if its a 88on 8v GTI, check the idle switch clicks just as the throttle shuts, and check the wiring to the back of the throttle body hasnt snapped. if the switch doesn't click, check the throttle shuts firmly to the stop, if it does you'll need to adjust the position of the idle switch a tad. the switch is right underneath the tb, so you have to remove the rubber boot and use an angle screwdriver to get to the screws. Good opportunitly to clean the throttle body and check all your vacuum hoses while the boot is off