Over rev ving Mondeo

[Gary Bills]

I have a 2.0ltr Mondeo ghia X that when you drive at normal speed the car will rev to 3000 revs and drive itself. When you put the clutch in it will rev for a while then return to normal. I have noticed that if you take it out of climate control this does not happen. I've had the diagnostics checked and it came up with no fault found in the memory. Any ideas of what it is and about how much it will cost to repair would be very much appreciated.

[rojam]

When you have the climate control on, the engine is under more load as the air con pump cuts in, the engine idle control cuts in to return the revs to the correct idle speed when you take your foot off the accelerator. The Zetec engine can suffer from the idle control valve sticking. It could be that the idle control valve needs cleaning out (they do gum up), or replacing.