Contents: A jerk at the start of movement 🠛 Jerks during acceleration 🠛 Jerks during steady motion 🠛
In relation to a car, a jerk is a short-term spontaneous change in the engine crankshaft speed regardless of the position of the gas pedal. In everyday operation, as a rule, there are series of jerks. The extreme case of a jerk is a dip - a noticeable delay in the engine's response to pressing the accelerator pedal.
Conventionally, three types of jerks can be distinguished:
- at the moment of the start of movement;
- during acceleration;
- during steady movement, i.e. with a constant position of the gas pedal.
To determine the causes of jerks when driving a car with an injection engine, special diagnostic equipment is required, so we recommend contacting a service center specializing in repairing fuel injection systems. However, as practice shows, in most cases jerks are caused by insufficient fuel pressure in the fuel rail or a malfunction of the throttle position sensor.
If you have some skills, you can identify the cause of the jerks yourself.
A jerk at the start of movement
At the moment of starting the movement, the extreme case of jerking - failure - often occurs. The most unpleasant sensations are associated with the delay in the engine's response to pressing the gas pedal. Sometimes the engine even stalls.
The jerk occurs at the moment the throttle valve begins to open, when the ECU determines the moment of transition from idle mode to load mode based on the signal from the throttle position sensor and must increase the amount of fuel supplied through the injectors. If the pressure in the fuel rail is insufficient, even with an increase in the injection duration, there is not enough fuel for smooth starting. For the fuel pressure testing method, see the subsection "Checking the pressure in the engine fuel system".
Jerks during acceleration
The cause of jerks during acceleration may be, as in the previous case ("Jerk at the start of movement"), insufficient fuel pressure in the fuel rail. The electronic engine control unit, having received a signal from the throttle position sensor about intensive opening of the valve at a large angle, tries to increase the fuel supply as much as possible, but is unable to do so due to the low fuel pressure. The reasons for this phenomenon and the method of checking, see the subsection "Jerk at the start of movement".
Jerks during steady motion
Such jerks are most often caused by a malfunction of the ignition system. Diagnostics and repair are necessary (see section. "Electrical equipment"). On the way, you can try to do the following yourself:
- carefully inspect the underhood space. Turn off the ignition and check the reliability of the fastening and fit of all wires and connectors at the ignition module and high-voltage wires. Start the engine and listen to its operation: the crackling sound when the high-voltage current breaks down to the "ground" is weak, but distinct. In complete darkness, the spark is clearly visible when breaking down;
- replace spark plugs regardless of their condition and mileage. Pay attention to the condition of the spark plugs: if it is not in compliance with the norm, it may be necessary to repair the engine or its systems.
A specific cause of jerks during steady movement of a car with an injection engine may be a failure of the throttle position sensor. Additional symptoms confirming the malfunction of this sensor are:
- uneven engine idle speed;
- reduction in maximum engine power.
The sensor is non-disassemblable and therefore cannot be repaired. If a sensor malfunction is detected (see "Checking and replacing engine management system sensors"), replace the sensor assembly.