Rough Idle and stall after warmup


#1

1993 Nissan Maxima. Great little car except when engine gets warmed up it begins to run rough and dies. At idle, when accelerating, while maintaining speed. Since its a manual it is a simple matter of dropping into neutral and it fires right up again… no problem. Then stall repeats.
Went through routine steps checking plugs, filters, wires, sensors etc… The O2 sensor (only one) is gone - as is the converter. At some point someone replaced it with a straight pipe.
After not finding a obvious answer, I began to ask around (auto-part stores, neighbors, some patrons at the saloon). Conclusion… OBD failure causing a “kill” routine which shuts off the engine. When I restart, it resets. Once it fails to enter a static state (engine is warm) it kills again.
I replaced the computer with a used one “known” to operate (so the e-bay seller said) Installed it and bingo, car ran like a top for 15 full minutes - best ever. Then after 15 minutes or so, suddenly began to run rough and stalled twice before I got back home. I runs now just as before (but oh those glorious 15 minutes).


#2

“The O2 sensor (only one) is gone - as is the converter. At some point someone replaced it with a straight pipe. After not finding a obvious answer,”

Um, since the oxygen sensor’s signal is important to the fuel metering, perhaps the answer is more obvious than you’re imagining. Is the O2 sensor plug still there? I’d start by replacing the pipe with the proper one and installing a new O2 sensor. If that didn’t work, I’d try putting the old ECU back in.