My 90 Jetta 8V/Digifant has developed a problem recently. The engine was
running great until a few days ago. Now, the engine runs rough, misses, and
has brown/black exhaust when hot. The exhaust smells like unburned gas, so I
'm assuming the mixture is too rich. The engine starts and runs fine when it
I replaced the cap/rotor/wires and the blue temperature sensor. I checked
the ground at the left rear cam cover stud, and it looks good. The wiring at
the Digifant control unit connector is dry and secure. When hot, the engine
runs just as poorly with the blue temperature sensor connected or not
connected. However, the hot engine will run well when the O2 sensor is
disconnected. There are no visibly broken wires in my engine bay.
Is this a sign of a failed O2 sensor? Has anyone had hot running, rich
mixture problems similar to this? I think the engine is running in open loop
or "warm-up" mode with the O2 sensor disconnected. Can the O2 sensor make
the engine run too rich if the sensor fails? The O2 sensor wiring looks OK
from the connector to the sensor, so I'm guessing the problem may be with
the O2 sensor.
How long can I run with the O2 sensor disconnected? My RV engine has the
sensor in the exhaust manifold. I can get a wrench back there, but is this a
job best left to a pro with a torch or other tricks? I don't want to round
the O2 sensor, then ask a professional for help.
Thanks for any help.