Mk2 GTI 16v K Jetronic

I have a problem with my 16v which is proving very difficult to sort. When hot it will suddenly start revving up and down between tickover and 2000rpm. I have reset fuel pressures, checked for Vac leaks, replaced Distributor, EMU, Idle Control Module (the one in centre console) the Idle Stab Valve is buzzing (all the time) and the only thing which seems to stop it happening is to remove the Wiring Plug from the back of the Overrun Cutoff Valve at the back of the Air Filter Box.
Does anyone have any idea if this Cutoff Valve would cause the problem or if it could be the ISV causing the problem.
Any help / advise would be much appreciated
Peter

 

When the throttle is closed is it clicking the micro switch on the throttle body to activate the idle control circuit. Check it when the engine’s warm to simulate the fault. A helper might be handy to actuate the throttle from the pedal while you listen for the click.

 

Thanks for the replies, the tho throttle arms move freely and the micro switch clicks and tests out correctly. The thing is this revving up and down happens with the throttle closed so it seems that is must be something to do with the ISV or the Cut Off Valve what i don't understand is why disconnecting the wiring from the Cut Off Valve stops it happening !!!!.
And should the ISV buzz all the time or only when the throttle is shut ?

 

Did you try unplugging the isv to see if there is any improvement?

 

Bri with the ISV connected it has an unstable tick over ranging between 1000 - 1300, with it disconnected the tick over drops to 800 but it is stable.
Does anyone think this may be that the mixture setting is to lean or it there anything else I could check ?

 

I had a similar issue on my 16v, but, it was more a consistent high idle rather than fluctuating idle. Was a while back and it turned out to be a vacuum leak. Took me ages to find it. After checking every mm of hosing and every nook and cranny in every boot it was found by chance. I sent my WUR off to KMI to get it refurbished and when I spoke to them they said they had found a massive vac leak inside the WUR. Might be worth checking again for a vac leak?

 

AFAIK with throttle switch closed, and rpm above 1400, the ECU opens the cut off valve to bypass the air metering flap and stop fuel injecting.
when this happen, RPM goes down, below some RPM cut off valve closes, fuel injection is restored, ISV opens to prevent engine from stalling, RPM goes high, ECU opens cut off valve.....and so on.

any chance your idle speed is high enough to trigger the overrun fuel cut ?

 

So could be a vacuum leak increasing your idle, then cycling as 2swe writes above.

 

Thanks everyone I think it was a combination of Ign Timing being way out and the Tick Over being set to high, it seems to be OK at the moment