① Direct causes refer to mechanical parts being dirty, worn, incorrectly installed, etc., which lead to changes in the power of individual cylinders, resulting in unbalanced power of each cylinder, resulting in unstable idling of the engine; ② Indirect causes, It means that the engine electronic control system is abnormal, resulting in poor combustion of the mixture, making it difficult to balance the power of each cylinder, and causing the engine to experience unstable idle speed.
These are specific, car owners can refer to them:
1. Intake system
(1) Leakage in the intake manifold or various valves
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When air, gasoline vapor, and combustion exhaust gas that should not enter enter the intake manifold, causing the mixture to be too rich or too lean, causing the engine to burn abnormally. When the air leakage position only affects individual cylinders, the engine will vibrate more violently, which will have a greater impact on cold idling speed. Common causes include: the intake manifold clip is loose or the hose is ruptured; the intake manifold gasket is leaking; the intake manifold is damaged or other parts have worn holes in the intake manifold; the injector O-ring seal is leaking; The vacuum tube plug falls off or breaks; the forced crankcase ventilation (PCV) valve has a large opening; the activated carbon canister valve is always open; the exhaust gas recirculation (EGR) valve is not closed tightly, etc.
(2) There is too much fouling on the throttle valve and the intake duct
There is too much carbon deposit and dirt on the throttle valve and the surrounding intake duct, and the cross-sectional area of ??the air passage changes, causing The control unit cannot accurately control the idle air intake volume, causing the mixture to be too rich or too lean, causing abnormal combustion. Common causes include: oil stains or carbon deposits on the throttle valve; oil stains and carbon deposits on the air intake duct around the throttle valve; oil stains and carbon deposits on idle stepper motors, duty cycle solenoid valves, and rotary solenoid valves.
(3) Idle air actuator failure
Idle air actuator failure results in inaccurate idle air control. Common causes include: the throttle motor is damaged or stuck; the idle stepper motor, duty cycle solenoid valve, and rotary solenoid valve are damaged or stuck.
(4) Inaccuracy of air intake volume
The control unit receives an erroneous signal and issues an erroneous command, causing the engine idle air intake control to be inaccurate and causing abnormal engine combustion, which is An indirect cause of unstable idle speed. Common causes include: air flow meter or its circuit failure; intake air pressure sensor or its circuit failure; engine control unit plug poor contact due to water inlet or internal computer failure.
2. Fuel system
(1) Fuel injector failure
The fuel injection amount of the fuel injector is uneven and the mist is not good, causing each cylinder to The power emitted is unbalanced. Common causes include: clogged fuel injectors, poor sealing, and linear fuel injection, etc.
(2) Fuel pressure failure
The oil pressure is too low, the fuel sprayed from the injector is in poor atomization state or the fuel sprayed is linear. In severe cases, only If oil drops appear, the fuel injection volume decreases, making the mixture too thin; if the oil pressure is too high, the actual fuel injection volume increases, making the mixture too rich. Common causes include: clogged fuel filter; clogged fuel pump filter; insufficient pumping capacity of the fuel pump; too small spring elasticity of the fuel pump safety valve; deformation of the fuel inlet pipe; faulty fuel pressure regulator; deflated and clogged fuel return pipe.
(3) The fuel injection amount is inaccurate
Each sensor or circuit failure causes the control unit to issue wrong instructions, causing the fuel injection amount to be incorrect, causing the mixture to be too rich or too thin. , which is an indirect cause of unstable idle speed. Specific reasons include: air flow meter (or intake manifold pressure sensor) failure; throttle position sensor failure; throttle idle switch failure; coolant temperature sensor failure; intake air temperature sensor failure; oxygen sensor failure; wiring of the above sensors There is an open circuit, short circuit, or ground fault; the engine control unit plug has poor contact due to water ingress or an internal computer failure.
3. Ignition system
(1) Ignition module and ignition coil
In recent years, various models have integrated the ignition module and ignition coil. A faulty module or ignition coil mainly manifests itself as weak high-voltage spark or misfiring of the spark plug. Common reasons include: missing ignition trigger signal; faulty ignition module; loose connection or poor contact of the ignition module power supply or ground wire; faulty primary coil or secondary coil, etc.
