This problem appeared on my 300TD today.
It happens only after a short on/off cycle with car running for less than 2 minutes from cold.
If I drive around for a bit, it shuts off as it should.

My assumption is that vacuum pump needs to build up "pressure" and it doesn't have enough vacuum for a brief on-off cycle.

I've seen numerous threads describing similar problem and most point to broken shut off valve or vacuum leak, if that was the issue in my case, why would it matter how long the engine been on for?

Appreciate any advise!

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1987 300TD (wagon)

Vacuum leak, the system needs to pressurize all the lines, if I have my heater blowing and try to turn off the car it most cases the engine will continue to run even though the key is off.
Solution: turn the heater off as i pull into the driveway, and once i am parked in the garage i am able to turn the car off without problems.

I would recommend renting a vacuum pump and searching for leaks in the system, as well as examining the connection pieces in the engine compartment. Pull them apart and reconnect them, the easier it is to connect them the more worn they are, also search for cracking.

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Nosce Te Ipsum "Know thyself"

During the time frame you describe, multiple systems, including climate control vacuum elements are cycling, or trying to = low or no vacuum if any have damaged diaphragms.

My experience and best guess is that the vehicle has at least three vacuum leaks.

.

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If it were a weak vacuum situation, I'd expect harsh shifts, ACC problems, possibly inadequate braking assist until there was sufficient vacuum. No mention of such corroborating symptoms. Try this - cap the small line from the vacuum pump on the supply side to isolate vacuum consumers except for the brake booster and shutoff actuator. Start the engine then shut off. Does the shutoff respond to the key? If so, the problem is among the other consumers. If not...

Sixto
87 300D

Well, today I was unable to replicate the issue. But knowing that there is a potential for future problems will drive me insane, so I'm determined to fix this leak. I'm a complete noob when it comes to troubleshooting vacuum leaks (never used a vacuum pump). Is there a step-by-step walkthrough on testing w124 for vacuum for leaks? Appreciate if someone could point me to some guides.

@Sixto, my brakes and are quite strong and transmission is smooth like butter. My central locking works fine also. However, the cruise control doesn't work most of the time and when it does it has trouble maintaining speed. I just don't use cruise, so never bothered to fix it, could it be part of the problem?

I stopped by my friendly MB dealership and picked up vacuum lines and rubber connecting lines. Once it gets a bit warmer I will start replacing all the lines in the engine bay with these new ones...

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1987 300TD (wagon)

Trouble shooting the Vacuum System at first seems baffeling.
However, one way to do it is you simply delet the various systems connected to the vacuum one at a time plugging off the Vacuum Supply end.

As an example you disconnect your locking system at the main connector and plugg off the vacuum supply end. If your Vacuum Shutoff improves after that you know you have some issue in the Locking System.

If there is no improvement when you did the Locking System you leave things as they are with the Locking System and move on and do the same disconnection and plugging to the Climate Control.

And so on

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84 300D, 82 Volvo 244Gl Diesel

I'd start at the 5-way connector that attaches to the small fitting at the vacuum pump and move down lines that won't hold vacuum. The two lines that go to the passenger side provide vacuum to EGR and ARV. The two lines that go to the driver side provide vacuum to the transmission modulator and climate control. Climate control is the trickiest to diagnose because of the number of actuators that can be routed to or isolated from the supply line. With the key out, only the defrost actuators should be routed to the supply line. Engine shutoff sources vacuum from the check valve along the big vacuum line to the brake booster. FWIW, central locking in an '87 TD has a separate electric pump

Here's a link to a 603.96 engine vacuum diagram - OM603 Vacuum & EDS Diagram . Note that some '87 300D/TDs have a split in the line at the VAC port of the transmission amplifier (item 123) that feeds a vacuum reservoir in the passenger fender... yes, the passenger fender!

Sixto
87 300D

I had the same problem with my 190D. I changed out the vacuum pump and all is well. One way to test the vacuum is to 'T' into any line and see what is the reading with a vacuum gauge. I do not suspect you have any leak.

Cruise control has nothing to do with vacuum. btw: a cruise controller from a 199x 190E would work for your car. There are load of them in the JY.

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