Engine Runaway
A dangerous condition in diesel engines where the engine draws in fuel from an unintended source and accelerates uncontrollably.
Engine runaway is a rare and highly dangerous condition specific to compression-ignition (diesel) engines where the engine draws fuel from an unintended external source (such as engine oil or combustible gas in the air) and accelerates uncontrollably. Because diesel engines control speed by fuel metering rather than air throttling, the engine will spin faster and faster, ignoring throttle inputs.
The runaway continues even if the ignition key is turned off, as the heat of compression keeps igniting the alternative fuel. The most common cause is a failing turbocharger seal that leaks pressurized engine oil into the intake tract. Once the engine starts running on its own oil, it creates a feedback loop: higher speeds pull in more oil, accelerating the engine until it suffers catastrophic mechanical failure or seizes due to oil starvation.
To stop a diesel runaway, the air supply must be physically choked off, or a CO2 fire extinguisher must be sprayed into the intake duct to displace the oxygen. Attempting to stop it by turning off the key will not work. Regular inspection of turbocharger shafts for radial play and monitoring crankcase ventilation systems helps prevent oil leaks that can lead to runaway.