I have seen several comments about blocking the crank case vent and the engine will stop. My 91 300D 2.5 doesn't do this but could someone explain why it happens and which engins it applies to?
I did a search but all I could find was that it happens.
Merry Christmas to all.

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84 500 SEL (307,xxx miles)

It applies to the 61x engines. (616,6171 etc.) Why? Not really sure, but they do.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

Apparently, the shutoff diaphragm on the 61x engines has vacuum from the ignition switch on one side and crankcase pressure on the opposite side. So, when you shut the engine down conventionally, the vacuum pulls on the diaphragm........against minimal crankcase pressure.........and the engine shuts down.

When you cover the breather and allow crankcase pressure to rise, the pressure acts against the back side of the diaphragm with nothing on the opposite side. If the pressure climbs high enough, the diaphragm moves in the normal shutoff direction and the engine stops.

The capability to stop depends on how much blowby the engine produces and how many leaks are in other areas of the crankcase. An engine with minimal blowby might not shutoff at all if the blowby gases can escape via other seals.

In any case, it's not a wise idea to perform this procedure.

Thanks, that makes sense. I always say, we whould strive to learn something new every day, and I have.

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84 500 SEL (307,xxx miles)

Indeed. It is a good way to blow out oil seals, especially the very difficult to replace crankshaft seals.

I dont think it has anything to do with the shutoff valve. The shutoff valve has nothing to do with crankcase pressure, it just pulls some linkage that shuts off fuel in the ip. I think it shutoff the engine becuase the pressure in the underside of the pistons becomes to high and the engine cant function. Im sure its not good for the engine, but I wouldnt worry about it blowing out gaskets or other seals, especialy if you just do it once to check the condition of the car. When ive done it, the back pressure aginst my finger is hardly noticable, maybe just a few psi if that.

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1981 300SD 512k OM603

Nope, shutoff valve is what does it. It has everything to do with crankcase pressure, it is crankcase pressure that shuts off the engine (The ones with a shutoff diaphragm). The vacuum system removes the atmospheric pressure on the opposite side of the diaphragm and the pressure in the crankcase pushes on said diaphragm to move it.

The seals ARE a major concern with that method, even once. If a seal is marginal all it takes is a little persuasion to cause it to shift and start leaking.

The test simply should never be done on any engine.

Well, you better inform the OP that he's got one unique engine because his 602 won't shutdown due to the "pressure on the underside of the pistons".

So the pistons can generate 400 psi going up but cannot overcome a few psi coming down?