This is a 1980 300SD.

I tried doing a search on this and came up with very little. And I know this is a common problem.

The engine will not shut off when the key is turned off. I put a hand vacuum pump on the shut off valve on the back of the IP and it shut off. I am getting about 18-19 lbs. out of the vacuum pump on the engine. When I turn the switch off I am getting about 4-5 lbs. of vacuum to the shut off valve from the ignition switch. So, my guess is the vacuum stuff at the ignition switch.

What vacuum stuff is in there and what needs to be replaced and how difficult is this?

I dont know about the switch mechanism at the ignitions switch. but you could check the brown line that goes into the firewall to the ignition switch and then to the fuel pump. You may have a vacuum leak with that line or elsewhere as vacuum loss is not discriminating.

So course of action
1 test the brown line that goes to the switch (have it all set up) Does it hold pressure? If it does you have a leak elsewhere. To double check start the car with your vacuum tester on the brown line. start the car then apply direct vacuum to only that circuit via the brown line. if the car stops then your leak is elswhere.
3. If the brown circuit is leaking try changing all the rubber vacuum hose that you can at the hose connection points.

4. This problem of vacuum leaks sucks...you have to investigate all of the vacuum systems as they all effect eachother. you have source vacuum and that is good. Your vacuum leak could likely be climate control related. How has that system been working. What you do is check all the systems one at a time and see which one is faulty. apply source vacuum directly to the system that you are checking while disconnecting the other systems (plug only the brown line into source vacuum aka vacuum booster line.)

That should get you started

Also I am guessing that your vacuum leak is not from your ignition switch. either a disonnected or poor connection or vacuum element from the climate control.

also there is alot of links to vacuum leaks.

good luck

bob

ps
I know your pain

The vacuum splits after coming from the booster supply line. The brown line is the supply for shut-off function. It goes to a vacuum switch on the steering lock. Once that switch is open the vacuum proceeds to the shut-off diaphram.

Montor vacuum in the brown line first while shutting off. Verify that the vacuum stays strong. If the source stays strong and the output line only has a small vacuum then either the switch or the coupling is leaking (based upon your statement that the shut-off diaphram worked).

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician