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Pelican Technical Article:

Run Away Diesel Engine

Peach Parts


5 min






20 pound CO2 fire extinguisher

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

HALON or CO2 fire extinguisher

Hot Tip:

Never use water

Performance Gain:

Stop a runaway engine

What is a run away diesel?
It is a diesel engine that will not shut down. It is out of control. This is a known problem with diesels, and any diesel engine can have it happen.

What causes it?
The answer is unregulated fuel entering the combustion chamber.

Why is it dangerous?
Because, the normal way you shut down a diesel engine is by stopping the fuel from getting to the combustion chamber. Unregulated fuel access will cause the engine to continue to accelerate until it basically self-destructs.

How can you stop a run away diesel?
There are three ways I know of to shut down a run away diesel:
1. Seal the air intake and starve it; this can be impossible if the duct work is on the intake.
2. Shove a 20 pound HALON or CO2 fire extinguisher in the intake and empty it.
3. Run for your life and come back if/when it has stopped naturally.

Danger: Never use water, it is non-compressible and will break pistons and rods; this can puncture the block and throw pieces hard enough to kill you.

Diesel engine cycle theory:
To understand the problem you must understand diesel engine cycle theory; intake stroke, regulated fuel is injected, compression stroke, power stroke, and exhaust stroke vents the cylinder.

A diesel engine will burn a very wide variety of fuel, especially when at operating temperature, fuel is fuel, if it burns the engine will run, crank case oil will burn as fuel. Diesels are called oil squeezers because the friction heat of compression causes combustion. There is no throttle; engine acceleration is controlled by the volume of fuel injected. A diesel will keep accelerating for as long as increasing volumes of fuel and air enter the combustion chamber, the end result is that it will reach destructive engine speeds that will break things or explosively disassemble your engine. Even professional diesel mechanics have had diesels run away.

General causes of a run away diesel:
- If the turbo seal leaks on the intake side, it feeds unregulated fuel into the combustion chamber.
- If the piston rings leak on the compression stroke, the crank case oil mist gets blown through the CCV (crank case vent) and into the intake, this feeds unregulated fuel into the combustion chamber, add a restricted or dirty air filter and you have a run away diesel.
- An over full crankcase oil level can cause massive unregulated fuel to flow into the combustion chamber leading to a run away diesel.
- If the intake valve guides leak and the head oil drains are clogged with sludge, it feeds increasing amounts of unregulated fuel into the combustion chamber, which can cause or contribute to a run away.
- If the piston rings leak on the intake stroke, it feeds unregulated fuel into the combustion chamber.
- The larger the volume of unregulated fuel going into the combustion chamber, the greater the risk of a run away diesel.

Your best defense against a runaway is:
1. A clean intake system and air filter.
2. Regular oil changes.
3. Keeping a log of oil consumption.
4. Watching for excess smoke in the exhaust

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Page last updated: Tue 6/27/2017 03:16:53 AM