B7 RS4 Lambda correction

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Jan 16, 2010 | 11:27 AM

silverRS4

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B7 RS4 Lambda correction

I log this occasionally and its usually -2% to -4% under load, which is fine. I had assumed that the (-) sign indicated that the correction required to achieve the target air-fuel ratio was negative. In other words, the actual injector pulsewidth was 2% to 4% LESS than the base table pulsewidth. However, recent logs have made me question my assumption. Is negative correction subtracting fuel, like I thought, or adding fuel?

Jan 16, 2010 | 05:53 PM

Dana @ Ross-Tech

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Please post an auto scan from this vehicle so we know what module is installed. Please include the measuring block are you referring to.

If you are referring to the data typically found in Measuring block 032, a Negative value is the indication of a rich condition and the corrective action would be to lean it out. Our fuel trim page:
http://wiki.ross-tech.com/index.php/Fuel_Trim_Info

Jan 18, 2010 | 06:36 AM

silverRS4

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Thanks. I'm logging Group 1 Field 2 "Lambda Control - Bank 1". It is not the Group 32 adaptive correction, it is the real-time, wide-band correction for cylinders 1-4. Each bank has separate wide-band lambda feedback. The RS4 does have adaptive trim, but rarely operates in open-loop and runs closed-loop constantly once the O2 sensors are heated, even at high rpm. Knowing the mass air flow, the RS4's tables merely have target AFR's based on RPM and load. I'd assumed that the Lambda Control was the "instantaneous" trim to maintain the target AFR and that negative control was leaning the mixture just as negative adaptive trim does. Your comment seems to verify the difference between positive and negative trim (or control). What threw me for a loop was a recent log from a different RS4. The mass air flow was ridiculously high, about 10% higher compared to a healthy RS4; but the Lambda control was -13%. The RS4 in question had a modified air box. I'm thinking now that the mass air flow reading was erroneously high leading to a rich condition, which the wideband feedback is continually correcting with a large, negative Lambda Control.