Recently my car will struggle to shutoff when i turn the key off so I got a vacuum tester and tested the shutoff valve at the injector pump. After about 2 pumps and 10" of vacuum it shuts down.
I then went up under the dash and hooked my tester up to the supply side of the shutoff valve connected to the ignition (brown line). With the engine running I pumped it to about 15" of vacuum and turned the key to the off position and nothing….other than my vacuum dropped to 0 and the car continued to run. So unless the shutoff valve behind the ignition switch is bad this procedure should shut down the car…Is this correct or am i missing something?

The top Nipple on the Valve is the Supply side Vacuum. Which should be the solid Brown Colored Vacuum Line.

You might T the Mighty Vac or a Vacuum Guage into the Supply Line where it comes off of the Y connector under the Hood and see what the actual Vacuum you are getting is and then test it.

The Hand Held Vacuum Tester/Mighty Vac can create the Vacuum but it does not produce what I am going to call a large Vacuum Volume like the Vacuum Pump does when it is running.

New Vacuum Valves used to be about $30+shipping.

That is cheap but you can get a new Steering Wheel Lock Assemblty for about $140 or so and it will come with a New Ignition Switch as well as the Vacuum Shutoff switch. I got mine from an online genuine Mercedes Parts Dealer. I figured it was a good buy as it cost less then the individual parts and I wanted the real Mercedes Ignition Switch (I have no idea what comes with the Myele or Febi versions if the Steering Colum Lock Assembly).


I say that because I had the inards of My Steering Colum Lock Break and 30 years use is a long time.
While My Vacuum Switch and the Ignition Switch were working I found that one of the little Arms on the Ignition Switch had the Contact Point burned off on one side.

In side of the Steering Colum Lock there is a Plastic cam that actuates the Vacuum Valve.

You may or may not be able to exchange the Vacuum Valve while the Steering Colum Lock is installed. It will depend on how tight the screws are and if they are Phillips Head or Blade type heads on the Screws.

Over on an Austrailian Benz forum someone had the Valve Plugged and removing the blockage fixed it.

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84 300D, 82 Volvo 244Gl Diesel

I'm having the same issue with my 1995 E300. Leak seems to be coming from the vacuum valve on the steering lock. I've found that I can get it to shut down if I finesse the key ever so slightly to the right after all the way left. We'll see how long that lasts.

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Remember how the steering lock valve works. Mercedes made it fail safe, so that there cannot be a failure to start, or cause a shutdown while running, due to a failure in the vacuum system.

The steering lock valve is not an "on-off" valve but a "3-way" valve.

There are 3 ports on the valve but only 2 you can hook hoses to. The third port is an internal vent.

The "common" port goes to the actuator.
The other port goes to vacuum supply.

When the key is in the off position the common and supply ports are connected together and the vent is dead ended.
This applies vacuum to the actuator, this pulls the rack to the "stop"/"no fuel" position.

When the key is in the running position, the common port is connected to the internal vent and the supply port is dead ended. This bleeds the vacuum off the actuator and releases the rack to allow fuel.

If you happen to get the hoses swapped at the actuator, this will cause a confounding problem.

The engine will shut down normally but you will not be able to start it unless you crank the key back and forth several times or it has sat for several days. Why?

With the key in the off position, the common and supply ports are connected together, and the actuator gets vacuum and shuts off like it's supposed to.

But with the key in the run position, the vacuum supply gets connected to the vent, causing a small vacuum leak, and the actuator has no place to bleed off. The only way the actuator can bleed off is if you turn the key back and forth several times, letting the vacuum system bleed down through the key switch vent (key on) and then releasing the vacuum in the actuator from the (now relieved) vacuum system when you turn the key back to off. Or you wait several days, the vacuum system bleeds down naturally, and then when you turn the key on, the actuator vacuum gets relieved into the system that is not under vacuum until the engine starts.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech