I've seen the volumetric flow test done at the Bosch service center, they lash the IP up to basically an engine lathe. It uses pipettes for individual ports, tiny pipes carrying the fuel (after purging air) and they read the RPM accurately and run for a specified time, all spelled out in some Bosch publication. Can one do this on the engine? I never thought of trying it but it would be easier (and cheaper) than yanking the IP and carrying it to the Bosch repair shop.
Incidentally the Bosch man (Gus) tells me that a nailing is from too much fuel. Or an open injector nozzle.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!