Mahindra Scorpio Diesel ‘runs away’: We explain what ‘diesel runaway’ is [Video]


The diesel engines are known for their longevity. However, badly maintained diesel engines can behave unusually and die on you. This incident from Goa shows a Mahindra Scorpio suffering from the “engine running away”. It happens with the diesel engines and can leave the engine dead.

The video shows a Mahindra Scorpio billowing thick smoke from the tailpipe on the public road. This is called engine running away and happens only with the diesel engines. There is nothing much that can be done to stop the phenomena and the engine keeps on revving uncontrollably till it blows out. The engine cannot be stopped even after turning off the ignition of the vehicle or cutting off the fuel supply.

Diesel engines are a quite complicated piece of engineering. The diesel engines can burn any combustible oil and the right temperature and pressure in the combustion chamber to work properly. If engine oil or any other combustible oil reaches the combustion chamber of a diesel engine, it may start running away. There can be various reasons behind the diesel engine running away but the most common and relevant cause of the engine running away is the badly maintained turbocharger.

Scorpio Diesel Runaway Featured

A failed oil seal in the turbocharger can prove to be fatal for any diesel engine. Turbochargers rely on engine oil to stay lubricated then there are sophisticated oil channels around the turbocharged to keep it well lubricated. However, if an oil seal breaks and the engine oil starts to enter the combustion chamber of the engine. If the leaked oil is in a small amount, it burns away in the combustion chamber. But if the flow of the oil is higher and the combustion chamber has required temperature and pressure, it will start burning the engine oil. It should be noted that diesel engines do not use spark plugs to ignite the fuel. The pressure and temperature in the engine chamber make the fuel burn. Once the engine oil starts to burn, it does not require the fuel to run and this is when the engine starts to run away.

The engine speed of the diesel engine is controlled by the amount of fuel it gets. While there is a way to regulate the flow of the diesel fuel in the engine, the flow from oil leaks cannot be controlled. As the engine starts to burn the engine oil on its own, it creates a vacuum and more engine oil rushes in. The suction created in the engine chamber ensures a steady supply of oil from the leak and the engine revs continue to rise and may even cross the red line limiter.

The engine runaway phenomena is not so common in modern engines. Many modern engines now feature cut-off valves that choke the engine of air and saves the engine. However, they are not the guaranteed way of saving the engine. There are certain conditions that need to be met for the cut-off valves to work properly.

How to save the diesel engines from running away?

The best way to avoid engine runaways is to follow the maintenance schedule of the diesel cars strictly. The engine oil should be changed at the mentioned intervals and only the grade of the oil mentioned by the factory should be used. Also, only the mentioned amount of fuel should be filled in the engine chamber. If the engine chamber is overfilled with the oil, it can create a higher pressure and break the oil seals.

If the engine has already started to running away, one can do a few things to try and stop it. In manual cars, one should never press the clutch when the engine starts to run away.

  • Manual cars can shift to highest possible gear when the engine starts to run away. The traction from the transmission and clutch may stop the engine. One can use the handbrake to apply more pressure on the engine and stop it but it puts a lot of stress on the transmission and clutch. It is basically stalling the vehicle to stop the engine.
  • In automatic cars, locating the air intake and choking it with fire extinguisher ensures that the air does not reach the engine chamber and the combustion stops.
  • The surest way of stopping the engine when it starts to run away is to locating the engine air intake and choking it completely with something solid. If the air intake is free of any leaks, it will stop the flow of air to the combustion chamber and stop the process.

One should never start the vehicle after an episode of a diesel engine running away. Always visit an authorised service centre or a local mechanic who can identify the problem and fix it.