1994 Jetta crazy MIL

I also own a 1994 Jetta. Mine is a 2.0L 5 speed. I don't know what you have there. Anyway, can you find out what the code is with the OBD-I connector and a OBD reader? You might have a bad data wire with an intermittant connection on the O2 sensor plug right above the passenger side motor mount. I had to solder mine back on after I realized that the new O2 sensor I installed was doing the same thing. It's the connector that the O2 sensor connection into above the mount.

the best thing to do with a MIL light coming on is to get the error code with a code reader. otherwise you will just be foolishly guessing and throwing money at the car blindly.

What's the fault code?????

I don't actually know the code, but I know my mechanic had to wire it up to blink out the codes manually because he tried like 9 readers and none of them would pull the code. But the exhaust was replaced because my catalytic converter was toast (you could hear the crap inside rattling) and the belt was the original after 125,000 miles. The mechanic isn't jerking me around as far as I know, but I am planning on returning to him soon (he promised to give a reduced charge if the light came back) I'll suggest checking the O2 sensor/wires and the PCV valve which I read somewhere else on this group might cause this.