How Do Diesel Engines Run Away?

Working in or near hazardous environments, such as those found in the Oil & Gas business, exposes you to dangers and risks on a daily basis. Between 2013 and 2017, 489 oil and gas extraction employees were murdered on the job in the United States alone, according to the Occupational Health and Safety Administration (source: https://www.osha.gov/SLTC/oilgaswelldrilling/). The severe occurrence known as diesel engine runaway is one of the lesser-known yet lethal threats. Engine runaway is explained in this video from AMOT’s Ask the Expert series.

To comprehend runaway, you must first comprehend the operation of a diesel engine and how it varies from that of a gasoline engine. Spark plugs ignite the fuel and air combination within the cylinders of a gasoline engine. Combustion in a diesel engine, on the other hand, takes place in a very different way. Clean air is drawn into a combustion chamber by a diesel engine’s intake. The air and fuel mixture in the chamber is squeezed to such a degree that it produces high heat and ignites.

The fuel delivered into the combustion chamber is regulated by a governor, which also controls the engine’s speed. The governor controls how much fuel is allowed into the engine. The more fuel allowed in, the faster the engine will run. A diesel engine can only be turned off by withdrawing the fuel supply or cutting off the air supply.

When a diesel engine ingests a hydrocarbon vapor, or flammable vapor, through the air intake system and uses it as an external fuel source, it is known as a diesel engine runaway. As the engine runs on these vapors, the governor releases less diesel fuel until the vapors are the engine’s sole fuel supply.

It can cause the engine to overspeed, the valves to bounce, and flames to pass through the manifold if not halted promptly. These flames can create catastrophic accidents and casualties by igniting the combustible gases present. The Deepwater Horizon explosion in the Gulf of Mexico on April 20, 2010, is a well-known example of this type of mishap.

Even modest concentration levels of gas pushed into the engine intake can cause runaway in 3-12 seconds, giving little time to react. A person’s first instinct when an engine starts to runaway is to turn the key off and stop the engine. Unfortunately, because the engine is now running on combustible fumes entering through the intake, this will not solve the problem. The engine will continue to run wildly, and cutting off the air supply is the only possible alternative at this time.

Thankfully, diesel engine runaway may be avoided. Devices that identify overspeed and shut off the air supply can be put on an engine’s air intake pipe to safely and quickly shut down a diesel engine.

How do you stop a diesel engine from running away?

Diesel engine runaway is a serious phenomenon that can be lethal when working in a hazardous environment. When a hydrocarbon vapor is swallowed through a diesel engine’s intake and becomes the engine’s uncontrollable fuel supply, engine runaway occurs. When this happens, the governor loses control of the engine’s speed, causing it to speed out of control. If the engine is not stopped, it can suffer irreversible damage or, worse, ignite the gases and explode. This condition is explained in further detail in our blog about diesel engine runaway.

The only proven approach to stop a diesel engine once it has begun to runaway is detailed in this video from AMOT’s Ask the Expert series.

The most usual reaction when an engine starts to runaway is to remove the ignition key and turn off the engine. Unfortunately, this step will be ineffective. Turning off the key will have no effect because the engine is now running on fumes. At this point, the only practical option is to turn off the engine’s air supply. Fortunately, there are safety systems that can cut off the air supply during a runaway situation.

In the event of a runaway situation, these safety mechanisms are known as air intake shut off valves, and they simply block off air entering through the intake. When shutting off the air supply isn’t an option, the engine shuts down since diesel engines work by consuming clean air through the intake.

Depending on the make and type of your engine, different systems are available, but they all work on the same principle: if your engine enters a runaway condition, the air intake shut off system will operate, either automatically or manually, and cut off the air entering through the intake. As indicated below, the systems are usually mounted between the engine air filter and the intake manifold.

Now that you understand how an air shut off system works, you should know which diesel engines should have one installed. Diesel engines utilized in hazardous areas, such as upstream, midstream, and downstream operations, are all susceptible to diesel engine runaway and should be protected at all times. This comprises both large and small equipment, such as fracturing blenders and coiled tubing units, as well as light towers and generators.

