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What Is Diesel Runaway?

Exploring reasons why it happens, and things you can do to keep your rig’s engine from “running away.”

Apr 28, 2020
More To Learn
What Is Diesel Blow-By?
What Is LINE-X?
We continue satisfying curiosities—some simple, some not-so-simple—about diesels, with this quick look at what might be one of the most unnerving mechanical problems a truck owner ever wants to experience: a runaway engine.
"Runaway" is another diesel term that is used quite often. But while saying "runaway engine" is somewhat self-explanatory for most enthusiasts, we have to allow for newbies and novices to oil burners, who could think an entirely different thing when they hear the term. Yes, that really can happen.
Although there isn't a single reason why diesels—typically turbodiesels run away—it's safe to attribute most of the dramatic instances that we see on social media and in YouTube videos to fueling failures of some sort. The occurrence doesn't happen frequently in new rigs—today's engine-management systems regulate fueling more accurately and can perform "checks" against other things that can contribute to runaway ever happening)— but it's not uncommon for older powerplants with high miles and blow-by to experience runaway.
The only way for us to effectively demonstrate diesel-engine runaway is through video. The clip accompanying this report, shot by our friend Chris Tobin, is footage of Tony Burkhard's unfortunate runaway on the dyno at Ultimate Callout Challenge 2019. Shrapnel from a failed turbo broke a fuel line, which led to diesel spraying directly into the engine and a raging inferno for several tense minutes.
Thankfully, Tony's driver was not injured. And, in one of the most dramatic overnight carnage thrashes we've seen, the Dirty Hooker Diesel team (and many friends, old and new) repaired the Chevy Silverado in time for the team to continue competing with it the following day.
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What Is Runaway?

Diesel "runaway" is a condition that happens when fuel is unintentionally introduced to a turbocharged engine, from a source other than the vehicle's dedicated fueling system. When this happens, the engine's speed (rpm) increases dramatically and uncontrollably, and it only stops when either the extraneous fuel flow is eliminated or the powerplant suffers severe damage (seized bearings, broken rods, crank, etc. ). Remember, oil-burners create combustion by using fuel and air only. There's no "spark." So, the more fuel, the faster that engine is going to run.
As we noted in our report on diesel engine blow-by, the combination of unburned fuel and engine oil can lead to a runaway condition if blow-by is excessive enough for that mixture to reach the combustion chamber. And, with turbodiesels, oil can sometimes cause an engine to run away.
Are you wondering, "how is this possible?" It's all about fuel. Most turbochargers' internals are lubricated by engine oil, which can be the catalyst for a runaway condition if or when it breaches a turbo's seals. Like blow-by, the lubricant enters the combustion chamber and promotes an rpm increase. In instance like this, oil is the "fuel" source, and the result is usually dramatic (and expensive).
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Stopping a Runaway Engine

Usually, after defining runaway, "how do you stop it?" is the question that immediately follows. Remember, we're talking about compression/ignition engines. While shutting the engine off by turning the key back from the Run position is an understandable knee-jerk thought, doing this won't work. Unlike gas engines, ignition for a diesel is not the same thing as a gasser's "spark," which also is called ignition. The key-turn tactic is futile. A runaway oil burner has to be denied fuel—completely and immediately—or choked to death, literally, by squelching all of its air supply, by any means possible. The latter method is more widely used and has saved many engines from total destruction. At Diesel Power Challenge 2019, Matt Maier used quick thinking and a Girl Scout cookie box at Bandimere Speedway to stop the runaway 7.3L Power Stroke in his 1997 Ford F-250.

Can Runaway Be Prevented?

The good news is, yes, diesel-engine runaway can be prevented by installing inline devices, such as Pacbrake's PH2 PowerHalt Shut-Off Valve, a tube that contains a butterfly-style valve plate plumbed into the intake airstream. When activated—manually, or electronically/automatically—at a prescribed rpm, the internal blade closes and cuts off airflow, which in turn chokes the engine.
Installing a shut-off valve is a good idea, especially for older rigs with high mileage and engines with excessive blow-by, or trucks with big horsepower that are used for dyno and sled-pull competition. Not only can a shut-off ward off catastrophic engine failure, it also could save a truck's occupants from being seriously or fatally wounded by shrapnel from an engine explosion.

Source

Pacbrake
800-663-0096
http://www.pacbrake.com

Sources

Pacbrake
800-663-0096
http://www.pacbrake.com
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