My 82 240D has always shut down with a nice violent kick as a healthy diesel should. Lately I've noticed that when I turn the key off the engine speed rises for one split second just a hair and then the engine shuts down.

Is this a sign of a failing shut off valve?

It shouldn't shut off with a violent kick if you don't have vacuum leaks

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Is your AC compressor running when you turn the key off? If it's running, it may be putting load on the engine. Then you turn off the key, the electrical system shuts down and thus the AC compressor releases, then it takes another second for the vacuum to actuate the shutdown solenoid.
I could see that causing a slight rise in RPM for a split second.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Really? I was under the impression from watching Mercedes source videos that a healthy diesel should quickly yet violently shut down as mine does.

It does not have AC so theres no compressor to be on.

A healthy diesel should give a "kick" when it shuts down, it's from the high compression and a tiny bit of fuel still left in the previous cylinder.

To test your theory about the shutoff pod, stop the engine with the stop lever and see if it does the same thing. If it doesn't, have someone operate the key to stop the engine for you and see how quickly the lever pulls down. It should be very quick. If it's slow, you may have a vacuum leak or a bad valve.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)