Imagine this situation: you stop, turn off your diesel car, but instead of shutting down, the engine suddenly starts to rev uncontrollably to extreme RPMs. The accelerator pedal no longer has any effect, the engine roars louder and louder, and thick smoke and strange noises may appear from under the hood.
Such a situation seems like science fiction, but it is a very real phenomenon known as diesel engine runaway. It happens relatively rarely, but the consequences are almost always extremely serious.
What is diesel engine runaway
Runaway is an uncontrolled increase in engine speed, where the crankshaft rotation frequency begins to exceed the design limits.
In fact, the engine stops obeying the driver and continues to independently increase RPMs until the fuel runs out, the air supply stops, or mechanical destruction of the power unit occurs.
In most cases, the process takes only a few tens of seconds.
Why this happens specifically with diesels
The main difference between a diesel engine and most gasoline engines lies in the principle of power regulation.
In gasoline engines, the amount of air is usually limited by a throttle valve, while a diesel engine almost always receives a full volume of air, and power is regulated solely by the amount of injected fuel.
Moreover, a diesel engine can run not only on diesel fuel. If another combustible substance, which ignites from the high compression temperature, starts to enter the cylinders, the engine will continue to run on it.
This is why there are two main causes of runaway.
Fuel system malfunction
The first option is related to the failure of the high-pressure fuel pump.
The high-pressure fuel pump supplies fuel in strictly metered portions, and a special mechanism is responsible for regulation.
If the pump rack jams in the maximum supply position, the engine starts to receive too much fuel.
Then a chain reaction occurs:
- RPMs increase;
- the pump continues to supply maximum fuel;
- the engine revs even higher.
It is no longer possible to stop such a process with standard controls.
Oil starts to act as fuel
Much more often, the cause is engine oil entering the cylinders.
For a diesel engine, oil is also a combustible substance. If it starts to enter the intake tract, the engine gradually switches to burning it, continuing to independently gain RPMs even after the diesel fuel supply is cut off.
The most common causes of this scenario are:
- wear of turbocharger seals;
- severe wear of the cylinder-piston group;
- excessively high engine oil level.
With a faulty turbine, oil under pressure enters the intake manifold and is then sucked into the cylinders.
If the piston rings are severely worn, crankcase gases create increased pressure, capture oil mist, and direct it back into the intake through the crankcase ventilation system.
Sometimes the problem also arises after a regular oil change. If the level significantly exceeds the norm, the crankshaft starts to foam the oil, which can also enter the intake system.
In such cases, cutting off the fuel supply no longer helps – the engine continues to run on engine oil.
What does runaway end with?
The consequences are almost always severe.
Within seconds, the engine goes far beyond the permissible RPM limits, after which its main parts begin to break down.
The most common consequences are:
- destruction of connecting rods;
- damage to pistons;
- crankshaft breakage;
- cylinder block penetration by a connecting rod;
- engine overheating;
- seizure of the power unit.
In addition, due to a large amount of oil entering the exhaust system, there is a risk of its ignition.
In many cases, after runaway, the engine is beyond repair and requires complete replacement.
How to stop the engine
In such a situation, every second counts.
The most effective way to stop a diesel engine is to completely deprive it of air access.
Without oxygen, the combustion process stops regardless of whether the engine is running on diesel fuel or has already switched to engine oil.
If such a situation arises, experts recommend acting as follows:
- if possible, find a dense material – thick fabric, a wooden board, or a sheet of plywood;
- completely block the engine's air intake;
- wait for the engine to stop completely.
However, you should absolutely not close the air intake with your hand. The suction force can be so high that it will lead to serious injury.
After the engine stops, it cannot be restarted. The car must be taken to a service center for full diagnostics.
Can the engine be stalled with a gear?
Sometimes it is advised to stop the engine by engaging a high gear, applying the parking brake, and slowly releasing the clutch to maximize the load on the power unit.
This method sometimes helps if the runaway has just begun and the engine has not yet reached critical RPMs.
However, this method involves a high risk of transmission damage and is not always effective.
Therefore, the most reliable method remains blocking the air supply.
Can runaway be prevented?
It is impossible to completely exclude such a possibility, but it is quite realistic to significantly reduce the risk.
To do this, you need to:
- regularly monitor the condition of the turbocharger;
- change engine oil in a timely manner;
- do not exceed the recommended oil level;
- monitor the serviceability of the crankcase ventilation system;
- service the fuel system in a timely manner.
On some modern diesel engines, manufacturers install emergency air dampers that automatically block the air supply when RPMs sharply increase. This once again confirms that the problem is well known to designers.
The main thing to remember
Diesel engine runaway is one of the most dangerous emergency situations that can occur during vehicle operation. Although it is relatively rare, the consequences are often fatal for the engine.
That is why at the first signs of uncontrolled RPM increase, you should not try to "rev" the engine or hope that it will stop on its own. The main task is to stop the air supply as quickly as possible and, after the engine stops, do not attempt to restart it until professional diagnostics are performed.