Boat Rat: Killing the Runaway Diesel

Captain John of tells us the best way to kill a diesel engine as fast and safely as possible.

If you have a small diesel engine, you may one day experience a “runaway diesel”. That means your engine throttle or fuel stop will not control your engine. Like a beast gone mad, your engine will keep running and running, no matter what you do with the throttle. Follow the steps below to prepare for this emergency.

Why this Happens

Spring an oil leak or overfill your oil sump and it can be sucked up into your engine air-inlet manifold. Other combustibles can cause problems too. In his book “Marine Diesel Engines“, Nigel Calder warns that poor propane locker ventilation can lead to vapor leaks into the engine compartment that get sucked into the intake.

That’s why no matter what you do with the fuel supply, your engine will continue to run. It’s no longer running off of diesel fuel, but oil or propane. And until those food supplies are used up, it will continue to run at increasing RPMs–no matter what you do with the throttle! Unless you know the single “take action” step that can stop any diesel engine.

Smother the Beast to Snuff it Out

Your diesel engine needs great volumes of air to run. Make this your #1 objective if you suspect a runaway diesel. Have an action plan already worked out, your crew briefed and ready, and conduct a simple mock drill or two so that all hands understand the procedure. There will be no time to explain in an emergency.

1. Locate the air filter on the side of your engine. Show your sailing crew or partner this location so that they know right where to go in an emergency. Seconds count!

2. Flatten a hatchboard, stiff foam cushion (not soft!), hard lifejacket, or large hardback book over the air intake. Take care not to get fingers or hands near the opening because of the high volume of air being sucked into the engine. Also, be aware that any loose gear can be sucked into the intake.

3. Hold your board or other flat object against the air intake until the engine dies. Do not attempt to restart the engine. Anchor and call for assistance or–if out at sea, heave-to if necessary to make repairs.

As a safer alternative method, aim a small CO2 extinguisher (make sure to use a carbon dioxide extinguisher) at the air intake. In an emergency, pull the pin on the CO2 extinguisher; aim the discharge funnel at the air intake; fire short bursts into the intake.


Follow these sailing tips in an emergency in case your sailboat diesel engine decides to pull a “runaway” on you. Train your sailing crew or partner to be prepared for the unexpected for safer sailing–wherever in the world you choose to cruise!

NOTE: This article was updated on May 16 to reflect a correction that the shifter will not become uncontrollable but the engine throttle will become uncontrollable.

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