The O2 heater is powered by the FP relay in 1990 so it should be heating
whenever the engine is running.

The ECU watches the Temp2 sensor and ignores the O2 reading unless the
Temp2 shows the engine coolant temp to have reached a warm enough level.
A bad or intermittent connection to the Temp2 could cause the ECU to run
in cold mode and I believe that will be a rich condition. Not rich
enough to kill the engine though.

The ECU uses a special ground to compare the O2 reading to and if that
special ground is weak or missing then it will get false readings from
the O2 sensor.

The special ground is the brown wire that comes out of the small bundle
going to the distributor hall connector. The brown wire has nothing to
do with the distributor. Make sure the brown wire has a good ring
terminal crimped on to it and is bolted down to clean metal on the engine.

Mark

David Clarkson wrote:
> Greeting vanagonauts. I recently (approx 500 miles ago) replaced my O2
> sensor after having a problem with my 1990 Wesfalia that caused it to
> bog down and quit running after warming up. I am assuming that this
> happened when the coolant temp sensor told the ECU to turn the heater
> off on the sensor. When I unplugged the sensor the problem went away.
> Since my sensor had more than 60K on it which is the VW replacement
> interval, I replaced the sensor. After Installing the sensor everything
> seemed to be back to normal. One other bit of info that is worth
> mentioning is that I have an O2 sensor monitor that indicated that
> everything went completely rich when the problem occurred. It showed
> this type reading with the original sensor issue and the new sensor
> issue....................