What is the reason why the idle speed of Wei Chai 340 fluctuates?

Hello, reason: 1) Air, gasoline vapor and combustion exhaust gas that should not have entered the intake manifold leaked from the intake manifold or various valves, resulting in the mixture being too rich or too lean, which made the engine burn abnormally. When the air leakage position only affects individual cylinders, the engine will shake violently, which has a great influence on the idle speed of cold cars. Common causes are: loose clamp of intake manifold or broken hose; Air leakage of intake manifold gasket; The intake manifold is damaged or other parts grind holes in the intake manifold; O-ring of fuel injector leaks; Vacuum tube plug falls off and breaks; The positive crankcase ventilation (PCV) valve has a large opening; The valve of activated carbon tank is normally open; Exhaust gas recirculation (EGR) valve is not closed correctly. (2) Too much dirt on the throttle and air inlet; Too much carbon and dirt on the throttle and the surrounding air intake; Moreover, the cross-sectional area of the air duct changes, which makes the control unit unable to accurately control the idle air intake, resulting in the mixture being too rich or too thin and abnormal combustion. Common reasons are: the throttle has oil pollution or carbon deposition; There are oil stains and carbon deposits at the air inlet around the throttle; Idle stepper motor, duty solenoid valve and rotary solenoid valve are polluted by oil and carbon. (3) Idle air actuator failure Idle air actuator failure leads to inaccurate idle air control. Common reasons are: the throttle motor is damaged or the card is issued; Idle stepper motor, duty solenoid valve and rotary solenoid valve are damaged or stuck. (4) Incorrect air intake control unit receives an error signal and sends an error command, which leads to the engine's intake misalignment at idle speed and makes the engine burn abnormally, which is the indirect cause of unstable idle speed. Common causes are: air flowmeter or its line failure; Intake pressure sensor or its line fault; The plug of the engine control unit is due to poor water contact or internal computer failure.