# Troubleshooting a lean condition, Command AFR unresponsive?



## Systemlord (Oct 4, 2015)

Hello everyone,

I have been troubleshooting a lean condition ever since I bought my 2007 Mini Cooper S R56 back in early 2013, bought it with only 45,000 miles and now I have 64,000 miles on my Mini and change oil every 5000-6500 miles using Mobil 1 0W-40 Euro blend. I have my valves walnut blasted three months ago, I don't burn much oil between oil change intervals. At first my completely stock Mini Cooper S knocking was barely audible and seemed like it was a single very short duration knock only on hard acceleration in cool and warm weather, it got worse as time went on. I have always used the highest octane available, 91 octane from Chevron or Shell V-Power.

Now it's getting progressively worse and is now happening in multiple cylinders simultaneously in secession, hotter weather exaggerates the issue and I strongly believe it's do to a lean condition. I have done a ton of research since then and have even logged files using Scan-XL Pro. I have two logs that captured the lean condition for which I have corresponded with someone very knowledgeable that finds the strange behavior perplexing, building boost, speed and RPM show that my AFR's seem stuck or fixed at 14.6 all the way to redline! The two log files are 1 frame apart from each other, Command AFR is pretty much a flat line 90 percent of the time.

However between 1-4 psi boost the AFR does dip ever so slightly but for a very, very short time probably measured in .2 seconds. This past Friday I installed a brand new MAF sensor after performing an oil change while my ECU was unplugged from the battery for a few hours, I have never had any CELs go off or and such codes stored in the ECU for my knocking and or pre-ignition events. I separate the knocks from the pre-ignitions because they sound different, knocks are minor, the pre-ignition is much more pronounced and grab my attention! Now the real shocker is yet to come, my Mini only knocks or has pre-ignition ONLY in third gear! Yeah, weird huh?! 

In my opinion I strongly believe the knocks are minor ones most of the time 1-2 90 percent of the time, but when these log files were recorded it felt like to me that the fuel ignited way to early (pre-ignition) which was followed by a knock rather than a knock caused by bad gas or low octane. It wasn't that hot when these log files were recorded, 85F isn't hot in my opinion. It means my AFR's at coasting speeds on a level freeway (14.6) using cruise control are the same when I'm attempting to building boost, RPM and speed. The two graphs will show me attempting a WOT run, then out of the blue knocking (pre-ignition followed by a knock) so bad across all cylinders that boost, RPM and the MAF readings instantly drop to idle conditions.

My educated guess is perhaps an O2 sensor either might not be far enough out of spec to cause a CEL or perhaps it's not out of spec long enough for the ECU to detect it. The research I have done has been on the sensors that would typically effect injection duration do to all the sensor data relayed to the ECU, mainly the pre-cat O2, post-cat O2, MAF, MAP, Crankshaft and Camshaft position sensors. Many sources state, "many O2 sensors that are badly degraded continue to work well enough not to set a fault code, but not well enough to prevent an increase in emissions and fuel consumption. The absence of a fault code or warning lamp, therefore does not mean the O2 sensor is functioning properly."

Any help would be greatly appreciated!

Thank you.


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