Remove the IP inspection plate and crank the engine to make sure you have movement of all the pump elements.

Rare, but the IP pump's crankshaft could have failed.
https://www.youtube.com/watch?v=o4KMb8YOr6Y

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022