If a diesel has black smoke, there is not enough air for the amount of fuel it is getting. If it was getting too much fuel, it would still rev when not under load. You may remember seeing big road tractors with black exhaust when pulling hills. That was caused by putting more fuel than the engine actually needed for the amount of air they were getting but they still had good power for pulling. In your case, I would say you are not getting enough air for the amount of fuel. There is a restriction somewhere. The turbo will not be in the game at this time as the engine is not pulling like it is when on the road. The restriction could be either the air intake or the exhaust. Not blocked completely, just restricted. If the fuel was restricted, the exhaust wouldn't be black.

My $.01 worth.

Paul

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84 500 SEL (307,xxx miles)