In perhaps an overabundance of caution, I am asking for more help before I replace the shut off valve in my 1981 300SD. How do I stop a runaway engine if I install it the wrong way?

DieselGiant says put a rattlecan top over the air intake. For one it does not fit and then I tested this theory with a larger block-off and it still did not stop the engine. See pics. I removed the ridged hose from the air cleaner and in pic #2 you can see how I blocked it off with an old coffee can plastic top. This did not phase the engine at all.

Is this the right place to attempt to shut off the air to the engine?

Think Vstech said to close it off at the turbo but I do not know where that is in the engine. How do I do this.

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Not a direct answer to your question, but perhaps it would be sufficient simply to test the movement of the shutoff lever with a Mityvac prior to starting the engine....that's what I did the one and only time I replaced a SOV.

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Putting a "stopper" where you are there in the pic would still allow for other connections to allow air into the engine. You probably wanna get closer to the intake manifold.

While the engine can runaway if you do not install the shutoff correctly, it's not that hard to install...and install it correctly. I think most times when one has run away after a shut off install was simply because careful attention wasn't paid to making sure everything was connected correctly.

Do you expect an unregulated fuel source? Runaways seem to occur when oil gets into the intake unexpectedly. I don't see how just replacing a shutoff valve changes much, or am I missing something.

If the shutoff is not installed properly then the fuel rack can "stick" open causing a full fuel scenario thus causing a max rpm when the engine is started up. It will run until it gives up.

That big tube down and to the right of your hand leads in to the turbo inlet. I wouldn't put anything less than a piece of thin plywood or siding in front of a turbo inlet, and for the love of all that's holy, don't get your fingers in there.
Edit: the actual turbo inlet is probably where a spray can top will fit. That sounds about right but I wouldn't trust myself to not actually jam a finger in there if in a hurry.

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1982 300sd from craigslist $800 greased on one tank with NO CONVERSION in the Hot Texas Sun. (currently dead & awaiting engine damage investigation and/or longblock swap)
new daily: '03 vw 5speed jetta tdi wagon. bagged&chipped

PG - thanks. Actually I always wondered what that big U-shaped plastic tube was for. Makes sense that it leads to the turbo. So I assume I can go out and remove it, start the engine with it gone, then test with a piece of plywood over the inlet to see if it stops the engine. Should not hurt to run the engine for a few minutes with that tube removed I guess? Apply plywood to the inlet on the top which, I think, goes to the turbo as opposed to the other end of the tube?

Take the air cleaner and the U-tube off so you can see the turbo itself. It's labeled "B" in the image below. Use a block of wood to cover the opening and starve the engine of air.

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As long as the area is fairly clean so no chunks of stuff get sucked in, the engine/turbo will be fine for your testing purposes. I know people who've run a pos $200 beater turbo car with no air filter for YEARS.
You can block off the top opening of that U elbow tube, or remove the tube and block off the actual opening of the turbo.

Edit: a picture is worth more than my bunch o' words. See mach4's post above. Lol.

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1982 300sd from craigslist $800 greased on one tank with NO CONVERSION in the Hot Texas Sun. (currently dead & awaiting engine damage investigation and/or longblock swap)
new daily: '03 vw 5speed jetta tdi wagon. bagged&chipped

Great, guys. Know exactly what to do.

Thanks for the help. Just waiting to receive the new shut off valve from Bill Grissom.

On a 602/3 remove the crossover tube and start it, you don't need boost from the turbo to test this. This is right on top of the engine and takes 2minutes with a 5mm allen wrench to remove.

But, in the rare case where the turbo seals are bad, you could potentially suck a lot of oil past the turbo seals if you block upstream of the turbo.

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Mine is a 617.

I cannot get my OM617 to stop running when blocking the intake at the air filter either.
When I block the air filter intake, it just starts sucking air in through every tiny gap in the air filter lid it can find. It makes the car sound like a tea kettle, as the air being sucked in through tiny gaps makes a whistling noise.

Do what others suggested and block it off at the turbo.

Be damned careful though:
The turbo will have no problem munching through your fingers, and it'll barely slow down doing so. Using a big, sturdy piece of plywood that you can slide in front of the turbo while keeping your hands clear of it will minimize the risk of losing any extremities.

Alternatively, you can use a CO2/carbon monoxide fire extinguisher (I don't think those come cheap though...) and spray it in the intake. It'll starve the engine of air without damaging anything, but honestly: I wouldn't know where to get one. I haven't seen stores carry them, and folks say you need a 20-pound one to be successful. All I've ever found was dry-chemical ones.

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Where you are blocking it would stop a run-away condition, since that is the only air inlet. It must be getting enough air to idle thru leaks, and initially from the crankcase (via PCV tube). Did you block it off tightly and hold it for ~20 sec? If everything is tight it would suck the crankcase down to ~10" Hg vacuum and then stall for lack of air.

If you want to be more assured, block "B" in post 8 photo. In know that works because I once stuffed paper towels there while installing a replacement engine, then missed them when I put the plastic duct back in the dark. The car idled, but wouldn't go faster than 5 mph. I started worrying that the engine was bad (it ran fine on the ground), but measured 10" Hg vacuum on the intake which tipped me to the problem.

I did like you first time I replaced a stop valve, based on the Diesel Giant article. Since then I wonder how anybody could install a stop valve thus. You would have to jam it into the IP if it was pushing on the back of the lever. It should go in easy and the arm grab the lever when you pull it back. Pretty obvious once you do it. Just install it angling in from the block side (where there is more room anyway) and with the stem sucked in.

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Menards: CO2, Foam, and dry-chem. I don't think I've seen a 20lb one though.

The trick would be to have enough CO2 to continue spraying until the engine has stopped turning so that it will not re-start, if it's truly a runaway even 5,000rpm will take a loooong time to spool down. I thought of using a halon extinguisher for the same reason, keep them in the car and garage, but enough halon to stop the engine will cost as much as a new engine!

I think it's pretty easy to verify that this is done correctly as Bill mentioned above.

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