What Happens When a Diesel Engine Runs Away?

A runway diesel is a diesel engine that has begun to rev out of control. An excess in fuel in one form or another has been introduced to the engine, and by no control of the operator, the engine will continue to rev until either the fuel supply or air feed have been cut, or catastrophic failure has occurred. If the cause is excessive fuel, massive amounts of black smoke will billow out of the engine, making the immediate environment toxic. Vision will also be highly limited and these two factors are often the reason why operators will have no choice but to stand back and wait for the problem to sort itself out.

There are fears of what could happen aside from blowing the engine in the case of a runaway diesel. The idea of an engine running away while driving will strike fear in your heart. If in gear, the vehicle would naturally accelerate and become hard to stop, so the best thing to do would be to get the vehicle stopped in order to prevent an accident. Getting the transmission in neutral and pulling off is the best measure if at high speeds.

If the vehicle is stationary, the risks are only to the engine unless you are too close. If a catastrophic failure occurs, rods or pistons may come out of the engine or the turbo can blow apart which puts anything within immediate range in danger. But it’s very unlikely for something like this to occur. One should be aware of this possibility, but more concerned that the engine may suck its oil system dry and damage the internal parts.

There are many causes for a runaway diesel engine. The root cause is typically excess fuel supply. Fuel pump failure will often be to blame when this event occurs. The mechanical fuel pump has an internal governor that will keep the pump at a stable rpm. If this governor is to fail, the chance of a diesel engine to runaway is very high.

Another major cause can be the crankcase breather pipe that feeds to the intake. Older engines with excessive blowby will have a decent amount of unburned fuel being fed into the oil, and this may make its way to the combustion chambers thus causing a diesel engine to runaway. This is also what can cause a runaway if a vehicle with a diesel engine were to turn on its side.

Turbo diesel engines face a potential threat due to the oil feed to the turbo. If this feed were to leak into the turbo, the engine will suck in the oil and burn it off just as it would if it were taking on oil through the crankcase breather.

Once one understands what causes a diesel engine to runaway the steps to take in order to prevent it from ever occurring become apparent. Luckily, a diesel engine running away is a very rare occurrence, particularly for passenger vehicles.

Most of the images and videos you will come across will feature fleet vehicles, and there is a reason behind this. Fleet vehicles see heavy wear and tear every day and if not properly maintained, the diesel engine will have a greater chance of running away.

So if you are afraid of the event of a runaway diesel engine, the best preventative step is proper maintenance. With a brand new vehicle, this means keeping up with gaskets and seals associated with the fuel system and making sure the pump is regularly serviced. With an older vehicle, you will want to consider running through everything before driving it to ensure no failures can occur.

The first thing anyone will want to know is whether or not you can shut down a runway diesel or stop it in some manner. Simply cutting off the engine’s power supply and fuel source may not be enough, and because the smoke will billow out of the engine, time is of the essence. This is no time to fly by the seat of your pants; you’re better off knowing what to do beforehand.

Diesel engines have an unrestricted air supply that only makes the situation worse. It is also a place to focus your efforts. Fuel cannot burn without air so if you can manage to cut the air supply or reduce it, you have a good chance of stopping the engine from running away.

This does not mean one should stick their hands anywhere near a runaway diesel engine. Instead, remote means of snuffing the air feed will be needed. This is why many recommend using a CO2 fire extinguisher. By shooting the air intake with the fire extinguisher, you can stop the combustion cycle.

Dealing with the aftermath of a runaway diesel engine can be quite the project. This is especially true if what stopped the engine was a catastrophic failure. In no case should one try to restart the engine after it has finally shut down. Instead, have the vehicle towed home or sent to a shop.

If the engine has suddenly halted, thus shutting down, you will need to find out why. If it threw a rod, you will either need to rebuild the engine or get a new one entirely. If a bearing spun, you will need to have the bottom end repaired or even rebuilt.

If you intend on reusing the engine that had run away, you will need to find out what the cause was and repair it. However, the safest measure would be to completely overhaul the engine to ensure it will not happen again.