You probably know that sometimes on the 300 diesels, the car sometimes won't turn off, so you have to open the hood and hit the 'stop' lever.
I have not yet had to do this in the last two years, but on two occasions I almost had to, but after a few minutes the engine cut off.
Most of the time it cuts of immediately.
Why would it do this sometimes, but usually not do it? Anything I need to check?

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. 300 TD Turbodiesel 1985-W123

can be a sometimes thing -- for instance, if you have been holding the brakes for a bit, or doing slow parking lot maneuvering for a while before you attempt to shut the motor down, the low rpm may prevent the motor's vacuum pump from keeping up with a small leak somewhere, whereas if you've been wailing down the highway and then coast into a reststop, you'll have sufficient reserve to shut it down on the regular vacuum operated shut-off system.

It's a vacuum leak somewhere. For instance, in my 77 300d I have a minor leak in the hose that goes to the drivers door. If I open the door before shutting off the engine, it is slow to shut down. On my 85300td I have a bad diaphragm in the acutator that runs the footwell heating vents. If I have the heat on, that vehicle is slow to shut off.
It's fairly easy to trace down these problems with a Mity Vac.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Dear Fimum Fit and Kerry Edwards,

Thank you very much for those insights. That does seem to explain it.

Do you know where I might find out more about a Mity Vac and how to use it?

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. 300 TD Turbodiesel 1985-W123

You should be able to buy one at most auto parts store. They come with directions and can be used for a number of things. You want to use it to pull a vacuum and test the various vacuum systems in your car. The doors and the climate control would be the obvious places to start since there are so many rubber acuators in those systems.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Thank you, Kerry. That is indeed helpful, and I plan to look into that.

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. 300 TD Turbodiesel 1985-W123