Where is diagnostics port on 1991 VW Golf?

There is no diagnostic port on cars fitted with the Digifant fuel injection system as this system predates the On Board Diagnostic era. Once the ignition timing, fuel mixture and idle speed have been correctly set, many Digifant running issues can be attributed to the blue Coolant Temperature Sender, which is a critical component in the Digifant system. Check and/or replace this sender before investigating further on Digifant equipped cars.

Note that Digifant systems do not rely on a Lamba/oxygen sensor to control the air/fuel mix except on catalyst-equipped vehicles.