Hi all,

A 2005 Fabia (auto) with a 1.4 16v BKY engine is, when revved, dropping to a low and lumpy idle momentarily  (c600RPM) before picking up again. At higher speeds, stationary and in neutral, revs fluctuate approx. 100 RPMs with the pedal held in a constant position. No dash warning lights show but a hand-held Fox scanner did show a fault with the MAP sensor (I forget the fault code number but it used the phrase 'implausible correlation' in the text box next to the code). Cleaned and put back in the problem persists, albeit less noticeably. Live data from the scanner shows manifold pressure cycling up and down; at more or less the same rate as the RPM changes. My question: is this a cause or effect of the fluctuation? That is, is the manifold pressure rising and falling as the revs change or is it supplying incorrect information to the ECU and thus causing it to adjust the mixture accordingly? Bit chicken and egg really.

All advice gratefully received...

False air from vacuum leak, probably. Start by feeling the brake servo hard plastic pipes at their terminations. Will probably discover splits.