The pump doesn't seem noisy (to me).

I was not able to measure the pressure yet, because the fuel pressure gauge set I've got is working for GM, Ford... no Bosch fitting.

I was trying to build an adapter by buying and extra set of screw + joint which close the top of the fuel distributor (to thread it and have my gauge adapted to it) but MB doesn't detail that screw...

What kind of fitting is required to monitor fuel pressure?

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From 300CE to SLK230K