Roger, one way or another I have had contact with a few Plus 4 SS owners and it seems that the issue did not apply to all cars. It seems that whatever caused the problem was right on the borderline of causing a fuel smell or not however it is almost certain that none of the production cars would pass an MOT emissions test before the calibration (more than just the fuel map) was updated.

The lambda sensor is absolutely critical to the correct funtioning of the fueling system on modern cars and the ECU adjusts the mixture within preset parameters to give the best power and to control emissions. If the ECU is not calibrated to the entire system of intake, cams, and exhaust system, the preset parameters of the calibration may be outside the range that can be adequately controlled by the ECU. Any changes to any of these elements such as the alteration of the cams, the addition of a sports exhaust, or even a different airfilter may throw the requirements of the fuel/air mixture outside of that which the ECU can cope with thereby requiring a revision of the calibration. In simple terms it may just be the fuel map but could also include the ignition (spark timing) map and other parameters to bring the calibration within the range that the feedback from the lambda sensor can accurately control the fuel mixture.

There is quite a good explanation of the functioning of lambda sensors at http://www.lambdapower.co.uk/TechNotes/Tech-1.asp. For anyone who is interested it is well worthwhile reading up to page 11 which covers petrol engines. From 12 to 15 covers diesel engines.

The index is at http://www.lambdapower.co.uk/TechNotes/technotes_index.asp

Peter

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