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Old 11-01-2001, 01:30 PM
Posts: n/a
Question Vacuum vent to passinger compartment

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.
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Old 11-01-2001, 02:00 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
First thing the source vacuum line should have two teed off souce lines. The trans control which involves the vent line should not be associated with the group that includes the brown line the carries vacuum through the ignition switch to the shut-off diaphram.

The important point is that what you have should not be able to exist if the two system groups are separate.
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Vacuum vent to passinger compartment-vacuum.jpg  
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 11-01-2001, 06:29 PM
Posts: n/a
Vacuum vent to passinger compartment

Thanks for the prompt responce Steve.

I've used that diagram from the service manual and yes, the two parts are seperated so I don't think thats the problem.

Does the order in which the two tees pull vacum from the large vacuum line going from the vacuum pump to the brake booster matter? I'm not sure why it would but who knows.
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