I've posted this problem here 2 years ago but havn't solved it. "90 f150, 4.9L, 5 speed, started running rough, like poor idle, missing, 6 years ago.. Not all the time. Most times runs oK, but I always get check engine (code 41 O2 sensor running lean).I replaced O2 sensor, EGR, tune up. I did KOEO check on TPS, MAP sensor, coolant temp sensor. Fuel pressure is OK. Last week, I put a volt meter on the O2 sensor and watched over and over again: signal is < .45 volts at startup. Then when engine is warm the signal for O2 starts shifting between .2 and about .65. System is in closed loop. After a while, the engine will idle rough likes its about to cutout then suddenly will smooth out. The rough idle doesn't happen all the time. But every time the O2 reading shifts to .7 volts (rich) after a few minutes and stays there. When I open the throttle, the reading shifts to lean and stays there. The O2 sensor reading does not switch hi-low after that. And the truck runs good, until I shut it off and then it starts all over again. I don't know where to look next. BTW the truck is exempt from emission inspection because of the yearly mileage.

 

Have you checked the intake for a vacuum leak? I have seen this on the 4.9L motor, where the two parts are bolted together. Try spraying choke cleaner around it while it is running and watch the O2 readings If you hit a leak, it will go rich.