The only established and dependable technique of stopping a diesel engine once it has started to run away is to use an air intake shut off device. Don’t wait until it’s too late to protect your workers and equipment from a calamity that could have been avoided.

Contact AMOT, download our selection guide, or use the filters on our products page if you need assistance choosing the right valve for your engine.

How common is diesel runaway?

Diesel engine runaway is a rare event in which a diesel engine absorbs extra fuel from an unanticipated source and overspeeds at higher and higher RPMs, producing up to 10 times the engine’s rated output until it is destroyed by mechanical failure or bearing seize owing to a lack of lubrication.

Can a runaway diesel engine be repaired?

The first thing to figure out if your engine had a runaway and it wasn’t caught before it seized on its own is why it seized. Buying an entire motor is usually less expensive than attempting to repair an engine that has seized due to metal-on-metal contact. However, the damage in a hydrolocked engine is usually limited to a few bent connecting rods. It won’t be inexpensive to fix the engine and get the car back on the road, but it should be achievable.

Can a non turbo diesel run away?

Although there isn’t a single reason why diesels—usually turbodiesels—run away, it’s safe to assume that most of the dramatic incidents we witness on social media and in YouTube videos are due to fueling failures of some kind. The phenomenon isn’t prevalent in new rigs—engine-management today’s systems regulate fueling more precisely and can perform “checks” against other things that can lead to runaway ever occurring)—but older powerplants with high miles and blow-by aren’t immune to runaway.

Why won’t my diesel engine shut off?

Stop Lever on 671 Diesel Engine: For those who are unfamiliar with the diesel engine, there is an emergency shutoff lever that allows you to manually shut it down. Look under your hood for a little lever with a red sticker that says “STOP.” Place your thumb on this lever and press firmly against the engine until it stops. If the linkage is old or misaligned, you may have to use a lot of force to get the engine to shut down. The label on certain engines may have worn off. Look for this lever near the linkages on the top drivers side of the valve cover on older 4 and 5 cylinder diesels. Look under the drivers side of the intake manifold, immediately above the injection pump, on modern engines built after 1985.

To fix a diesel engine that won’t shut off, you’ll need to first have a basic understanding of how it operates. With a Mercedes gasoline engine, this problem nearly never occurs. That type of engine is powered by a 12 volt electrical spark from your car’s battery. When you turn your key off, the electrical power is cut off, and the engine promptly shuts down. Because a diesel engine does not use electric spark to start, it must be turned off in some other way. You may have observed that your Mercedes diesel engine does not turn off quickly when you turn the key off. It may even continue to operate for a second or two after you have turned the key to the off position before ultimately shutting down. That should be enough to indicate that a diesel engine shuts down in a different way. From the first 300D in 1975 to the mid-1990s, vacuum (suction) is utilized to turn off Mercedes diesel engines – not electricity!

In simple terms, the diesel fuel injection pump has a valve positioned on the back or side. This valve is linked to an arm that extends down into the pump. The lever moves and shuts off the fuel flow inside the pump when vacuum (suction) is applied to this valve. The engine will not be able to continue to run without fuel. The engine will shut down immediately if the valve reacts rapidly. The engine may take a few seconds to shut off if the valve is sluggish. The engine will not shut off if this valve does not get vacuum or is broken. A mechanical vacuum pump placed on the front of the engine provides the vacuum for this valve. This pump pumps vacuum (suction) into the system using either a rubber diaphram or an aluminum piston. The vacuum then returns to your ignition key assembly under the dash before exiting out the back of your injection pump’s shut-off valve. Vacuum is fed through the back of the switch and on to the shutoff valve when you turn your key off. Your engine should shut off fast if the valve is working properly and the vacuum is not being drained out by a leak anywhere in the system.

The loss of suction to the fuel injection pump shut-off valve is the most common cause of your diesel engine failing to switch off. A leak anywhere in the lines is the most common cause of unexpected loss of vacuum. This leak might be the result of one of the rubber fittings becoming loose or being knocked off by mistake, or it could be the result of a leak elsewhere in your vacuum system (the most common on the 123 chassis is leaks in your door lock system). You’ll know the two are connected if your door locks cease working at the same time the engine shuts off. To solve your problem, think vacuum!

