A very common place for a vacuum leak on the E55’s is that port on the rear of the supercharger with a strange multi-piece plug in it. I assume that port is used for the brake booster on cars that don’t have the SBC system, but it is beyond me why the engineers at Mercedes-Benz decided to make a three-part plug to fill it instead of using a simple rubber plug. This is especially strange since there is an existing Mercedes part that seems to work perfectly.
I was at Mercedes Benz of Portland today, trying to get a new plug for my car since the old one is leaking. For some reason the parts diagram on the computer only shows two of the pieces required. It was clear that it would not work without that third piece, so after some head-scratching the parts guy said “Hey, I’ve got an idea!” He ran into the back and two minutes later came out with a plug for the windshield washer on a CLK320 (part number 210-987-00-45) that is the perfect size for the port in the supercharger!
