Karl,

Chuck's boat, like many if not most in this forum, does not suffer the
indignity of housing a GM diesel. The shutdown procedures vary according
to engine type and manufacturer and even installation so what I write
may or may not apply to an engine with a GM on the header tank.

The oil seals on the GM blower are simple lip seals and the greatest
differential they could ever see is 14.7 psi. That is far from a
damaging pressure. I have several GM service manuals and none of them
say a word about damaging seals by testing the air door shutdown.

A "runaway" engine is one which continues to run after the normal
shutdown devices have been selected to "stop" and is increasing in
speed. It is an emergency condition.

An engine that does not stop but continues to run at idle speed or
slower is probably not an emergency and may be dealt with as an
inconvenience. It does not require heroic measures to secure the engine.

CO2 will not harm an engine. A halon fire extinguisher will produce
poisonous gas (phosgene) and contaminate the lube oil with phosphoric acid.

If the engine is a true "runaway" the best option for most small boat
operators is to secure the fuel from a safe position away from the
engine and standby with fire extinguishers and a radio. Considering the
limited access to most small boat engines and the effect of the loudly
accelerating engine on the psyche of the boat operator there is very
little chance that many readers - myself included - will have a very
great desire to bend over that engine and try to unscrew injector pump
vents or open the high pressure side of the fuel system (which is very
foolish anyway) in order to stop the engine. Chances are the cause of a
true runaway is not fuel related so removing the fuel may not do much to
alleviate the problem. The governor would have already reduced fuel to
minimum unless the governor is the problem and this is no time for
intensive troubleshooting.

If you feel you can safely approach the engine you can attempt to block
the intake with rags, a jacket, a rain slicker, anything you can wrap
around the intake filter or screen. This will probably reduce the speed
considerably and give you time to think about your options. If the
engine is running on lube oil but has been slowed enough to work around
it go ahead and add oil, there is no point in just standing there
watching it destroy itself completely.

If you have slowed it down and have a fire extinguisher available, not
dry chem, go ahead and squirt it down the intake. Don't be modest about
this, it will take a lot of CO2 applied very quickly. If that doesn't
stop the engine make sure you re-cover the intake to restrict the air
available. If the engine continues to run at all it will slowly build up
so much oil in the cylinders (if that is what it is running on) that it
will stop eventually.

If the engine simply does not stop when the key is turned but continues
to run at idle or slower there are several steps that can be taken.

Put the boat in gear, if it is running at less than idle that might
overload it enough to stop the engine.

Shut off the fuel and wait until the engine quits. Nothing wrong with
this, it won't hurt the engine and is no cause for panic or
handwringing. If you are at the dock and the engine is idling, put it in
gear and run it up a bit if possible, that will burn off the fuel
faster. If you are not at the dock why do you want it shutdown? Maybe
this is a sign you should head for a maintenance dock or someplace where
loss of power will not create headaches.

If you can reach the engine easily, try tapping the shutdown solenoid
mounted on the injector pump with the plastic end of a good sized
screwdriver. Professional marine engineers use a hammer but calibrate it
first which you may not have time to do. If that doesn't work try moving
the lever on the injector pump. Which lever? I don't know what you have
so just move any lever you see there and if the engine speeds up, move
it the other way. It may stop the engine. There may be a separate
shutdown lever near the throttle lever on the pump.

The bottom line of this is, if you have a true runaway, keep away. If it
just won't stop and you are not inclined to work on a running engine,
park the boat somewhere safe, shut the fuel off and call a mechanic at
your convenience.

Rick