Why does mondeo rev on its own when turning full lock?

the engine should speed up a little to compensate for the power steering, likewise the revs should increase as the car starts moving, but the change should be subtle and hard to spot.

check for an over tight throttle cable. look at whether there are any air leaks that would confuse the system, and maybe a clean of the throttle body.

I'm guessing this bit, as I know next to nothing about power steering - but when the wheels reach full lock, the pressure being produced by the pump needs to be dispersed, to stop it ramming the wheels hard against the stop. I'd say that the engine is working hard to get to that point, running the pump at full pressure, then when the pressure is dumped the pump suddenly becomes very easy to turn - and thus the engine over revs, until the management system works out what is going on.

Perhaps the steering rack / wheel assembly connections are 'tight' making the pump struggle to move them.

As I said, this is pure speculation on how I think a power steering probably works - could quite well be a red herring!

I've noticed some old (not fresh) black greasy deposits where the pipes attach to the power steering pump. Nothing fresh and not loosing any fluid.

It wouldn't need to lose fluid, the pressurised fluid would pass through a relief valve and back into the reservoir.

Whats the fluid like?

Looks dark.

Should be thin, usually red. Main thing is that there isn't a great deal of frothyness which might indicate bubbles in the system

"Mrcheerful" wrote in news:nK8Lm.4500$ snipped-for-privacy@text.news.virginmedia.com:

Isn't there a gizmo on the firewall that raises the revs to compensate for load? Maybe it is being a bit exuberant.

AFAIK it is a pressure switch, it is either on or off, it isn't variable. certainly the op could just pull the connector off and see if that improves drivability, but it won't be fixing the fault, just disguising it.

In message , Mrcheerful writes

You are right, its just an on/off pressure switch. It sounds like its working fine. When it fails, the revs drop not rise when the steering is moved.

The revs shouldn't rise to 2k though. Looks like the OP has another problem..... ICV looks favourite.

When on full lock it revs up and down between 1000 and 2000 rpm just like they do in films at traffic lights when they want to race someone. Even worse if aircon on too.

That sounds as if the idle speed control valve's sticking. If cleaning fixes it temporarily then change it.

Duncan Wood pretended :

I thought, as everyone had failed to mention the IACV, that this was a very old model without one. Now it has been mentioned - I agree. The IACV is sticking open. Remove it - just two bolts and one electrical plug, give it a good clean out inside with carb cleaner, then squirt some thin oil on the rod inside.

Thanks I'll do that. Should ahve said its a 1999 1.8 mondeo