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 accelerator pedal position. 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 accelerator 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 car service that specializes 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 built into the throttle assembly cover. In addition, a specific cause of jerks in a Ford Fusion equipped with an electronic throttle control system may be a malfunction of the accelerator pedal.
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 accelerator 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 fuel pressure in the fuel system".
In addition, a jerk may also occur due to a malfunction of the electromechanical part of the throttle assembly and contamination of the throttle valve with deposits from crankcase gases.
Jerks during acceleration
The cause of jerks during acceleration may be, as in the previous case, 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 causes of this phenomenon and the method of checking can be found in the subsection "Jerking at the moment of starting to move".
In addition to the above reasons, jerks during acceleration of a car with an automatic transmission can be caused by either an insufficient amount of working fluid in the transmission or a malfunction of the transmission itself.
Note: A car with an automatic transmission may accelerate jerkily at the beginning of the first trip after disconnecting and reconnecting the battery. This is not a malfunction, since after several accelerations the electronic unit will calibrate the transmission control and gear shifting will become smooth.
Jerks during steady motion
Such jerks are most often caused by a malfunction of the ignition system. Diagnostics and repair are necessary (see section 10 "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 coil 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 jerking during steady movement of a Ford Fusion may be a failure of the throttle position sensor built into the throttle assembly cover.
Additional symptoms that confirm a faulty sensor include:
- uneven engine idling;
- reduction in maximum engine power.
The throttle body cover is non-separable and therefore cannot be repaired. If a sensor fault is detected (see "Checking and replacing engine management system sensors"), replace the throttle assembly with the cover.
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