Karl Denninger wrote:

If the cutoff is on the vacuum side of the supply, then shutting the valve
will not instantly shut the engine down. In fact, it will run until the
fuel in the lines is exhausted and the vacuum drawn by the lift pump
exceeds what can be pulled through. If the valve is in the wrong place,
this could be a considerable amount of fuel (e.g. if you can draw air into
a filter cannister, you could conceivably have to run thruogh the entire
cannister's contents before the engine shut down.)

It doesn't matter. IF you have a runaway engine you must secure the fuel
to the engine as a matter of safety and good practice. You do not
necessarily know why the engine is running or if it will suddenly
destroy itself and create a much more serious problem, like a fire
burning off the open fuel supply. Do not hesitate to shut off the fuel.
You should have remote fuel shutoffs in any event.

Spraying a CO2 extinguisher into the intake will definitely shut it down.
It will also likely destroy the engine. CO2 is stupid cold coming out
of an extinguisher and the shock cooling will hose the motor fairly
reliably.

Nonsense. It will not damage the engine and it will not definitely shut
it down either. What happens depends on the reason for the runaway and
the condition of the engine air system. It will probably shut down the
engine.

BTW, air doors are popular emergency shutdowns on Detroits. The problem
with tripping them is that above idle the engine will attempt to pull
whatever it can through whatever is in the way after the air is cut off.

The usual consequence of pulling the air doors is that the seals in the
blower are destroyed and significant crankcase oil is pulled through.

Which seals? It is common practice to use the emergency shutdown when
testing the operation of emergency devices at full speed on 6-71 powered
emergency diesel gensets on ships. I have done this hundreds of times
and have yet to destroy any sort of seal.

That beats having rods flying all over the engine room, but it still
requires a teardown if they're used under load before the engine is
restarted.

Again, nonsense. If the engine did not overspeed for an extended period
or run without oil pressure it is probably perfectly OK once the reason
for the original runaway has been addressed.

DO NOT put it before any significant reservoir of fuel, such as a filter.

Most little diesels have a shutdown solenoid on the injector pump anyway
so you don't need to add another valve. The solenoid most likely
requires power to remain in the run position. Consider that the scenario
will almost inevitably include the operator attempting to stop the
engine by normal means (the switch) and then discovering that it will
not stop. That may mean the solenoid has failed in the run position or
there is another source of fuel, most likely lube oil. Shut the fuel
valves off at wherever point in the system you can reach one. If you are
able to approach the engine you may be able to move the speed lever on
the pump/governor to the stop or low idle position. If you have a
2-stroke GM then trip the air door if it hasn't already tripped.

If the engine is "running away" slowly and is showing no signs of major
lube oil consumption, just wait till the fuel is exhausted. It will
either stop or slow even more. If it does not stop then cover the intake
filter or screen with rags, plastic bags or something to block the air
supply. It should stop. If it doesn't and/or seems to be gaining speed
then use a CO2 extinguisher into the intake. You will NOT harm the
engine by using CO2. Even if you were somehow able to squirt liquid CO2
into the intake it would not somehow shock cool the engine and damage
it. There is no little boat floating that has a large enough source of
liquid CO2, or a means of delivery to supply enough liquid to damage the
smallest of engines.

Rick