1998 Citroen Xsara engine revs keep rising and falling

Any lights on?
Does it do it when you press the brake?
If no lighter are on then listen for air leaks whistling.
If you can hear something try wiggling the vac pipes.
Could be idle stepper motor

I'd think you have a vucuum leak it could be your brake servo leaking when you press the brake. That would make it rev for sure.

Come back with your thoughts and I'll try to help some more

Plz let us know if you have solved the issue

thanks for your suggestion, having lights on no change.
cannot hear an air leak.
braking does not cause it,but my previous xsara had more assistance on the brake pedal,this car has a harder pedal, could the fault be the servo

If the servo is leaking internally you will have a hard pedal. And it will not retain the vacuum after the engine is turned off.

After turning the engine off you should have 3 or 4 pumps of the pedal before it goes hard.

Can you hear a hiss when you press the pedal with engine just turned off?
Try removing the servo vacuum pipe and blank it off. See if it runs and idles better.
But beware you will have no brakes or very little so don't drive it like that!

i tried the pedal after the engine was switched off, it took 4 pushes before the pedal was hard.
so it seems it is not the servo ?
any other ideas please.

Without any em lights on its gonna be hard to diagnose over the net.
It could be a sticking throttle body/stepper motor
Sticking idle control valve

I just don't know where to suggest.

Have you had someone look at it?

thanks for your help, ill get it looked at.
goodbye

Ok sorry I couldn't be of help