Have You Heard Of A Runaway Diesel?

One of the best things about a diesel engine is the fact it doesn’t require an ignition system. The basic design of the engine provides the heat necessary to start combustion. This reduces the complexity of the system and helps make things more reliable. Unfortunately, there can be some drawback to not controlling ignition.

runawayDiesel engines are designed to run on a wide range of fuels. Unfortunately, some of those fuels are oils and the engine isn’t real picky what type of oil. Ignoring catastrophic failures, oil can find its way into the combustion chamber via a leaking oil seal in the turbocharger and/or a broken piston ring to name a couple common ways.

Out of the three required elements needed for ignition (heat, fuel, air) the engine can now supply two of them and the third oxygen isn’t regulated. So, an engine will rev as high as it can until it self destructs. Really, the only way to turn the engine off is to block the air supply. Now with the typical diesel engines we use in our trucks, when they runaway you cannot put your hand over the intake and kill them. You need to block the intake with something that cannot get sucked in and won’t let air pass.

The video above shows how scary it can be when first firing up a new engine. Lucky for the owner he was quick on his feet and found a way to snuff the engine out. Judging by the smoke and what he was doing, this was probably a stuck rack and not true oil runaway. But you can see how quickly an engine revs up with virtually unlimited fuel.

Having been in a situation where an engine was running away, it can be very scary. Especially when you have something blocking the intake, but there is just enough of a hole or leak to keep the engine running at a few hundred RPMs. You know as soon as you let go, the engine will scream back up until it blow up. So, you don’t move and instead just pray that you can figure out where the leak is coming from and block it before you lose your grip.

How common is this in our trucks that we drive today? Well, Josh Smith, a member of Powerstrokenation.com recently found out. “So the other day I was driving down a feeder road about 45 mph when all of a sudden my truck started to take off like I stomped the gas pedal down to the floor. I panicked because a red light was approaching fast. I hit the brakes but it only slowed it down a little bit. So I put it in neutral to slow down and my RPMs where pegged. I then turned off the key, but the truck wouldn’t turn off either! After bellowing black smoke for what it seems forever but in reality was maybe 30 seconds until it shut itself down. I coasted over to a side road and got towed home. After that I tried turning the key but it seemed hard to turn over like the batteries were weak. It now sits in my garage and I’m scared I blew the motor and I’m puzzled what could of caused all this.”

With millions of light duty diesel trucks on the road, this isn’t super common, but is happens enough that you need to be aware of it. Also, make sure you at least have an idea what to do if it happens. It is your reflexes that will make the difference in the outcome.

About the author

Chad Westfall

With diesel running through his veins from childhood, Chad has more than a decade of experience in the automotive industry. From editorial work to wrenching, there isn’t much he hasn't conquered head-on. When he’s not writing and shooting trucks and tech, you’ll find him in the shop working on turning the ideas floating around in his head into reality.
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