It sounds like your idle air bypass valve is not working properly. It
is located on the intake manifold and is hard to get to on the Zetec.
I used to remove and clean the one on my Escort when it had this
problem. It might not be getting the right electrical signal from the
computer either, but more likely it just needs to be cleaned or
WHAT IT DOES: 1) Fast idle...It passes air past the throttle to rev
the engine at idle when it is cold. 2) When a load is put on (like
your power steering/AC, etc, it maintains a normal idle speed. When it
is working ok, the idle will be pretty stable when turning the wheel,
putting on the AC, lights, etc. You can try disconnecting the battery
for an hour or so to have the engine "re-learn" how to idle. Worth a
As to the battery corrosion, you do not likely need a new battery. Put
on some latex gloves as the acid is nasty and take off the terminals.
Soak the terminals in a baking soda paste solution (add water) and
neutralize all the white deposits. When the bubbling stops, rinse with
clean water. Clean the battery top and dry. Don't get baking soda in
the battery, as it will weaken the acid strength. Dry the terminals
and clean the contact parts of them until shiny with a battery brush
from an auto parts store. Put the terminals back on, tighten and coat
them lightly with petroleum jelly to retard future corrosion. Don't
overfill the batterey with water (distilled only) as this causes acid
vapor to blow out of the vents and you will be doing this again.