Don't Let A Runaway Diesel Get The Better Of You


A guide to understanding what a runaway diesel engine is, and tips on how to prevent or stop it.

Read in this article:

Understanding Diesel Engines

Diesel is made from the distillation of crude or other oils and ignites automatically under extreme pressure in compression engines. A runaway diesel can occur because engine oil can also combust under these conditions - but what is a diesel engine runaway exactly? A diesel's torque output is determined by the fuel supplied by the injection system and it normally runs very leanly on an excess-air mixture, which is one of the reasons for the fuel efficiency of diesel engines.

This is why there are several diesel SUVs around and why the best diesel trucks can return around 30 mpg on the highway, such as a used Chevrolet Colorado. The combined gas mileage of a large SUV such as even a used Chevrolet Suburban 3.0-liter diesel is over 20% better than the equivalent 5.3 gas V8 model's. For a great gas/diesel comparison, read this.

Because of the high diesel combustion temperature, combustion requires no spark plug, which is key to understanding diesel engines - and what causes a diesel engine to run away.

What is a Diesel Engine Runaway or Overspeed?

A runaway diesel engine overspeeds to beyond its redline by sucking in extra fuel or a high concentration of fuel vapors from an unintended source, causing the engine to speed up by itself to very high rpm until mechanical failure occurs. If extra combustible fuel is drawn into the combustion chamber on the intake stroke, it is compressed and ignites on the compression stroke. It feeds the runaway diesel engine and can damage or destroy it - a runaway diesel engine can blow up explosively. Most car owners will never experience it, as it is a problem only affecting diesel vehicles, which are less popular in the United States. It is less likely to happen to most modern diesel engines with advanced electronic controls, as there are more safeguards in place to shut down the engine in case of an emergency.

The engine will remain in its runaway state until:

  • The air supplied to the engine is cut off
  • The fuel supply is interrupted or the source runs out
  • The engine dismantles itself
  • Bearing or cylinder surfaces seize due to a lack of lubrication.
Runaway Diesel

What Causes a Diesel Engine to Run Away?

As mentioned earlier, extra fuel from an unintended source must be sucked in by the engine for it to run away, and this can occur in various ways:

  • Fueling problem: Overfueling caused by a fueling system that pumps too much diesel to the engine can cause a runaway event.
  • Oil leak: Oil leaks may have various sources, but if there is a possibility that oil anywhere in the lubrication system can be sucked into the intake manifold or combustion chamber, it can trigger a runaway event.
  • Failed turbocharger seals: Failed turbocharger seals will cause oil to flow through the turbocharger and into the combustion chamber, potentially resulting in a runaway.
  • Introduction of combustible gases: In rare cases, and mostly in industrial settings with stationary diesel engines, the presence of a gas leak close to the engine can cause the engine to ingest combustible gases that can also cause the engine to run away.
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How to Stop a Runaway Diesel

Turning off the ignition is not how to stop a runaway diesel truck or car engine, because a diesel engine does not have spark plugs and, therefore, needs no electrical power to sustain the runaway event. Cutting off the source of the fuel will stop the runaway, but if oil is sucked into the combustion chambers internally, that's impossible. You could select a high gear in a manual car and let out the clutch, but this is not guaranteed to work and might damage the clutch. Another solution is to block the engine's air intake completely with something like a towel so the engine chokes and dies through a lack of oxygen to sustain the combustion process.

Spraying a CO2 fire extinguisher directly into the engine's air intake might also deprive the engine of enough oxygen to shut it off. Some stationary diesel engines have a decompressor that can be operated via a lever, which opens the exhaust valves to prevent combustion pressure from building up, also causing the engine to stop. Remember that you don't have much time to act and trying to stop a runaway engine might be dangerous, especially if you can't do it swiftly.


Diesels are called 'oil-burners' because their high combustion pressures and temperatures are great for combusting oil-based diesel, as well as its own motor oil. Because no spark is needed, engine runaways can occur, but it's unlikely if your engine is left standard and if you properly maintain and repair it when parts fail. If it ever happens to you, remember that a catastrophic failure can endanger life and limb, so if the hood is not already up and you have a towel ready to stuff into the air intake, it's best to stand well clear and wait for the engine to expire.

Compression Engines

FAQs About Diesel Engine Overspeed

Can a diesel runaway condition be prevented?

To prevent a runaway condition from escalating to an engine-destroying catastrophe, an inline shut-off valve can be installed in the diesel engine's intake system. Such a valve can be activated manually by the driver, but automatic systems exist that activate when certain conditions are met. A butterfly valve shuts off the engine's intake tract completely, depriving the engine of the air it needs to feed the runaway combustion. The engine will simply choke and die.

How can I tell if a diesel engine is at risk of running away?

Diesel runaway is a rare condition and unlikely to happen, but age and poor maintenance substantially increase the risks. Old engines with worn piston rings can suffer enough blow-by that oil can be sucked from the crankcase into the combustion chamber, setting off a runaway condition. Worn turbocharger seals and other sources of oil leaks can contribute. A properly maintained engine that is overhauled before it becomes too worn out should not suffer a runaway event.

How quickly does a runaway event happen?

Usually, you don't have much warning and only a few seconds are necessary for a runaway event to start. As soon as the extra fuel enters a cylinder, the engine speeds up and starts to suck in more fuel from that source in a self-propagating cycle.

What must I do if a runaway event happens while driving?

Your car will start to accelerate automatically and unexpectedly, so it's important not to panic. Scan for a safe spot to pull off the road and select neutral on the transmission to disconnect the engine from the wheels. Pull off the road at once, get yourself and your passengers to safety, and get away from the car in case of a catastrophic engine failure, which may ignite a fire or, in rare cases, cause a small explosion.

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