I’ve said it before and I’ll say it again: There is nothing quite like a high-performance diesel motor. They peg the cool meter, all the way up until something goes screwy inside, and that engine transforms into a runaway diesel. At which point it becomes completely unrestrained, and subsequently tries to kill itself and take everything around it with them.

As the title of this brief diatribe clearly states, runaway diesel engines are potentially as dangerous and disastrous as it gets. Oh, you’ve never heard of a runaway diesel? Surely you’ve heard of a runaway train before, right? No, not the Grammy award-winning track by Soul Asylum. That classic hit is about a totally different kind of dangerous situation, and a damn sad one at that…

No, a runaway diesel is basically an engine that refuses to slow its roll. An engine that won’t stop, and one that seems to just keep on going no matter what you do to cut it off. In this scenario, the engine will keep burning fuel until either it exhausts its resources, or combustion levels are somehow brought back under control. That, or the engine goes full kamikaze and disassembles itself out of sheer zealotry.

Runaway Diesels: Rare, But Not Unheard Of Either

In extreme scenarios, or on older vehicles not equipped with an ECU failsafe, the diesel engine will keep burning anything it can until it grenades. At this point, you may want to call your friendly insurance agent. Photo Credit: Truck Master/YouTube

 Thanks to modern ECU systems and safety “kill-switch” setups, runaway diesels are not nearly as common as they once were.

However, that’s not to say that they still don’t occur on occasion. And while many are quick to assume that this issue is caused by some form of electronic or mechanical failure (or some combination of the two), a runaway diesel engine can be caused by a plethora of different problems.

Defective components can be the cause. A stuck fuel pump, a defective or improperly calibrated fuel pressure regulator, compromised oil seals, overloaded crankcases, and faulty fuel lines or linkages/couplers are the primary problematic points to ponder. Additionally, propane-converted diesel engines are equally prone to this problem, as are high-performance engines running the wrong tune or improperly calibrated tunes.

Photo Credit: Truck Master/YouTube

But over the years, one of the most common reoccurring causes of a runaway diesel engine is the classic combo of crapped-out piston rings and a motor that is either overloaded or running entirely too hot. The explanation for this sort of situation is fairly straightforward. As oil gets past the rings, it vaporizes. The steamy solution seeps into the combustion chamber, and if the engine is still under a heavy load, the oil vapor uncontrollably combusts.

Of course, this results in a massive spike in power and engine speed, for within a diesel engine, compression alone is more than enough to cause this volatile cocktail to combust. Typically, something called “quality torque manipulation” is utilized via the use of an ECU to keep a diesel engine’s speed and rotating assembly pace within check.

Photo Credit: Truck Master/YouTube

But spew some hot oil onto the situation via entirely too much blowby or from a faulty seal somewhere and you’ve got a raging inferno on your hands, both internally and externally if things get truly out of control! Allow me to illustrate in brief…

Since almost all diesel engines are turbocharged nowadays, a leaky turbo housing is another area of concern as the oil once used to lubricate the turbo can quickly cause a runaway diesel. Last we checked, turbos required oil for cooling and lubrication just like many other forms of automotive engine components. Incompletely combusted fuel is another risk, as it also can cause RPM spikes to occur suddenly and sporadically.

How To Slow Your Roll On A Runaway Diesel Engine

Diesel engines rely upon highly compressed fuel and air to create combustion, hence them not having spark plugs. So cutting off fuel and/or air from that engine cycle is your best bet at getting that runaway powerplant back under control. Photo Credit: Truck Master/YouTube

Since the unwanted fuel continues to be fed into the combustion process against the engine’s will, removing the key from the ignition won’t stop a runaway diesel engine. Additionally, the motor will more than likely continue to run at extreme RPM, and the only way to get it to shut off is by either eliminating its fuel source or intake air.

But this can be tricky for those of us who are rolling in older rigs, piloting farm equipment, or controlling locomotive trains. Unless you have an inline fuel kill switch integrated somewhere, getting to a line and cutting the engine off from its fuel source is pretty damn risky and scary. And cutting off the air means getting dangerously close to the high-revving engine to seal off the intake or simply suffocating the motor by way of inundating it with CO² via a fire extinguisher.

Physically cutting the fuel off during a runaway is a dangerous proposition. However, it might work. Photo Credit: Truck Master/YouTube

Another option, for those of you planning to build a diesel project, is to install an electronic or manual turbo shut-off. This throttle body look-alike serves as a safety valve and fits inline onto the turbo housing. Those who opt for an electronic unit can either program it to automatically close at a certain RPM, when engine temps reach a certain level, or via the use of a toggle switch inside the cab. 

Oil Burners Are Here To Stay

With a manually shifted transmission, one option for bringing a runaway diesel engine under control is to upshift into the highest gear you’ve got and stall the motor. Photo Credit: Truck Master/YouTube

Fortunately, runaway diesels are increasingly uncommon. Most modern diesel vehicles have electronic fuel pump cutoff switches that safeguard against these sorts of scenarios.

But that doesn’t mean you can slack off on your routine maintenance. Even modern diesel motors require upkeep over time, so keep those regular turbine compressor wheel checkups and leak-down tests in mind when it comes time for a tune-up.

And if you are genuinely concerned about a leaky snail, don’t fret. When in doubt either have your turbocharger rebuilt or install a fresh turbocharger. Especially if the vehicle has an odometer with more digits than Charlie Sheen’s outstanding bar tab at the Bellagio.

Photo Credit: Truck Master/YouTube