What Makes A Diesel 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 runaway?

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 a runaway diesel?

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 be fixed?

To bring a diesel engine to a complete halt, the air or fuel supply must be cut off. Because the fuel in a runaway engine is uncontrollable, the only effective technique to halt a diesel engine is to cut off the air supply. To accomplish this, the vehicle’s air inlet must be closed or sealed in some way. To do this, a positive air shutoff is used.

A positive air shutdown (PAS) is a throttle value (buttery fly or guillotine) that is often put in a diesel engine’s intake tubing. The valve is set to open by default and does not operate unless directed to. The user can activate the ‘PAS’ in the event of a diesel engine runaway by pressing a button that slams the throttle blade shut, starving the engine of air and causes it to stall.

Can a runaway diesel explode?

Depending on the richness of the vapor cloud, a runaway diesel engine can explode in seconds. There is no time to take remedial action, and approaching a runaway engine to try to stop it is perilous.

What happens when a diesel engine runs out of oil?

Your vehicle’s lifeblood is engine oil. It is required for the proper operation of your engine. Any lack of engine oil, or even unclean oil, will result in extreme engine wear, and running a car low on oil can result in some fairly disastrous circumstances.

Your engine will break down if you run out of oil. Fast moving elements with a lot of potential friction can be found inside the engine. The engine oil is the lubricant that keeps parts sliding smoothly against each other rather than grinding them down. When the engine runs out of oil, it begins to grind and ultimately seizes, causing the car to stall. Your engine will be harmed, if not completely destroyed. Many drivers will not be aware that their engine oil is low until it is too late. More diagnostic instruments, such as an engine oil pressure sensor unit and dashboard tools, are available in newer vehicles, which may assist in identifying low oil issues. Drivers in older vehicles may often have to manually check the oil because the “check oil” dash light will not alert them to a low oil problem.

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!

What causes diesel knock?

Diesel knock occurs when injected fuel auto-ignites and combusts in the premixed stage of combustion, as opposed to spark-ignition knock. While this is a regular element of diesel engine functioning, there are times when excessive amounts of fuel combust in a premixed manner due to a variety of factors.

Why are most diesels turbocharged?

By pushing more air into the combustion chamber, a turbocharger improves the compression of an engine. Because of the larger air mass, more injected fuel may be burnt. This has two effects: it improves engine efficiency while also increasing air mass. The torque output is improved as a result of this. Because the torque production of diesel engines is regulated by a forced flow of the air-fuel combination, they are suitable for turbocharging.

Can a non turbo diesel runaway?

On social media recently, there has been a lot of discussion about footage of diesel trucks that are emitting white smoke and then suddenly die. The vehicle isn’t performing a massive burnout. You’re probably looking at a rogue diesel engine. No, it isn’t solely a problem with Power Strokes, Duramaxes, or Cummins, so stop slamming brands. It can happen to any diesel engine in any vehicle, but turbo-diesel applications are more common.

So, before you go commenting on that video in your newsfeed about how much better your Chevy is than the Ford, take a moment to learn more about what a runaway is and, more importantly, how to stop one if it ever occurs to you. You must either cut off the air supply or the fuel supply to halt a runaway diesel engine. The easiest alternative is to turn off the air supply because it is getting fuel from an unknown source. Finding something to block the intake can lower the RPM and cause the engine to shut down.