(2) Spark plugs and high-voltage wires
Faulty spark plugs and high-voltage wires lead to a decrease in spark energy or misfire.
Common reasons include: incorrect spark plug gap; ablation or damage to the spark plug electrode; carbon deposits on the spark plug electrode; cracks in the spark plug magnetic insulator; excessive resistance of the high-voltage line; leakage of the high-voltage line insulation skin or plug; ablation or poor insulation of the spark plug electrode.
(3) The ignition advance angle is inaccurate
Since sensor and circuit faults are indirect causes of unstable idle speed, the control unit issues wrong instructions, causing the ignition advance angle to be incorrect, or Causes ignition advance angle to fluctuate in a wide range. Common causes include: air flow meter or intake pressure signal failure; Hall sensor failure; coolant temperature sensor failure; intake air temperature sensor failure; knock sensor failure; open circuit, short circuit, or ground fault in the circuits of the above sensors; engine control The unit has poor plug contact or internal circuit damage due to water intrusion.
(4) Other reasons
The clogged three-way purification catalytic converter causes unstable idle speed. This fault is most easily found when driving at high speed. Failure of the automatic transmission, air conditioner, and steering booster will increase the idling load and cause unstable idling. The idle speed boost signal between the engine control unit and the air conditioning and automatic transmission control units is interrupted, and there is a bus system failure in vehicles equipped with CAN-BUS. With the increase of new technologies and new structures, there will be more factors causing unstable idle speed, and the diagnostician must consider the problem comprehensively.
4. Mechanical structure
(1) Valve distribution mechanism
The failure of the valve distribution mechanism causes the power of individual cylinders to drop too much, resulting in insufficient power of each cylinder. balance. Common reasons include: the timing belt is installed in the wrong position, causing the opening and closing time of each cylinder valve to change, resulting in inaccurate valve timing and abnormal combustion in each cylinder. There is too much carbon deposit on the valve working surface and valve seat ring, and the valve seal is not tight, causing the compression pressure of each cylinder to be inconsistent. The cams of the camshaft are worn, and the wear of the cams in each cylinder is inconsistent, resulting in inconsistent air intake into each cylinder. The valve-related parts are faulty, such as the valve push rod is worn or bent, the rocker arm is worn, the valve is stuck or leaking, the valve spring is broken, etc.
I have encountered 2 cases of intermittent idling vibration due to broken valve springs. Various instruments were used to detect the cause but the cause could not be determined. The cause of the failure was discovered after disassembling the valve spring. In addition, for engines equipped with hydraulic lifters, a pressure relief valve is installed on the oil passage leading to the cylinder head. When the pressure is higher than 300kPa, the valve is opened. If this valve is clogged, the hydraulic lifter will extend too much due to excessive pressure, causing the valve to not close tightly. There is a large amount of carbon deposit on the back of the intake valve, which causes the newly injected fuel to be absorbed when the car is cold and cannot enter the cylinder. Because the mixture is too thin, the fast idle speed of the cold car is unstable.
(2) Engine body and piston connecting rod mechanism
These failures will cause the power of individual cylinders to drop too much, resulting in an imbalance in the power of each cylinder. Common causes include: cylinder liner ablation or damage, causing air leakage in a single cylinder or between two cylinders; piston ring end gap is too large, aligned or broken, and the piston ring loses elasticity; excessive carbon deposits in the piston ring groove; The piston and cylinder are worn, and the roundness and cylindricity of the cylinder are out of tolerance; the connecting rod is bent due to water intrusion into the cylinder, changing the compression ratio; carbon deposits in the combustion chamber will change the compression ratio, and serious carbon deposits will lead to unstable idle speed.
(3) Other reasons
The dynamic balance of the crankshaft, flywheel, crankshaft pulley and other rotating parts failed, the engine foot pads were broken and damaged, and the engine bottom guard plate was deformed and in contact with the oil pan. Impact, etc., these causes will only cause engine vibration but not affect the speed.