Trying to solve an engine shutoff dilemma in my 1978 300D, the dilemma being that it doesn't shut off when I turn the key. I assumed the problem was with the tiny vacuum shutoff valve at the ignition switch, because I can shut the engine off by sucking on a hose connected directly the vacuum shutoff on the injection pump. This leaves the possibility of a vacuum line error or failed shutoff at IP. One of the hoses that goes to the valve behind the ignition is not connected to anything, but it seems to be the same configuration in my friend's 300D, which I've been parting out.
If you can shut off the engine by sucking on the line going to the shut-off valve on the IP, the shut-off valve is OK.
It is most likely the valve at the ignition or a vacuum leakage elsewhere which causes the problem.
The brake booster is working?
The ignition valve definitely works because my friend never had a problem shutting it off. Parting out his car because the tranny is borked. I would imagine the brake booster works...is there a way to test this or should it be obvious?
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The ignition valve definitely works because my friend never had a problem shutting it off. Parting out his car because the tranny is borked. I would imagine the brake booster works...is there a way to test this or should it be obvious?
I am missing the logic of the sentence above in Red.
If your Friend never had a problem shutting it off that meant that the Shutoff Valve was also working as well as the Ignition Vacuum Valve and there must have been sufficient Vacuum; at that time.
Vacuum leaks in other places tend to have an effect on the Shutoff first. If it is a big Vacuum leak it has an effect on your Brakes.
If when you disconnect the Vacuum Line from the Shutoff Valve on the Fuel Injection Pump and your find Crankcase Oil in it the Valve is on the way to need replacing. But, they sometimes will work along time like that.
You need to systematically check each system using a handheld vacuum pump after you've studied the vacuum diagrams (check the metal support above driver side headlight for vacuum diagram) and determined how to isolate each system from the other.
As Govert said, if you can suck on the hose at the injection pump for the shut off valve, and it shuts off as intended, then the valve is fine. You need to check upstream of the shut off valve (check where else it connects to) - the next item is the ignition switch.
Plumb the handheld vacuum pump in place of the shut off valve and check to see if the ignition switch is holding vacuum at both key positions (running - position 2, and shutoff - position 1 or 0). When the key is in position 2 as in the engine running, the shut off valve should NOT receive vacuum (otherwise it will shut off). So in position 2, the handheld pump should have NO vacuum. When you turn the key to position 1 to shut off the engine, the ignition switch will relay vacuum from the main vacuum hose (large hose going to brake booster) towards the shut off valve. If position 1 holds vacuum in the OFF position at the same line, AND if position 2 has NO vacuum, then the switch is fine. Does this make sense?
If this is not the cause of the problem, you need to move on towards another system, because the next step is the main hose to the brake booster. This is the main vacuum supply from the pump and at this point, a leak from a different system will exhibit symptoms in other systems.
Disconnected all of the vacuum lines near the firewall except the ones that go to the shutoff, blocked off the open lines with golf tees and gave it a test. Still won't shut off and I'm thinking it's probably related to the climate control system, which doesn't work and is partially disconnected (York AC unit has no belt and was zip-tied to reduce rattle). I'll have a look at that diagram and see if I can figure anything out.
Sold this car to a friend awhile back, still haven't managed to fix the shutoff. Does anyone know where to find a pre-1980 vacuum diagram? The one in the engine compartment is gone.
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Engine shutoff problems in '78 300D 
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