I was trying to solve this problem for a couple of days and found what causes it. I have an 1998 fiesta with the Endura-e engine (the same is one installed in most KA cars):
- it is caused by the throttle position sensor (tps) - a small black plastic piece with a three-wire plug which is located on the back side of the throttle body.
- in my case it was not faulty because i've taken it out and measured it with an ohm-meter, nor was the connector faulty. I've read that sometimes the connector itself can fail, and that there exsists a revised one.
- it was the wrong resistance of the tps in the idle throttle position that was causing it - it seems that this is a design flaw and that TPS is set to work near the "edge" and over time when the material changes its properties this causes it to go over the ege - and overrev.
The TPS in endura-e is a potentiometer which is connected like this:
- one pin is connected to the minus (chasis)
- the second one is connected to constant 5 volt source from the ECU
- the middle pin is the slider and changes the voltage as the throttle turns. When you turn the key (give a contact) and measure the voltage between it and the chassis in idle, it has to be 0.4 V. In my case it was 0.55 V
You can adjust this sometimes by loosening the screws on the tps and moving it to the side (it has a little play)
What I did (and I don't recommend it
is to solder a 5k ohm trimmer pot between the slider (middle pin) and a minus pin (the brown wire in my case) and with it I can regulate the idle voltage from 0 to 0.55 V and by experimenting i could clearly see:
- if I set it above 0.4 V it has a tendency to revv a little longer
- if I set it above 0.48V it is overevving madly
- if it's at 0.4V - its fine
Of course you shouldn't turn the trimmer too low not to short-circuit the ecu source, and if it's set too low the engine might have a tendency to stall at idle. AND NEVER DO THIS if you don't know how to fit it firmly and to insulate it properly.
First try loosening the screws (just a little) with a star screwdriver and tapping the tps so that it moves in the same direction where the throttle turns when increasing the gas. Be careful not to spray the tps with wd 40 because it will make the problem even worse (it affects it a little) and if you're removing it remember to look carefully where the axle sits in it's slot because if you mount it back just a little off it could make it worse.
PS My car is still sometimes stalling when idling, but I think, This is an another problem (with the idle air control valve)