I have a 1999 Focus 1800 Zetec manual estate and had exactly the same
problem in 2003 after 34,000 miles. The problem was then cured by
cleaning the "throttle body and idle control valve". This after a mobile
computer diagnostics check revealed no engine management fault.
However, the problem has now reappeared at 48,000 miles and the above
cleaning has again been carried out by a different garage. They say that
whilst cleaning may cure the problem, the underlying cause has something
to do with a malfunction of the air conditioning switch which should make
some revs adjustment when turned on.
The above cleaning was carried out today and I will know in a few days
whether it has been successful again.
I should also mention that both the above situations have been preceded
for a time by what feels to be a reluctance for the revs to reduce when
taking the foot off the accelerator to change gear.
Any comments on the above by anyone having the same problem would be