ONE of the more unique and frightening things a diesel engine can do is, without warning, apparently develop a mind of its own and accelerate uncontrollably until it destroys itself, despite every attempt to stop it.
The phenomenon is called diesel engine runaway, a fortunately rare but extremely dangerous condition in which the engine fuels itself, revving faster and faster until it finally fails, usually violently.
In a diesel engine, unlike a gasoline engine, there is essentially no restriction on airflow. While a gas engine has a throttle body with a butterfly valve that regulates airflow to the engine and controls power through the air-fuel mixture, the diesel engine, which fires due to compression rather than electric spark, regulates fuel flow alone to control engine speed and has unrestricted airflow.
While modern diesel engines have better fuel system controls, ventilation systems, and seals to prevent most runaway conditions, it hypothetically can happen to any engine.
The most common cause, according to most diesel mechanics and service guides, is worn piston rings or cylinders. These can allow combustion gases to blow past the piston into the crankcase, creating an oil mist that is vented through the crankcase breather tube. The breather is, of course, connected to the intake manifold, so the oil mist is drawn back into the engine; remember, a diesel is capable of running on even very poor fuel, and engine oil in a mist form is actually not too different in terms of combustibility than actual diesel fuel. The oil being drawn into the engine causes it to run faster, which in turn forces more oil into the intake, created a positive feedback loop that quickly has the engine overspeeding well beyond its redline.
Other causes include worn turbo seals, which allow oil to leak from the turbocharger into the intake, and on rare occasions, a blown head gasket, oil overfilling, or worn valve seals. One mechanic pointed out, however, that if enough valve seals are worn to cause an engine runaway, the engine has problems that would probably cause a breakdown long before it reaches that point.
Once a runaway begins, it is nearly impossible to stop. Turning off the ignition makes no difference, as a diesel runs on compression rather than electric spark. And unless one of the two available methods to stop the engine don’t work within a minute or so of a runaway starting, the wisest course is to move a safe distance from the vehicle and wait for the motor to destroy itself; after a minute or two, the damage the engine will suffer from a runaway ruins it anyway, so letting it blow apart or seize is not necessarily a greater loss.
If a runaway occurs while you are driving, try not to panic: Shift the vehicle into neutral, carefully apply the brakes, and pull over at a safe spot. Leave the transmission in neutral, set the parking brake firmly, and move away.
To stop a runaway, there are two things you can try, although they may not work. In a small vehicle, it is sometimes possible to stall the engine with clutch friction – provided the clutch is in good condition. First, set the parking brake, then step on the brake pedal as hard as possible and hold it. Depress the clutch, shift the car into a high gear (5 or 6), and then dump the clutch. DO NOT take your foot off the brake.
If that doesn’t work, the only other way is to try to stop the airflow, but this requires a desperate sort of bravery that is NOT recommended. Stuffing rags or covering the air intake with something solid may work (keep your hands away from the turbocharger, which is now spinning at a speed that will remove fingers). Alternately, spraying a carbon dioxide fire extinguisher directly into the air intake may starve the engine of oxygen and cause it to stop.
If these methods do not work, or you don’t wish to have internal engine parts exploding in your direction at waist level, then simply stay away, warn others in the area, and wait for the engine to fail. Sometimes it will simply seize up and stop due to extreme heat and friction, but more often it will blow in spectacular fashion once a rod bearing or wrist pin finally gives up.
Again, a runaway is a relatively rare sort of failure in a modern diesel engine, but it can and does happen, particularly given the number of diesel engines that are decidedly less than modern on Philippine roads. Knowing what to do if it happens to you won’t necessarily save your engine, but it certainly can save you and others from serious injury.