Runs fine when cold and then when warm loads up on 1995 Ford E-350

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Starts fine and runs great for 10 to 15 minutes. Then it starts running rough and idle goes up a little and has black exhaust. Definatly loading up with fuel. Lack of power. Feels like tranny slips when running rough. May be just lack of power. Feels fine when cold.
(1) Answer
Basically one of two areas needs addressed.

My first thought is basic - has to do with the amount of air verses fuel ratio that is required for a cold engine verses a warm engine. As the engine warms, it gets more air introduced to mix with the supplied fuel. It is possible that as it warms it is not taking in enough air. Check for sticking idle air control valve or an air intake restriction.

Second thought, and more likely probably -- as the engine warms, it goes from "open loop" to "closed loop" in the engine's processor. The processor (PCM) looks for a certain engine temperature to be achieved, and then goes into "closed loop", that is, depends on its various sensors to send information in that helps the PCM adjust fuel, air, spark, transmission shifting commands, etc.

Sounds like your problem does not occur during cold, "open loop" running. When it warms and hits "closed loop", your oxygen sensors, for example, figure into the fuel / air ratio calculation, and could be throwing things off.

You need a professional diagnosis. A qualified technician would probably monitor short and long fuel trims with a proper high quality diagnostic tool.

Good luck!