Mercedes-Benz s320 has a history of about ten years and uses an L6 engine. Idle speed is unstable, about 600-800. What are the general situations?

1. Intake system

(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.

2. Fuel system

(1) injector failure

The fuel injection quantity of injector is uneven and the fog is not good, which leads to the power imbalance of each cylinder. The common reasons are: the injector is blocked, the sealing performance is poor, and the ejected fuel is linear.

(2) Fuel pressure failure

If the oil pressure is too low, the fuel sprayed by the injector is in poor atomization state or the fuel sprayed is linear. In severe cases, only oil droplets are sprayed, and the fuel injection quantity is reduced, making the mixture too thin; When the oil pressure is too high, the actual fuel injection increases, making the mixture too rich. Common reasons are: fuel filter is blocked; The fuel pump filter screen is blocked; The pumping capacity of the fuel pump is insufficient; The spring force of fuel pump safety valve is too small; Deformation of oil inlet pipe; Fuel pressure regulator is faulty; The oil return pipe is compressed and blocked.

(3) inaccurate fuel injection quantity

The fault of each sensor or line causes the control unit to send out wrong instructions, which makes the fuel injection amount incorrect, and the mixture is too rich or too thin, which is the indirect reason for the unstable idle speed. The specific reasons are: air flow meter (or intake manifold pressure sensor) failure; Throttle position sensor failure; Throttle idle switch failure; Coolant temperature sensor failure; Air temperature sensor failure; Oxygen sensor failure; The lines of the above sensors have open circuit, short circuit and grounding fault; The plug of the engine control unit is due to poor water contact or internal computer failure.

3. Ignition system

(1) Ignition module and ignition coil

In recent years, the ignition module and the ignition coil are mostly made into a whole. The failure of ignition module or ignition coil is mainly manifested as weak high-voltage spark or non-ignition of spark plug. The common reasons are: the ignition trigger signal is missing; The ignition module is faulty; Loose connection or poor contact of power supply or grounding wire of ignition module; The primary coil or secondary coil is faulty, etc.

(2) Spark plugs and high-voltage lines

Spark plugs and high-voltage lines fail, resulting in a drop in spark energy or a fire. Common reasons are: incorrect spark plug clearance; The spark plug electrode is ablated or damaged; Spark plug electrode has carbon deposit; The magnetic insulator of spark plug has cracks; High voltage line resistance is too large; High-voltage line insulation sheath or plug leakage; Electrodes of the distributor are ablated or have poor insulation.

(3) The ignition advance angle is inaccurate

Because the sensor and line faults are indirect causes of unstable idle speed, the control unit sends out wrong instructions, which leads to incorrect ignition advance angle or large-scale fluctuation of ignition advance angle. Common causes are: air flow meter or intake pressure signal failure; Hall sensor failure; Coolant temperature sensor failure; Air temperature sensor failure; Knock sensor failure; The lines of the above sensors have open circuit, short circuit and grounding fault; The engine control unit is flooded, resulting in poor plug contact or internal circuit damage.

(4) Other reasons

The blockage of three-way purification catalyst leads to unstable idle speed, which is most easily found when driving at high speed. Failure of automatic transmission, air conditioner and steering assist will increase idle load and cause idle instability. The idle acceleration signal between the engine control unit and the air conditioner and automatic transmission control unit is interrupted, and there is a bus system fault in the vehicle with CAN bus installed. With the increase of new technology and new structure, there will be more factors causing idling instability, and the diagnostician must consider the problems comprehensively.

4. Mechanical structure

(1) valve train

The failure of the valve train causes the power of a single cylinder to drop too much, which makes the power of each cylinder unbalanced. The common reasons are: the timing belt is installed in the wrong position, which changes the opening and closing time of the valves of each cylinder, resulting in inaccurate valve distribution phase and abnormal combustion of each cylinder. There is too much carbon deposit on the valve working face and valve seat ring, and the valve seal is not strict, resulting in inconsistent compression pressure of each cylinder. The cam of the camshaft is worn, and the inconsistent wear of the cams of each cylinder leads to inconsistent air intake of each cylinder. Faults of valve-related parts, such as valve push rod wear or bending, rocker arm wear, valve sticking or air leakage, and valve spring fracture.

I have encountered two cases of intermittent idling jitter caused by valve spring fracture, and the reasons can not be determined by using various instruments. The cause of the failure was discovered only after the valve spring was removed. In addition, for engines equipped with hydraulic tappets, the pressure reducing valve is installed in the oil passage leading to the cylinder head. When the pressure is higher than 300 kPa, open the valve. If the valve is blocked, the hydraulic tappet will be stretched too much due to too high pressure, resulting in the valve not closing normally. There is a large amount of carbon deposit on the back of the intake valve, which makes the newly injected fuel absorb when the car is cold, but it cannot enter the cylinder. Because the mixture is too thin, the fast idle speed of the car is unstable.

(2) Engine cylinder block and piston connecting rod mechanism

These failures will reduce the power of a single cylinder too much, thus making the power of each cylinder unbalanced. Common causes are: the cylinder liner is ablated or damaged, resulting in air leakage between one cylinder or two cylinders; The end clearance of piston ring is too large, butt joint or fracture, and the piston ring loses elasticity; Excessive carbon deposition in piston ring groove; Piston and cylinder wear, cylinder roundness and cylindricity out of tolerance; Because water enters the cylinder, the connecting rod bends and the compression ratio changes; Carbon deposition in combustion chamber will change the compression ratio, which will seriously lead to unstable idle speed.

(3) Other reasons

The dynamic balance of rotating parts such as crankshaft, flywheel and crankshaft pulley is unqualified, the engine footstool is broken and damaged, and the engine bottom guard plate is deformed and collides with the oil pan. These reasons will only lead to engine vibration, and will not affect the speed.

Third, the diagnosis method of unstable idle speed

Engine idling instability will be caused by mechanical failures of intake system, fuel system, ignition system and engine. Therefore, it is a wide-ranging and difficult job to diagnose the cause of engine idling instability, and it is not appropriate to replace parts easily. There are many reasons for the unstable idle speed, and correct judgment should be made according to the test results, theoretical analysis and maintenance experience, so the diagnosis work is regular.

observational check

Open the engine hood for inspection: observe the engine operation and jitter, and observe the swing amplitude of the engine tachometer pointer, whether it deviates from the idle speed expectation; Observe whether it is normal idle jitter or load idle jitter (turn on the air conditioner, light, shift gear, turn the steering wheel, etc.). ); Whether the external parts of the engine are abnormal; Whether the vacuum tube falls off or is damaged; Whether the wire connector is loose; Whether there is oil leakage, water leakage, air leakage and electric leakage; Whether the exhaust pipe is "abrupt" (indicating poor combustion), emitting black smoke, smelling of gasoline and other abnormal phenomena; Whether the throttle cable is adjusted correctly.