What is the reason for the high idle speed of freight cars?

(1) Leakage of intake manifold or various valves.

When air, gasoline vapor and combustion waste gas that should not enter enter the intake manifold, the mixture is too rich or too thin, which makes 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) There is too much scale on the throttle and air intake.

There is too much carbon and dirt in the throttle and the surrounding air intake, and the cross-sectional area of the air passage changes, which makes the control unit unable to accurately control the idle intake, resulting in the mixture being too rich or too thin, which makes the combustion abnormal. 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

The control unit receives the wrong signal and sends out the wrong command, which leads to inaccurate engine idle intake control and abnormal engine combustion, 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.