When idle speed is fast when slow, it is often said that the idle speed is not stable, or idle speed jitter and so on the problem, idle speed is idling the lowest speed, after the car generally want to idle hot car to transition, to avoid immediately accelerating damage to the engine. Common general formula of idle speed control system, idle speed when the throttle is closed, the air through the idle speed control valves and related system to complete the air traffic control, at the same time, fuel injection quantity and ignition advance Angle and condition also have difference, do so on the one hand is to reduce fuel consumption, energy conservation, environmental protection, on the other hand is to let the engine as soon as possible into the normal working state, the transition of the unstable state, often difficult to find the problem immediately at a draught, because generally won't have any fault code can be diagnosed.

1, idle speed at high and low speed, the essence is determined by the idle valve size, and this valve, generally through the step number of stepper motor to complete positioning. The greater the valve opening, the greater the air flow in, the higher the idle speed; On the contrary, the valve opening low idle speed is low. If the valve is worn out, or if there is a problem with the stepper motor and control system, the valve may not be positioned in a timely and accurate manner, causing severe idle fluctuations.

2, after start-up, ECU will adjust the idle speed according to the water temperature sensor of water temperature is low at the beginning, will let idle high hot car, such as water temperature up after idle drop down again, if the water temperature sensor is broken, idle speed will be according to the abnormal temperature signal to output the corresponding rotational speed, it can also cause abnormal idle instability.

3, ECU oil injection in addition to considering the speed signal, load is also an important link. Although the engine idling nominally idling, but there will still be a load, such as a generator, sometimes open air conditioning, load measurement often rely on the intake pressure sensor and to complete, if the pressure sensor problems, will cause the same idle fluctuation. In addition to the sensor itself is broken, also do not rule out some plug contact is not good, or loose thread caused by problems.

4, generator and battery charging, when idling is considered to affect the idle speed of the load, if the generator output leakage, will cause severe changes in the resistance of the generator, resulting in idle sloshing; Or when the rectifier breaks down and the battery loses power, the voltage supplied to the ECU and the stepper motor is unstable, and the idle speed is definitely fluctuating.

5, throttle and other links, if there is carbon deposition, may cause the throttle close lax and other problems, resulting in too much air intake, so it will also cause idle instability, clean it up, but if there is a leak in the pipeline, it is easy to be ignored, in fact, will cause the same problem. Air filter element dirty, will cause insufficient air intake, may idle low easy to flameout.

6, ignition system energy is insufficient, or cut off the fire caused by idle when fast when slow, ignition is not normal, any working condition will make the engine combustion is not ideal, thus causing the fluctuation of speed, idle is the same, spark plug gap is not reasonable, or spark plug with high voltage cabinet performance is relatively common, generally easy to replace.

7, caused by oil road blockage, such as the nozzle dirty, pump problems, oil is not smooth, or cause spray oil atomization is not ideal, resulting in air fuel ratio is not ideal, when thick when sparse, idle speed will follow the fluctuations.

8, engine problems, cylinder cylinder wear caused by insufficient pressure, throttle valve gap is not reasonable and other problems will cause idle instability, these need to find a professional repair shop to help check.