90 Jetta hot running problem

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 is cold.
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.
Andy
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Had the same problem with my 90 Golf.
It was the 02 sensor.
Very easy to change. It's a 11/16 wrench or was it a 13/16.
Has yours been replaced before? Does it still have the original wiring harness?
I ask as you can get either the OE type which will plug right in or the universal one which require you splice in 3 wires.
The savings between the two can be over $100 depending on where you get yours. I got a universal Bosch for $52 with shipping.
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wrote:

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as for how long you run the car without changing the O2 sensor, the sooner the better...a rich mixture will continue to burn as it goes down the exhaust and will cause the cat to overheat and destroy itself....had the same problem with a 90 golf, bought the car and it failed the emission test due to a rich condition. The cat was just a hollow shell.
Glenn
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Universal is Bosch 13912. On my '89, the sensor screws into the exhaust manifold. I used a 22mm wrench :)
Place the closed end of the wrench over the sensor. Place a pipe on the open end of the wrench to make up the distance and hammer away. If it's on the cat., can't help.
Darryl.
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