What does it mean for an engine to throttle?
Diesel engines are robust, but they are not immune to occasional failure. This is especially true for newer diesel engines that have various legally mandated exhaust systems that can limit performance if a problem is detected.
This load-limiting feature is called “derating,” and all diesel owners should be familiar with it. If ignored, derating issues can leave truck drivers feeling helpless.
What is engine throttling?
As already mentioned, engine throttling is a self-protection or load-limiting feature that is pre-programmed into the operating software of modern diesel engines. The feature was first introduced early in the new century when the federal government enacted stricter emissions standards for diesel engines.
The principle of engine throttling is very similar to that of the “limp home” feature used in many gasoline vehicles. When the engine is throttled, its overall performance is significantly reduced as a self-protection measure. In this throttle state, the engine typically produces enough power for the driver to pull off the road and seek further technical assistance in a safe location.
The exact extent to which the truck throttles and performance is reduced varies by manufacturer. Such increased performance degradation is also common in Volvo brand trucks and some pickup trucks for private customers.
If previous power derating procedures are ignored, the power derating can actually shut down the truck and will almost certainly require the affected vehicle to be towed to a repair shop for repairs.
Additionally, some manufacturers have been known to void certain parts of a vehicle’s powertrain warranty if such performance degradation has occurred in the past.
What causes engine performance to degrade? Poor engine performance can be caused by a variety of specific faults, most of which are related to the truck’s exhaust system. Some of the most common of these issues include DEF quality and inefficient DPF/SCR catalysts. Generally, this type of error affects the truck’s ability to successfully complete an automatic regeneration cycle.
“Regeneration” is the term used to describe the conversion of soot and other particles trapped in the truck’s DPF (diesel particulate filter) to ash. This conversion is achieved through the application of heat and special chemical compounds. Diesel exhaust fluid is also injected into the truck’s SCR catalytic converter to reduce nitrogen oxide emissions.
Modern diesel engines are equipped with numerous sensors in the exhaust pipe to monitor the efficiency of such reactions. The most common of these sensors are back pressure sensors, differential pressure sensors and temperature sensors. If any of these special sensors provide feedback outside of specification, this can lead to a reduction in engine performance.
Beyond the usual emission-related failures, a reduction in performance can be triggered as a self-protection measure based on a number of additional criteria. However, the exact point at which this reduction in performance occurs varies widely from manufacturer to manufacturer. Many trucks experience a loss of performance when the coolant or oil level falls below a certain level or when overheating is detected.
How to fix the loss of performance
Unfortunately, the loss of power issue is not something that the average driver or amateur mechanic can easily solve. This is because most manufacturers require the derating condition to be repaired and reset using OEM-specific diagnostic tools (not a regular OBD2 scanner).
Therefore, most drivers have no choice but to visit a certified vehicle repair shop equipped with proprietary software.
Furthermore, the underlying fault that caused the engine loss of power must be fixed before full performance can be restored. In many cases, a technician can perform a forced regeneration (cleaning the emissions system) and reset the truck’s derating status. However, if the exact cause of this excessive soot exposure is not identified, further performance degradation will occur in a short period of time.
In some cases, a forced regeneration can also be used to buy time before initiating a more extensive performance degradation protocol. While such functionality is not always accessible without a diagnostic tool, some trucks are equipped with a forced regeneration switch that allows the driver to manually initiate a regeneration sequence.
This should be done while parked in a safe location away from other vehicles. In any case, the cause of the performance loss must be eliminated as soon as possible, which will prevent further performance losses from being recorded and in most cases avoid engine damage.
If you are not sure whether you can carry out such repairs yourself or simply do not have the appropriate equipment, you should make an appointment with a qualified service center as soon as possible.