I let off it would go to 16-17ish and then correct back to 14.7. It would sometimes go towards the rich route too, but I knew something was a little off as it normally wouldn't go all over the place like that. My tuner said, since I am on E85, that it can tend to lean things out a little, but it was doing it on my 91 tune as well. Other option was a vacuum leak of some sort. As it turns out, the seal on my oil cap was bad and my BOV had a slight leak. Fixed them and all is well again. So I would maybe suggest doing a smoke test to make sure there are no vacuum leaks anywhere. Just an idea.

 

I don't think it's a problem with my AFR gauge.....because when it starts creeping up and showing an excessively lean mixture (around 18.0), the motor will actually start to shutter and run rough........then as it richens back up, the motor smooths out. I think the gauge is right on target......just need to figure out what the DME thinks it needs to start leaning the mixture out for no reason.

 

It does sound like the narrowband hooked up to the ECU is going bad, or has a flaky connection. Plug in an OBDII scan-tool to check the output of that sensor along with if there are any pending codes.

 

I didn't say your gauge was the issue. Just stated my tuner doesn't like the AEM AFR gauge because he believes they don't read the exact correct measurements. My issue wasn't the gauge, but as stated it was a vacuum leak causing me to lean out. Could not be your issue though, but was just sharing my experience with my AFR acting a bit strange. Once I took care of the leaks, the AFR is now acting normal.