Conventionally, three types of jerks can be distinguished:
- at the moment of the beginning of the movement;
- during acceleration;
- with steady motion, i.e. at 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, therefore, in this case, we recommend contacting a service 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.
With some skills, the cause of jerks can be determined independently.
Jerk at the moment of the beginning of the movement
At the moment of the beginning of the movement, the limiting case of a jerk, a failure, most often takes place. The most unpleasant sensations are associated precisely with the delay in the engine's response to pressing the accelerator pedal. Sometimes the engine even stalls. A jerk occurs at the moment the throttle valve opens, when, according to the signal from the throttle position sensor, the ECU determines the moment of transition from idle to load mode and must increase the amount of fuel supplied through the nozzles. In case of insufficient pressure in the fuel line (even with increased injection duration) there is not enough fuel for smooth starting. Checking the fuel pressure, see subsection «Checking the pressure in the engine power system».
Jerks during acceleration
The cause of jerks during acceleration can be, as in the previous case (see Jerk at the moment of the start of the movement), insufficient fuel pressure in the fuel line. The electronic engine control unit, having received a signal from the throttle position sensor about the intensive opening of the damper at a large angle, seeks to maximize the fuel supply, but due to the reduced 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 in steady motion
Such jerks are most often caused by a malfunction of the ignition system. Diagnosis and repair needed (see «Ignition system»). On the way, you can try to do the following yourself:
- carefully inspect the underhood space. Switch off the ignition and check the reliability of the fastening of the wiring harness pads to the ignition coils. Start the engine and listen to its work: crackling during breakdown of high voltage «to ground» weak but distinct. In complete darkness, a spark is clearly visible during breakdown;
- replace spark plugs regardless of their condition and mileage. Pay attention to the condition of the candles: if it is abnormal, it may be necessary to repair the engine or its systems.
A specific cause of jerks during the 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 idling;
- reduction in maximum engine power.
The sensor is non-separable and therefore non-repairable. If a sensor malfunction is detected, it is replaced as an assembly (see «Engine management sensors»).