How Does 300SD Diesel Engine Stop?

I was wondering how the diesel engine is stopped in Mercedes diesels. The ignition switch doesn't appear to mechanically valve off the fuel line, so I assume there must be an electromechanical relay of some sort
to stop the fuel flow.
Anyone know how this accomplished?
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Yeah, when you turn the key to the off position it closes a vacuum circuit that pulls vacuum to a silver mushroom shaped device on the back of the injection pump that pulls a lever inside the IP that cuts off the supply of fuel.
When the vacuum pump goes south, even marginally, oil gets in the lines and eats the diapgragm in the shutoff valve (as the mushrooom thingy is propelry called). You need a nee shutoff valve if this happens and a vacuum pump rebuild kit or a new vacuum pump. You have to clean the oil out of the vacuum lines, too.
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A diesel engine is stopped by cutting off its fuel. That's done in one of two ways. You can manually press the STOP lever on the throttle linkage or turn the key to OFF. When you turn the key to OFF vacuum is applied to a small bellows inside the injection pump, the "shut down device," which pulls the fuel rack to its OFF position and the engine, deprived of fuel, stops. Remember this, for a vacuum leak (anywhere) or worn out shut down device will allow the engine to keep running no matter what you do with the key and until you open the hood and press the STOP lever which will always stop the engine. As a new diesel driver you should know how to manually stop the engine. Try it, you won't hurt anything.
As a matter of interest, a diesel can "runaway" that is, run uncontrolled to the point of self destruction if, but only if, its lube oil is somehow sucked into its intake manifold. The engine, being an oil engine runs on its lube oil rather than its fuel. I've heard of this happening with stationary engines when their turbos' seals failed but haven't heard that happen with our engines.
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Thanks to both of you for the information!..
I will find this emergency stop valve first thing in the morning.
Does the key switch apply the vacuum to the bellows directly, or is there some sort of electric relay which does this? This really seems like a stupid design. It should take vacuum to allow fuel to flow, shouldn't it? It seems to me that you would want the fuel flow shutoff to be failsafe.
T.G. Lambach wrote:

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Not hard to find, it's tucked away at the back of the injection pump.

There's two vacuum hoses that go to the ignition switch. One is vacuum source the other goes to the shutoff valve. With the key in the off position the two are "connected".
Idealy you have a vacuum tester like a mighty-vac to test the shutoff valve. You pull the vacuum hose off, put the mighty-vac on it and pull vacuum. More than a couple of psi is all you should need to actuact it.
If you pump and pump and pump and it wont shut off you need a new shutof valve; moreso, the shutoff valve should hold whatever vacuum you assert for 5 minutes without wavering.
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I can see how this would happen! The only way to stop it would be to stop the airflow or to stall the engine with a load, and it would sure be hard to have the time or ability to do either of these things before something broke loose!
What do you think would let go first in a Mercedes engine if this happened? The top end, or the bottom?
T.G. Lambach wrote:

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I'm having this shutoff problem with my 1980 300SD now. It's been too cold to start figuring out what's causing it. But over the last 6 months it's taken longer to shutoff after turning off the key. Now it's to the point where if I've been driving for awhile and shut it off right away, it will shutoff. However if I've been driving and then let it idle for a couple mins, then it won't shut off. I have to idle it at 3000 for 30 secs or so to build up more vacuuum, then it will shut off, though it still may take 5 secs or so. It's even worse when started up at first.
I also can hear a new faint sound in the cabin, which sounds like it could be air being sucked through a small crack in a vacuum hose. So, I suspect there is a vacuum leak somewhere that I'm going to have to find. I did pull the main hose off the vacuum pump, and just from putting my finger over the pump fitting, it feels like quite a bit of vacuum, so I suspect it's a leak. But it'll probably be a pain in the ass to find.
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On 2005-03-06 04:35:10 -0800, " snipped-for-privacy@optonline.net"

Not at all, follow the sound you said you here... The whistling sound is the leaker.
Good Luck, Marty
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The main vacuum lines going to the door locks, trunk lock and fuel door pin lock are usually under the mat on the driver's side of the car. The lines are usually yellow and yellow with a brown stripe. I think there is also a brown line but can't recall at the moment just where that goes. Anyway, make sure all of these lines are properly connected. There will be one color of line which opens the locks and one which closes them. I can't remember which is which but you can check them rather easily to isolate your problem. I use a rubber "cap" to close off the ends of each line coming from the engine compartment and driver's door lock to check for leaks without the use of a small, hand-operated vacuum pump. You can usually find these pumps at places like Pep Boys, etc. I paid about $35 for mine which came with a bunch of connectors for various applications. Like a previous post said attach the vacuum pump to the diaphragm at the rear of the fuel injector pump under the hood to see if your engine will stop. If it doesn't the problem is the diaphragm itself. If it does you have one of two problems: 1 - the vacuum pump is shot or, 2 - (more likely) you have a leak in the system. If you have one of these small vacuum pumps you can check the engine vacuum pump by simply isolating the main vacuum line from the remaining lines with the exception of the large line going to the vacuum brake booster. Plug the vacuum pump (or a vacuum gauge) onto the "Y" connector which goes to the balance of the vacuum system. This should pull down a rather good vacuum (20-25 inches of mercury) pretty quick. If you can't get much vacuum here the pump needs rebuilding or replacing, your choice. If your engine will shut off after running a while but doesn't if you sit idle for a bit my best guess is you have a vacuum leak somewhere. The most probable suspects are door lock actuators. It can get pretty wordy explaining exactly how to check it out but let me know if you're game.
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