Mercedes Diesel Engine Won’t Shut Off When Turning the Key Off: Probable Cause is another video worth seeing.

Additional information is available at: If our quick repair tip doesn’t work, you’ll have to perform more debugging and work to address the problem. The first stage entails thoroughly testing your vehicle’s vacuum system. Check for dangling vacuum lines in and around the oil filter housing when you open the hood. Check the top of the valve cover for any loose lines. Examine the rubber connector fittings to ensure that all hard lines are securely connected. Those connectors should be replaced if the hard lines are loose in the rubber connections. A vacuum service kit with an array of those connectors is provided. Locate the vacuum lines for your door locks on a 123 chassis diesel and plug them off to see what happens (they are the two large yellow plastic lines going into the firewall just inboard of your brake booster). If you find that all of your vacuum lines are in good working order, you may have additional work ahead of you. It’s possible that it won’t be a quick cure after all!

Many people mistakenly believe that if their lines are correctly connected and the diesel engine would not shut off, all they need to do is buy and install a new shutoff valve. You should have figured out by now that the problem might be caused by a variety of system problems. It might be something as basic as a broken vacuum hose or something as complex as a vacuum pump rebuild. “Don’t just throw pieces at a problem,” I’ve always stated. Be warned: you can waste a lot of money and never have it fixed! To identify which parts, if any, need to be replaced, first diagnose the problem. We strongly advise you to get Kent’s “Diesel Vacuum Source Troubleshooting and Repair” guidebook. This guidebook will guide you through the process of diagnosing your diesel’s vacuum system step by step. Most 240D 300D 300CD 300TD 300SD 190D 300SDL 350SD 350SDL and S350 with vacuum controlled engine shutoff are covered by this manual. We have a convenient kit if you don’t have a vacuum hand pump tester. We have everything you’ll need to fix the problem, but we strongly advise you to troubleshoot it first!

How do I remember the diesel in my car?

Many different model names are used by car manufacturers, such as ‘TDI,’ ‘HDi,’ ‘GTD,’ ‘dCi,’ ‘JTD,’ and so on. Your automobile is most likely a diesel if it has a badge like this on the back with a ‘D’.

How do you stop a Duramax runaway?

Registered. Defueling will not stop a diesel engine that is running away owing to gas or oil in the intake. The only way to turn it off is to turn off the air supply.

What causes a 7.3 to run away?

Answer: In a runaway condition, engine oil makes its way into the intake route of a diesel engine, where it is then burned as if it were fuel. While runaway situations are uncommon, when they do happen, the cause is usually a failed oil seal within the turbocharger (although oil slipping past piston rings on the compression or intake stroke can cause it, too). Take, for example, a journal bearing turbo. Oil fed to the center cartridge from the crankcase will effectively be vacuumed into the compressor housing if the center cartridge seal that covers the shaft fails on the compressor side (intake side of the turbo). When oil enters the compressed intake air stream, it condenses into a fine, heated mist that is driven into the intercooler pipework, intercooler, and eventually the engine. Assuming the turbo’s seal failure is serious enough, the combustion chamber will be given an unregulated amount of oil—at least until the engine runs out of oil, at which point it will seize (if it hasn’t already self-destructed internally).

Turning off the ignition won’t help because a diesel engine is generally shut down by cutting off the gasoline supply to the combustion chamber. Unless the vehicle has a manual gearbox and can be stalled out, cutting off the air supply to the engine is the only option to stop a runaway. With an engine revving to the moon, however, it may be difficult to summon the bravery to open the hood, remove the air filter, and block the air intake path—not to mention the risk of being sprayed with hot oil or debris if a hard part decides to emerge.

Are you scared yet? Don’t be that way. Although runaway situations frequently result in catastrophic engine failure, they are relatively uncommon. To put things in perspective, most diesel mechanics will go their entire careers without seeing a runaway vehicle. Do not let the possibility of a runaway situation deter you from entering the diesel business.