After changing O2 sensor, car eventually (15 mins?) quits running smoothly on 1992 Ford Explorer

Took my 1992 Ford Explorer XLT (4-door, auto 4WD, auto tran, 4.0 V6) for smog test (California) and it failed. It seemed to run fine, and indeed everything passed EXCEPT "NO (PPM)" -- Max 699, measured 2342! So, it's a "GROSS POLLUTER." (I now suspect this was caused by an unoticed missing 8" "intermediate" pipe in the exhaust system.) I'm over a barrel (very broke, NEED to get the Ford running and licensed), so the tech suggested it probably is a bad Oxygen Sensor (the primary one, before the catalytic converter). I coughed up the $50 for a new one and replaced it. After replacement, as always, the Ford ran fine ... until about 15 minutes later, while driving. Suddenly, a clunk, power drops, engine runs rough and smells of fuel, and the "check engine" light comes on. I nursed it home. The next morning, I try the Ford again. It starts, runs okay, then after about 15 minutes of idling, the same bad thing happens: sudden clunk, power loss, rough running, fuel smell, 'check engine' light. I then re-installed the original oxygen sensor, hoping to solve the problem. Now the same problem, after another day: it starts, runs, then after a few minutes, loses power, check engine light comes on, it chugs and knocks. Summary: I put in a new O2 sensor, and this problem began. I removed the new O2 sensor and put the original one back in, and the problem continues. BEFORE I changed any O2 sensor, the vehicle ran good and was reliable. Anyone have any idea what I probably did wrong, and how to fix it? Could it have something to do with the computer not recognizing the sensor?

by in Fresno, CA on February 06, 2014
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1 answer
ANSWER by , February 07, 2014
sounds like poss the exhaust is plugged which gave you the o2 code. have the exh ck'd by a muffler shop to start with