I've been tooling with my 82 300 CD Turbo and using this amazing web site to help. What a great resource!!

Anyway, it started out as troubleshooting the engine cut off. Engine would diesel on for a moment or two before shutting down. Using this site I learned all about the vacuum system, adjusting the ALDA, checking and cleaning the boost lines and switches, all kinds of stuff. To make a short story long, I've narrowed it down to the following problem.

There is a vent tube that runs from the vacuum system, through the firewall, and into the passenger compartment. All references tell me this is just a vent for the system and is supposed to just hang out up under the dash.

I can accept this.

However, if I plug up the vent tube, it fixes the shut off problem, but it lowers the turbo performance. If I unplug the vent tube, the turbo works like a charm, but the engine won't shut off.
It's possible that the vacuum pressure is just a little too low to shut off the system, the needle on my vacuum tester flips back and forth really fast between 10 and 15 hg while idling. When I plug the vent, the vacuum moves up closer to 20 hg. I had the pump replaced about 2 years ago. (hole in diaphram) I guess it could have gone bad again... but I'm wondering if I might be missing something simple. Any help would be greatly appreciated.

This is my second Mercedes. My first 79 300D Sedan was turned into an accordion by a mid life crisis, girl watching Corvette driver who purchased animals for zoos as a living.