Idling means that the engine is at the lowest speed, or when the gas pedal is not pressed and the air is supplied to the cylinders bypassing the throttle valve.
As a rule, in idle mode, the motor does not transmit torque to the wheels, since this mode is activated when the car is parked with the engine running in neutral gear, or when coasting, and as well as at the moments of pressing the clutch to change gear, etc.
During the operation of a vehicle with various types of engines and power systems (petrol, and diesel), drivers face the problem of an engine that jerks while idling.
In other words, a typical symptom of a jerking car at idle is when the engine works unevenly, twitching can be sharp, short-term or prolonged, strong or barely noticeable, with perceptible vibrations on the body.
At the same time, when the speed goes up or after the engine starts to load, the symptoms disappear. Otherwise, the twitching of the motor disappears after the internal combustion engine switches to another operating mode, or archives the optimal functioning temperature.
What are the causes of jerkings at low and idle speeds?
If the car jerks when the engine is idling, there may be several reasons for this malfunction. Most often, problems affect the ignition system and the power system, less often the elements of the engine control system or parts inside the internal combustion engine are to blame.
First of all, damage to the engine mounts should be excluded, since the intensification of vibrations from the internal combustion engine in this case often occurs precisely at cold.
If everything is normal with the engine mounts, then engine twitching may occur due to failures in the operation of one cylinder.
1. Bad spark plugs
The most common cause is spark plugs as well as high voltage armored wires. Often, a spark plug can be damaged, dirty from carbon deposits, etc., as a result, the spark on it is either very weak or unstable.
2. Problems with the ignition system
Damage to the insulation of high-voltage wires when the spark “breaks through” also leads to similar symptoms in the form of motor jerking. The ignition coil, switch, ignition module, and distributor deserve additional attention.
Depending on which ignition system is installed on a particular engine, all its elements must be checked.
3. Dirty air filter
We also add that in cases where the engine jerks while idling, it is necessary to make sure that not only the fuel filter but also the air filter is in good condition.
If the air filter is dirty, the engine may not have enough oxygen to prepare the correct fuel-air mixture. The same is true in cases where airing occurs in the power supply system or excess air is sucked in at the intake.
In any case, the mixture may turn out to be too rich or lean, while normal and timely combustion of such a charge in the cylinder does not occur. At the same time, in a similar situation, spark plugs are even more covered with carbon deposits, and, logically, sparking further deteriorates.
4. Problems with electronic engine control system – ECM
Another possible reason for jerking a car while idling, in some cases, is the electronic engine control system (ECM). In this situation, it should be borne in mind that the specified system includes a whole group of various electronic sensors and actuators that communicate with the electronic control unit (ECU).
If any sensor is out of order, cylinder misfires or other malfunctions occur, then the ECU on some cars immediately informs the driver about this.
At the same time, in case of various malfunctions of ECM elements and other breakdowns, an error code is displayed on the dashboard computer screen.
The error is recorded only in the ECU memory, but to read the error code, it will be necessary to carry out computer diagnostics separately.
5. Faulty idle speed control sensor
Problems with the idle speed control sensor are other possible causes of engine jerking when idling. The idle speed controller itself closes the air supply channel bypassing the throttle valve.
In case of improper operation and malfunctions of this sensor, as well as the accumulation of a large amount of dirt and carbon deposits on the damper, the idle speed becomes unstable. In this case, the engine not only jerks, but the idle speed often floats a lot.
RPM floating can appear on a cold engine and disappear when it warms up. Also, RPM in some cases jumps and floats both on cold and hot.
To solve the problem, you need to clean the throttle, and also check the idle air control actuator (IAC). It is also important to consider that on many cars, after cleaning, it is also necessary to adjust the throttle valve.
What to check
After diagnosing the ignition system, you need to also check the fuel supply and air supply systems. Typically, injector nozzles may be faulty or dirty. Fuel filters are also checked, as well as the fuel lines should be inspected for leaks.
The fuel system is the culprit for dips and jerks during the operation of the internal combustion engine under load, however, even in idle mode, the fuel may not be enough or it is supplied to the engine intermittently. Step by step it is necessary to check the injector and individual elements of the system, the fuel pump, etc.
Quite often, an engine twitching at idle speed can manifest itself more when it is cold. In this case, pay special attention to the motor temperature sensor.
Failure or malfunctions in its operation often lead to the fact that the ECU receives a premature signal that the motor is warming up, while it is still cold.
Taking into account the incorrect readings of the temperature sensor, the ECU automatically reduces the amount of supplied fuel to exit the mode of the so-called increased “warm-up” revolutions, however, in this case, the cold engine starts to work unstably, and the engine jerks, and vibrations increase.
A similar situation may be when air leaks or problems arise with the mass airflow sensor (MAF). In this case, the control unit incorrectly calculates the actual amount of air that enters the cylinders, as a result, the correct ratio of fuel and air will be violated.
If the diesel engine jerks while idle, then problems with the high-pressure pump should be added to the general list of possible malfunctions. There are many moving parts in the pump, damaged or clogged, that can also lead to jerking.
As you can see, at the initial stage of diagnostics, in the event of engine jerking while idle, you should:
- check spark plugs and high voltage wires;
- check the condition of the air and fuel filter;
- check the injectors ;
- perform computer diagnostics of the engine;
Finally, we should note that in cars with LPG equipment, the cause of twitching may be problems with gas equipment or settings of the gas control unit.