1991 Jetta idle adjusting

First clean the idele stablizer thing on top of the valve cover. Then clean the throttle body. Use carb cleaner for both.

Joe

the throttle body. Use carb cleaner for both.

That's a good start but there may be more to do.

I just went through this on my 1991 GTI 1.8 but the issue was that my idle was fast, not slow. I did the above and it made the issue worse (idled even faster). Nevertheless a cleaning of those items is a good thing.

I then pulled off the crankcase vent hose from the emission control valve (the round plastic thing sticking out of the valve cover) and removed the emission control valve and cleaned that too.

Then, I also reset the Digifant unit. However I have Digifant 1 (California) and the Bentley doesn't list any reset procedure for the Digifant 2 unit (Canada and 49-state US). But just in case it works for both Digifant 1 and 2, here's the procedure.

1. Warm up the car to normal operating temperature and then shut it off.
2. Remove the crankcase vent hose from emissions control valve as described above. Plug this hose with something (I stuffed a screwdriver handle into it).
3. Start engine.
4. Disconnect the blue temperature sensor wiring harness (located on the coolant fitting between cylinders 3 and 4 (there are two of them...a blue and a black one...just do the blue one)). But do not do this if the radiator fan is running...wait until the fan is not running.
5. After unplugging, let idle for at least 1 minute.
6. Reconnect coolant sensor harness (but again do not do this if the radiator fan is on...wait until it stops).
7. Stop engine.
8. Reconnect hose.
9. You might have a fault code stored for the coolant sensor if your ECU stores fault codes (not likely for Digifant 2...only some California Digifant 2s had this and also California Digifant 1s). No big deal but you should erase it eventually. If you don't have a "check engine" light your ECU probably doesn't store fault codes.

I did all of the above (haven't reset the fault code yet) and my idle is spot-on now and is super smooth.