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 accelerator pedal.
To determine the causes of jerks when driving a car with an injection engine, special diagnostic equipment is required, so in this case we recommend contacting a service center specializing in the repair of fuel injection systems. However, as practice shows, in most cases jerks are caused by insufficient fuel pressure in the fuel rail, malfunctions of the throttle position sensor or mass air flow sensor.
If you have some skills, you can determine the cause of the jerks yourself.
A jerk at the start of movement
At the moment of starting the movement, the most common case of jerking is a failure. The most unpleasant sensations are associated with the delay in the engine's response to pressing the accelerator pedal. Sometimes the engine even stalls. The jerk occurs at the moment the throttle valve begins to open, when, based on the signal from the throttle position sensor, the ECU determines the moment of transition from idle mode to load mode and must increase the amount of fuel supplied through the injectors. If there is insufficient pressure in the fuel line (even with increasing injection duration) there is not enough fuel to start off smoothly. To check fuel pressure, see. in the subsection "Checking the pressure in the engine fuel system".
Jerks during acceleration
The reason for jerks during acceleration may be the same as in the previous case (see Jerk at the start of movement), insufficient fuel pressure in the fuel line. 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 due to the low fuel pressure it is not able to do this. The reasons for this phenomenon and the method of verification are the same as in the previous case.
Jerks during steady motion
Such jerks are most often caused by a malfunction of the ignition system. Diagnostics and repair are required (see "Ignition system"). 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 wiring harness connectors to the ignition coils. Start the engine and listen to its operation: the crackling sound when the high voltage breaks down "to ground" is weak but distinct. In complete darkness, the spark during breakdown is clearly visible;
- replace spark plugs regardless of their condition and mileage. Pay attention to the condition of the spark plugs: if it is abnormal, you may have to repair the engine or its systems.
A specific reason for jerks during steady movement of a car with an injection engine may be a failure of the throttle position sensor. Additional symptoms that confirm the malfunction of this sensor are:
- uneven engine operation at idle;
- reduction in maximum engine power.
The sensor is non-disassemblable and therefore cannot be repaired. If a sensor fault is detected, it is replaced as an assembly (see "Engine Management System Sensors").