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.
What makes a diesel engine 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.
Can a fire extinguisher stop a runaway diesel?
Diesel engines are capable of running on nearly any fuel. This is a huge advantage, but it also has a deadly weakness. When a turbocharged diesel’s oil-feed line or seal fails, oil is pumped directly into the combustion chamber. That’s a diesel engine on the verge of exploding. Unless the air intake is blocked or the air is displaced with CO2, the operator cannot stop the engine because it is now operating on motor oil and will be destroyed by mechanical failure or bearing seizing. Directing a CO2 fire extinguisher into the air intake is the safest approach to halt a runaway diesel engine. Take away one part of the fire triangle, and the flames will go out, just like we learnt in fire school.
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!
Is it good to warm up a diesel engine?
Mr4X4: Is a longer (than suggested) warm-up time beneficial or detrimental to the engine’s longevity? Or are they simply spending unnecessary hours on the engine and burning fuel?
Tony: Because older diesel vehicles lack the pollution controls seen in newer diesels, longer warm-up times do not harm the engine. All this accomplishes is add hours to the engine’s life and waste fuel. Excessive idle times can cause DPFs and EGR valves in modern diesels to function in ways that the manufacturer does not advocate. This practice may cause the intake manifolds to soot up more than usual, and the DPFs to choke up more quickly, resulting in more burns and excessive fuel use. Modern diesels are entirely computer-controlled; some lower performance by limiting fuel flow until the vehicle is warm enough. The engine will not be harmed by going off at a steady pace and taking it easy for the first few minutes of the journey. Taking off and excessively increasing the RPMs and load on a cold engine will result in undue wear and damage. Modern diesel automobiles have more advanced cooling systems than older models, and they are engineered to warm up rapidly. Allowing the vehicle to start and idle for a minute or two would not harm it and will only benefit it, but anything more is, in my opinion, needless. It simply creates extra noise in the caravan park, needless odors, and so on for no benefit.
Mr4X4: Is there any benefit to letting your four-wheel drive idle for five minutes after pulling up for cool-down? It made sense when turbos were exclusively oil-cooled, but with newer turbos that have both water and oil cooling, is there really any point?
Tony: Idle-down depends a lot on the conditions you’ve been driving in. Five minutes is well worth it if you’ve been working hard right up until you pull up to turn it off. It would be good to just shut down if you idle around town, then get to the caravan park and reverse your van into its position. You’ve basically done the job of the turbo timer anyhow. When compared to older wastegated turbos, VNT (Variable Nozzle Turbine) turbos spin at idle and at a pretty high speed. Idle time is more about regulating temperature and allowing it to drop before cooling down.
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.
Is a diesel more expensive to maintain?
Diesel maintenance is less expensive in the long run since diesel engines have fewer problems than gasoline engines; nevertheless, diesels are more expensive to repair when they do. Depending on the sort of diesel car you have, maintenance will not be more expensive than for a gasoline-powered vehicle.
What is CO2 fire extinguisher?
CO2 fire extinguishers contain pure carbon dioxide, which is a residue-free extinguishant. Recommended for use on live electrical equipment and suitable for class B flammable liquid fires (petrol, oil, solvents).
Our CO2 extinguishers are BAFE-approved, fully charged, and come with a wall bracket. Visit our fire extinguisher page for additional information on the many types of fire extinguishers.