Good idea, 2 years ago, at the first couple of runs (full throttle on the testing machine) the soot from the exhaust was over the MOT limit. The guy thrashed it on it's road test, then in ran clean when he tested it again. Last year, I used some diesel cleaner for the 3 or 4 tanks preceding the MOT (and thrashed it a bit on the M8) and it passed ok...