Josh Rowles profile picture Josh Rowles 4 years ago 70
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Runaway Diesel - What is it? And How to Stop it.

Runaway Diesel - What is it? And How to Stop it. - Blog

A diesel owners nightmare. Runaway diesel. It’s exactly how it sounds. A diesel engine that just wont stop. Causing many problems, such as damages, injury or worse.

I hope this will help educate someone who is in the market for a diesel or currently has a diesel powered vehicle. This is most definitely not meant to discourage current or future diesel owners. My intention is to help those who may come across this rare, but possible occurrence.

Before I start, I want to stress something. Don’t value your truck or car over your own life or safety. I know it’s hard to hear, and I’m even cringing writing this. But you can always repair/replace the car. (Crying is okay, just don’t let the tears water-log your phone.)

Runaway Diesel - What is it? And How to Stop it. - Blog

First off, lets start off by going over the basics of a diesel engine.

A diesel engine does not operate in the same manner as a gasoline/petrol engine (go figure). A gasoline engine uses a spark plug to ignite the air-fuel mixture. Whereas a diesel uses compression-ignition and constantly heated glow plugs. In a diesel, the air-fuel mixture happens in the combustion chamber and not the ports like a gas engine. Diesel fuel is injected into the cylinder just as the piston is nearing Top Dead Center (TDC). A standard gas engine usually uses a 8-12:1 compression ratio. Most diesels have anywhere from 17-23:1 compression ratio. From the beginning, diesels have had direct injection, and they don’t use butterflies to manage air flow. They instead throttle the fuel going into the cylinders, leaving the intake with little to no obstruction. This will cause problems, for which I’ll elaborate upon later.

Now we can move onto what a runaway diesel is, as we’ve discussed the basics.

The intake butterfly is the flap that restricts the airflow and thus the fuel going into a gasoline engine. Diesels don't use this part in the same manner, if they have one.
The intake butterfly is the flap that restricts the airflow and thus the fuel going into a gasoline engine. Diesels don't use this part in the same manner, if they have one.
UPS truck with a runaway diesel condition.
UPS truck with a runaway diesel condition.

A runaway diesel used to be a relatively common occurrence. But now as times have changed, it’s a rare situation in modern diesels. Most Electronic Control Modules (ECMs) can meter the fuel more accurately and sensors warn the ECM and allow it to prevent things like this from happening. But there’s an exception to everything, and parts do fail.

Now what causes a runaway diesel condition? Well its many factors. Pumps that stick and meter too much fuel, oil seal failure in a turbocharged engine, overfilling the crankcase, broken/damaged internal fuel pipes, incorrectly assembled or faulty fuel linkages, and even dual fuel engines that use bottled gas (like propane), or are in an environment saturated with airborne vapors. But most usually age and wear or vapors are the common cause.

Runaway diesel is most noticed in fleet vehicles that are used constantly, poorly maintained, and almost always running, such as the unfortunate UPS truck above. But age and neglect seems to be the most common cause of runaway diesel with one exception that can effect diesels new and old. Since a diesel engine meter’s the fuel instead of the air, this leaves a completely unrestricted air flow. So if hydrocarbons such as diesel, gasoline, propane, or other airborne fumes that are at the right concentration, can make the engine run uncontrollably to its doom. Sometimes the vehicle will catch on fire due to the intense heat. So if you’re going to let it die, get as far away as you can.

Another way the engine can run uncontrollably is caused by wear and neglect. As piston rings wear, they allow a minute amount of oil into the combustion chamber, as with most internal combustion engines. But if the wear gets to the right level, and the diesel engine gets hot enough, it starts to vaporize the oil in the crankcase and it leaks passed the piston rings. Once in the cylinder and combustion chamber, the vaporized oil acts like diesel fuel and ignites under the extreme compression. Once this happens, the engine will go until the oil has been exhausted and/or it blows up, whichever comes first.

Ford 6.0L Power Stroke Diesel brochure page.
Ford 6.0L Power Stroke Diesel brochure page.

Now before I get into this, I’m going to stress again. DON’T value the vehicle over yourself and others. And if you are going to attempt to stop it, be very, VERY cautious. If you don’t think you can or are too afraid, don’t even try. It’s not worth it.

So we get to the question of how does one stop a runaway diesel?

Well now I know what you’re thinking, “just switch off the ignition.” That won’t work, as a diesel has unrestricted air flow and uses compression-ignition. This leaves very few options to work with.

Your only feasible option to stop the engine is to smother it, and this can be done by one of many ways.

You can use a CO2 extinguisher to smother the engine. This is the easiest and one of the safer options, but does require you to get to the intake, which could be tricky. You can also physically cover the intake, weather it’s by the throttle body, or the turbo if the vehicle has one that is easily reached. Now you should aim to try the throttle body, but in a snap the turbo will do the job all the same. Just be weary of that turbine. It’s spinning at 10s of thousands of RPMs, and it will eat limbs like a blender. If you can avoid this, I would recommend doing so.

A third option only applies to vehicles that have a standard (manual) transmission. This will stop the engine with the best and safest results. As you are slowing down, or while you’re stopped, which is preferable, shift into the highest gear and push the foot brake with relatively hard force and engage parking brake if possible. Slowly let off the clutch and let it grab the flywheel. The engine should stumble and die. This method may work, depending on your vehicles condition and if the clutch doesn’t glaze over and slip.

After you get the engine to stop, UNDER NO CIRCUMSTANCES should you start it again. Even after a small or mild runaway condition. It will just start it all over again. Call a tow truck and, if need be, push the vehicle out of the way of traffic. Also don’t sit in the vehicle, just in case the heat causes a fire.

Now you know what a runaway diesel is, what causes it, and how to stop it. Of course there are too many factors at play to cover everything. And every situation is unique. But knowledge is power, and knowing how to possibly prevent and deal with a runaway diesel can save lives, and possibly your pride and joy.

Thanks for readin’. This is also my second article, so feel free to constructively criticize it. And your thoughts are always welcomed.