I agree with BC. It has to be some kind of restriction, like the turbo seized up. A waste gate stuck open would result in loss of power, but I don't think you would have the super fuel-rich condition you are describing because the ALDA would not be getting the boost signal. Physically check the turbo shaft/impeller by removing the U-tube and touching it (engine OFF of course).

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'83 300D Turrrbo 295K miles