doc518 presented the following explanation :
My Xantia has an intermittant problem. Sometimes, at low speed , when I
start to brake the engine seems to race and the car starts to speed up. Very
disconcerting !
Problem goes away the instant the brake is released and then I can brake
normally.

Heard of this before ????

Not that particular car, but ECU controlled cars tend to be programmed
to speed up their tickover a bit, whenever the car is moving - could
that explain it?

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk