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Your ALDA may be leaking or have a failed internal capsule. It's easy to test for leakdown with a MityVac, but harder to check shaft travel unless you have something to compare to. The other possibility is that your injection pump needs calibration and the ALDA is fine. This is not uncommon after 100-200kmi, the IP's don't stay "in tune" forever you know.
Simple ALDA test: Disconnect it entirely and test 0-60 again. If the power drops even more, the ALDA is doing *something* and is either partially failed, or the IP is bad. If the power is the same with the ALDA disconnected, most likely the ALDA is not functioning at all. You can't buy a new one from Mercedes but you can buy one from Bosch if you try hard enough (they're NOT cheap though.)
I have rebuilt the old style ALDA, used on turbo diesels up through 1987. The 1990-up type are difference and I have never seen one.
Good luck,
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