I have tried my best to avoid posting on this topic because there is a wealth of information on it, and I have certainly done my due diligence in researching and diagnosing up until this point.

Vacuum pump is good, shutoff valve is good (did an isolated test), all vacuum systems are working perfectly.

If I drive the car around for 10-15min, and then attempt a shutoff, it will do the oscillatory, low RPM dance until I hit the stop lever.

After shut down, if I start it back up and then shut it off again, it will shut off normally. Hot or cold does not matter. If it is turned on and I haven't driven it, it will shut off all by itself.

This is making the problem harder to diagnose because when I try to replicate in my garage, there isn't a problem at all.

Has anyone experienced something similar? Why would it only *not* shut off after driving?

If the shutoff valve tests good, look for where driving is letting vacuum bleed off. Perhaps Tee a vacuum gauge in close to the valve.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

My guess is blow by, oil is being sucked into intake, causing it to run.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

Is this a 116, 126 or 140 SD?

If it always shuts off with the stop lever, it probably isn't running on blow by fumes. Carry a Mityvac and vacuum gauge. When it doesn't shut off with the key, see how much vacuum registers at the vacuum hose to the actuator, and how much it takes to shut off with the Mityvac. If you discover there's less supply than the actuator needs, take a vacuum reading the source - the check valve along the brake booster line.

Sixto
98 E320s sedan and wagon

Thanks for the tips, I'll do some more investigating and update with either a solution or more puzzlement

Car is a W126 daily driver, have put about 12k miles in the past year. Keeps me on my toes!

Do you have an actual gauge reading of the Vacuum?

It is possible for everything in the Vacuum System to work when it leaks just a bit but for it to cause a cumlative issue with the shut off.
To troubleshoot that disconnect all the vacuum stuff going into the Fire Wall (accept for the the vacuum line going to the vacuum shutoff on the Fuel Injection Pump) and plug off the vacuum side and see the shutoff is normal after that. If so you have a vacuum leak inside of the Car.

If your vacuum is not leaking and your Vacuum from the Vacuum Pump is high enough all that leaves is the Vacuum Shutoff Valve that is on the Steering Colum Lock and on my Vehicle there is little 2/3 way valves on the valve cover that control the EGR. Those 2/3 way valves can leak especially if the little levers are worn off.

See post #23 for some pics of the steering colum shutoff valve.
http://www.peachparts.com/shopforum/diesel-discussion/382612-84-300d-why-cant-i-keep-thing-running-2.html

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84 300D, 82 Volvo 244Gl Diesel

I discovered, after not finding a leak through testing with mityvac, that my intermittent vacuum leak in a 1985 w123 only occured when the center vents were activated. I've got a leaking climate control vacuum pod.

I don't know if the W126 climate control system or if it is different from mine, but it is simple to see if your symptoms change depending on what vents you select on your climate control. I also have a leak if I bump the driver door lock button down half way with my left arm. Plan to replace that actuator also.

1985 300TD, 363K, daily driver

So far, three pumps with the handheld vac will shut her off immediately. From the brake booster I am reading 18-20 inHg while idling. I plugged everything to the firewall and will get a normal shutoff, but then again that is only because I have already shut it down previously.

Climate control is my next suspect..

UPDATE: I have found the culprit. If I block off the thicker white line that runs into the drivers side fender, she shuts off. Picture is attached.

This leads to a vacuum reservoir...?

Attached Thumbnails

 

For education's sake (there doesn't seem to be much coverage on this part of the vacuum system), I will post my findings.

The vacuum reservoir would not hold any vacuum, so I removed the fender liner and sure enough, the grommet at the top of the res was completely disintegrated. With the line plugged at the "Y", the car still seems to retain full functionality and shuts off immediately..which makes me wonder what the purpose of the reservoir is?

Regardless, a couple of pictures for the "visual", and I will be on the hunt for a new grommet.

Thanks again for the help!

(for some reason photos aren't working at the moment...)

I know that OE and aftermarket grommets are available for the windshield washer reservoir.

I'm getting an order up and forgot the grommet. Thanks for the reminder.

Good job trouble shooting.

On a W123 the reservoir is for the locking system (check valves isolate it and the locking system). When your licking system hold vacuum the reservoir allows you to operate the locks a bunch of time with out having to run the Engine to restore vacuum.

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84 300D, 82 Volvo 244Gl Diesel