1980 300SD Engine shut off problem

My 1980 300SD has a new problem where it won't always shut off when the key is turned off. The main symptom is that if the car has only been running for a short time, like from cold start to a couple of mins, if
the key is turned off, the engine keeps running. After maybe 15-20 secs, it finally stops, but even then it takes another few secs for it finally to quit. If it's been driven for 5 mins or so, then it usually shuts off normally, or at least a lot faster.
I know the engine shut off is vac driven, so my first thought was maybe the vac pump was going bad. I've also noticed that I hear a click from the AC system maybe 1 min after driving off. I think this is one of the vac gizmos kicking in that then allows the AC system to operate. I think this click and action used to occur almost immeadiately on starting the car, so this may be another indication of lacking vac.
I disconnected the main vac line at the vac pump. I didn't hook up a gauge, but just holding my finger over the opening, it felt like there was significant vacuum. My next step will be to try hooking something up so I can apply vac to the injector pump directly to see if that will stop it, to rule out that. Then with a vac gauge, I'll start working through the sys looking for a leak.
Just wondering if anyone has seen this before and knows where to look. I guess one possiblitiy would be the vac control on the ignition switch. Also, if anyone knows what the vacuum measurement should be in the main line going to the vac pump, that would be helpful. TIA for any ideas.
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On 2 Dec 2004 09:12:31 -0800, trader4 wrote:

Are your glow plugs sticking on maybe? Mike
--
W126 300SE
W115 220/8
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I've gone through similar probs with my '79 240D. In my last case it was the vacuum line behind the console had a split in it. What you need to do is just go through the lines one by one manually check the integrity, either with your mouth or a vaccum pump, until you nab the culprit. It's time consuming and a bit of a pain, bit it is EASILY do-able.
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Your symptoms are typical of vacuum loss somewhere. The reason it takes a while to shut off after just a short engine run time is that the vacuum pump hasn't had a chance to build up a sufficient vacuum reservoir. I have both a 240D and a 300D. The 300D will shut down instantly regardless of how long it's been running. However, the 240 takes about five minutes of running to shut down immediately after turning the ignition off. I know I have leaks in either the diaphrams which operate the venting system or some of the vacuum line connections because the door locks (and everything else) work fine when the engine is running. When I leave it off for a while the door locks will not operate or operate very slowly. Overnight? Forget it. Nothing works in the vacuum systems until the engine runs at least five minutes. The outside temperature also affects the system so I'm thinking it's possibly the rubber vacuum line connectors, of which there are many.
My advice would be to block every line going everywhere except the vacuum shutoff on the injector pump and see what happens. My guess is that the engine will immediately shut off. I doubt it is a problem with the vacuum pump although that is always a possibility.
On my wife's 300D I actually blocked off the vacuum lines going to the door locks since that is where her vacuum leak problem is located, probably one of the door actuators. Found that out after sufficient time eliminating the various vacuum systems one by one. Good luck!

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I have an '82 300D 123. You have a vacuum leak, obviously. When the car is first started, it takes a few minutes to overcome the leak and build up enough vacuum to shut down the injection pump. I would suggest that you NOT mess with the main components of the vac system or the ignition switch. That is not where you will find the problem. It will be an AC diaphragm or a door lock diaphragm.
The easiest place to start is just in front of the firewall. Follow the main vac line to just in front of the brake booster. On mine, a couple of lines come off the main line. One of those lines feeds vacuum to the transmission, injection pump, and other engine-related items. The other feeds the ignition switch, the door locks and the AC system. The AC system line is green and the switch system is brown (on mine) and feeds from a Y connector just behind a small round plastic control valve. Pull the green line from the Y connector and plug the connector. Start the car. If it shuts down OK, then the leak is in the AC circuit. Could also be in the door lock circuit which is yellow, I think. One of those two circuits will be bad.
Of course, you should also look for the simple things first. Like a line that is loose from its connector. Sometimes, just making sure the lines are plugged in will fix it.
Larry In the back yard, under the oak.
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I had the shop replace the "shut down device" when my '80 300SD had this problem a few years ago.
The shut down device is a vacuum powered bellows inside the injection pump, specifically its aft end. The bellows wears out and that's just the problem - the bellows is perforated and no longer holds vacuum.
The part costs about $75 and there's about one hour of labor to install it.
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Thanks to everyone for the tips. Soon as I get some decent weather here, I'll do some more investigating and let you know how it turns out. I was going to start pulling apart the vac lines and isolating sections at a time the other day, but was amazed at how hard it is to pull apart the vac lines where they start splitting up near the engine. The main lines are made of some kind of hard plastic type tubing that won't pull off the T or Y fittings. Thinking I'm probably going to try heating them a bit with a heat gun next time.
I'll try connecting the injection pump shut off direct first, and see if it shuts off that way to rule out the diaphragm.
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