revs up and down

[S3 chj]

hi my old mk3 golf gti 16v keeps reving up and down its goling from 500rpm to 2000rpm any ideas what causes this and how to fix

cheers chris

 

[a19hur]

This may be a bit late now but i the same problem with vw polo mk4 16v people call it 'hunting' for revs and i managed to cure it, firstly i removed and cleaned out my throttle body which didnt actually cure it but when fixed it revved smoother, i then reset my ecu memory which i did by disconnecting my battery for a day (may have to hook urs up to diagnostics im not sure) Still not fixed!
Then i changed my engine coolant temp sensor which was defective and giving wrong signals to my ecu, all sorted for £30 and a little bit of time and effort.
I found this info on a polo forum and it was process of elimination for the three things i did. Hope your problem is all sorted now, cheers Andy