Fixing Those Vacuum Leaks

The following VFAQ was created to help people replace their leaky or incorrectly hooked up vacuum systems. As always remember the lovely legal disclaimer that I do not accept any liability for yours or anyone’s actions who may follow any of my guides.

As a quick explanation of the main hoses you see in the engine compartment look at the labeled picture below:

Here is a picture of the correct hook up for your hoses.

The inline fuel filter was repositioned to off the IP shutoff valve, it serves the purpose of stopping any potential oil leaks into the vacuum system at the cost of a couple extra seconds delay before shutting off.

Vacuum Diagrams:

Courtesy of Peter Schmid and Phantoms

General Vacuum System Diagrams:

General Diesel 1
General Diesel 2
IP Shutoff system diagram
Heating and AC diagram

W123 Chassis:

Cabin
240D AC diagram
Central Interlock System diagram for Chassis 123.1, 123.02/03
Central Interlock Circuit diagram for Chassis 123.1
Central Interlock Circuit diagram for Chassis 123.02/03
Central Interlock System diagram for Chassis 123.183/190
Central Interlock Circuit diagram for Chassis 123.183/190

Engine
616 & 617.91 Motors
617.95 Motors:
1980 California
1981-84 Federal, 1981-1983 California
82 300D Fuel & EGR vacuum diagram
82 & up 240D Fuel and EGR (Automatic trans) vacuum diagram
82 & up 240D Fuel and EGR (Manual trans) vacuum diagram
1984 California, 1985 Federal
85 300CD Federal emissions Fuel & EGR vacuum diagram
1985 California

W126 Chassis:

80 300SD diagram
85 300SD California emissions Fuel & EGR vacuum diagram
Earlier years can be found here.

More information on vacuum related shifting quality can be found here.

If you don't know how old your rubber fittings are or especially if they are showing slight cracking it is time to replace them. Yes I agree that it normally doens't leak through the cracks but instead it typically leaks where the hose meets the fitting. A MityVac is critical for any vacuum system troubleshooting. The black line going to the transmission should hold vacuum for several minutes, if not you have to start replacing things. If this is allowed to leak just a little bit it will change the vehicles shifting vastly. Start off with a new cap, then if that doesn't work either get a new modulator (supposed to work better) or the modulator seal for your current modulator.

Tools:
MityVac

If you know of some good w123 related DIY pages, would like me to host a DIY page you wrote, have any questions, comments, or suggestions feel free to contact me.