Ford Ranger 1999

Having replaced the ECM with a life time warranty rebuilt unit the engine starts and runs smoothly. Hold it at 3000 rpm and after a few minutes it drops to 2200 rpm and runs very very rich - black smoke from the exhaust, disconnecting the air flow meter has no effect, disconnecting the cool start has no effect, disconnecting the oxygen sensor has no effect. After about ten minutes the check engine light comes on. When it is cold the same process happens, when it is hot it runs rich. Why would it run perfectly then suddenly run so very rich?

What OBDII code(s) are you getting? The codes will lead you in the direction to troubleshoot. This could be a misfire, a mass air flow meter issue, an idle air control issue, O2 sensor problem...etc...etc. All blind guesses until you scan the codes.
G/luck
Joel