Ok, I was just about to ask about this.

I see where the line from the fuel pumps coming in at the rear of the fuel rail. Just in front of the FPR is a cap fitted onto the rail. I'm assuming this is where the pressure would be measures.

What type of fitting is this? Is there a special Mercedes tool that has to be used, or can a regular gauge be hooked up?

Cheers

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1991 Japanese S600
Resides in Kampala Uganda