I recently did a major tune up on my Volvo 850. As part of the tune up, I replaced the 02 sensor. Not 1000 miles after the tune up, my 850 started blowing black smoke and getting roughly 11 MPG. I think the smoke is the result of too rich fuel mixture. I removed the 02 Sensor and it was completely caked up. I cleaned it with a torch and the problem went away. I'm convinced it will shortly return.
My question is:
What is causing my engine to run too rich thus clogging my 02 Sensor? Is there anything I can do to reduce the fuel mixture?
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