06 discovery 3 occasionally shakes when lifting the fuel tank and touching the car.

(1) air supply system.

Air intake system is an important part of electronically controlled fuel injection system. Faults that lead to inaccurate gas supply mainly include:

1, the leakage of intake pipe and various valves, the common ones are the broken intake manifold, poor seal of intake manifold, vacuum tube falling off, and the PCV valve /EGR valve not being closed properly. For example, under normal circumstances, the opening of the idle speed control engine and the air intake should strictly follow a certain functional relationship, that is, the air intake will increase when the idle valve is opened. When the air supply system leaks, the air intake and the opening of the idle speed control valve can't follow the original functional relationship, which makes the air flowmeter unable to accurately measure the real air intake, resulting in inaccurate near-intake signals obtained by the engine electronic control unit (ECU), which leads to misjudgment and unstable idle speed of the engine.

2. When there is too much fouling in the idle port and throttle, the intake cross-sectional area changes, which leads to inaccurate control of the idle port, which makes the air quantity entering the cylinder deviate from the normal value, resulting in too rich or too lean mixture, abnormal combustion and unstable engine idle speed.

3. The sensor and other circuits that control the idle speed are abnormal. If the idle switch can't be turned off, ECU misjudges part of the engine load, resulting in an intake control error; The idle valve is delayed or stuck due to oil pollution and carbon deposition, and the throttle valve is improperly closed, which will cause ECU to be unable to adjust the engine idle speed. In addition, short circuit and open circuit of air temperature sensor, air flow sensor, water temperature sensor and sensor circuit will cause engine idling instability.

(2) Ignition system.

The poor ignition system is mainly due to the weak high-pressure fire or the non-ignition of the spark plug, which directly affects the combustion of each cylinder, resulting in different power of each cylinder, thus making the engine idling unstable. The specific performance is as follows:

(1) Low secondary voltage; (2) high-voltage line leakage; (3) High-voltage lines are short-circuited or have large internal resistance; (4) Incorrect early ignition; (5) Carbon deposition and falling off of spark plug; (6) The spark plug electrode gap is wrong; (7) The ignition coil is damaged or the ignition control circuit is faulty; (8)ECU failure.

(3) Fuel system.

Faults that may lead to inaccurate fuel supply in the fuel system include: (1) fuel injector leakage or blockage; (2) Electrical damage of engine oil; (3) The filter screen of the fuel pump is blocked, and the spring elasticity of the fuel pump safety valve is small or the pump oil pressure is insufficient; (4) The fuel filter is blocked; (5) Fuel pressure regulator failure; (6) poor fuel quality; (7) The fuel pipeline is deformed. For example, the poor atomization of the fuel injector and the poor mixture mixing of the corresponding cylinders lead to poor combustion, and the power of each cylinder is different, which leads to unstable idle speed of the engine. In addition, the oxygen sensor will generate a low potential signal, and the ECU will enrich the mixture according to this signal. Once the increased fuel quantity exceeds the set adjustment limit, ECU will mistakenly think that the oxygen sensor is faulty and record the fault code.

(4) Mechanical part.

(1) The camshaft cam is seriously worn and the wear is inconsistent, which makes the power of each cylinder different;

(2) The timing chain (belt) is loose or worn, resulting in valve timing dislocation;

(3) Abnormal valve-related parts, such as worn and bent valve push rod, jammed special door, air leakage, broken valve spring and damaged valve seal;

(4) The cylinder pad is ablated or damaged;

(5) The end clearance of the piston ring is too large, and the piston ring is centered and broken;

(6) Excessive cylinder wear.

(5) Other circuit failures.

Mainly refers to the failure of poor contact of power supply circuit or control circuit related to air intake system, fuel system and ignition system. Usually, the oil supply is short or bad in an instant. The abnormal combustion of mixed gas in each cylinder makes the power of each cylinder different. If the engine ECU is poorly grounded and the power supply voltage exceeds 9- 16V, it will cause engine failure.

Detection Steps of Engine Idle Jitter Instability Fault

1. After starting the engine, does the "Check Engine" warning light go out?

2. 1 If the warning light does not go out, check the cause and location of the fault according to the fault code.

2.2 The warning light goes out, and the idle speed matching setting is determined.

3, check whether there is a lack of cylinder, cylinder line is correct, joint connection is reliable.

4. Check whether the idle actuator is normal.

5. According to the signal voltage of the oxygen sensor, judge whether the idle mixture is too rich or too thin.

6. 1 is too strong, check whether the system oil pressure and sensor are normal, check whether the activated carbon tank works normally, check whether the fuel system actuator works normally, and check whether the ignition system works normally.

6.2 If it is too thin, check whether the ignition system is normal, check the system oil pressure, check whether the vacuum leaks, check whether the sensor is normal, and check whether the fuel injector, EGR valve and cylinder pressure are normal.

7. Check whether the engine bracket and buffer rubber pad are damaged.

Second, troubleshooting methods

By analyzing various reasons for the unstable idle speed of the electronic control system, check as follows:

1 Used for cylinder breaking test.

Unplug the high-voltage line of 1 cylinder, but the engine speed will increase. Unplug the high-voltage line of cylinder 4, and the engine has no obvious response. When the high voltage lines of cylinders 2 and 3 are unplugged, the engine speed will decrease. When four spark plugs were removed, it was found that the center electrodes of spark plugs in cylinders 1 and 4 were all ablated. When all spark plugs are replaced to start the engine, the idle speed is slightly increased.

2 Check the air intake system.

No air leakage was found, so the idle valve was removed for inspection, and the idle channel and valve were cleaned with cleaning agent and put back in its original place. The problem was not solved. Considering that the engine is accelerating normally and the instrument has not read the fault code, it is considered that it cannot be caused by a circuit or ECU fault, and the fuel system should be fine.

3 carefully analyze the reasons for the failure again.

It is still considered that the air intake system has excessive air intake. According to past experience, is there a problem with the exhaust gas recirculation valve? So the vacuum tube on the exhaust gas recirculation valve was pulled out, and the engine did not change, so the exhaust gas recirculation valve was removed. It was found that there was scale in the exhaust gas recirculation valve, and the valve was improperly closed, which led to the exhaust gas always entering the intake system to participate in combustion, resulting in unstable engine idle speed. Everything is fine.