I've had this problem for quite a while. Fixed many other problems but this one just wont go away.

Watching the oxygen sensor via snap-on scan tool, when it shows Lean, the car idles smooth. The second it switches to Rich, the car stumbles, sometimes even stalling out. But if it switches back to Lean, it idles perfect again. This happens sometimes in Park/Neutral but more so in Drive while holding the brakes.

I've changed almost every sensor in the car, tune-up, ecm (when it died a while back). Plenum gaskets, injectors.. List goes on. I've heard there may be an updated PROM chip. Mine is #7221. If needed I'll write out the whole list of what I put into it. Any information would be greatly appreciated! Thanks

 

I always tell people to get a 3 or 4 wire O2 sensor when problems like this come up. Check to make sure the base timing is at factory spec (disconnect ESC wire...) and check for vacuum or intake pipe leaks.