I have a 1988 Jetta with 1.8 Digifant II engine. I have an odd problem. When ever I use the defrost (I live in Canada) the engine idles at about
2500 rpm when at a stop. When I switch it to the heater it goes back down to a normal resting idle. I can than change back to the defrost and the rpm will stay normal until I start driving again. What is causing this?
I would check the auxillary air valve for problems. I'm not 100% familiar with Canadian modeels, but it should have an electrically operated valve near the throttle body. It should have a 1" hose coming from the air boot in one side & going out the other to the backside of the throttle plate. It should have a two pin electrical connector. Do you experience idle problems when the engine first starts or has the idle screw been changed due to prior problems? You can clean out the valve with carb or throttle body claner. With engine cold the valve should be partially open, then as the engine warms it will close off. I beleive when you run the A/C (as in defrost position) it will open slightly to raise the idle. With the engine running you can check there is battery voltage across the two pins of the harness (unplug it). There should also be resistance (read with an ohmeter) across the two pins of the valve, indicating the electrical coil is still intact.
