Causes of unstable idle speed of engine

Causes of unstable idle speed of engine

idle

Idle speed usually refers to the condition that the throttle is closed, the accelerator pedal is completely released, the engine has no power output, and the lowest stable speed can be maintained. ?

When the engine is idling, the loads include: internal friction between crank-connecting rod mechanism and valve train, auxiliary driving devices (water pump, generator, etc.). ).

Idle speed control system?

The function of the idle speed control system is to realize the stable operation of the engine at the target speed, and make the engine warm up quickly after starting and adjust the idle speed after turning on the air conditioner, so that the engine can meet the requirements of different idle working conditions and achieve good economy, emission performance and driving performance.

Idle speed control system can be divided into two types according to the control mode: one is the throttle direct-acting type that controls the closed position of the throttle, and the actuator adjusts the throttle opening through the throttle operating mechanism to achieve the purpose of controlling the idle air intake.

The second type is the bypass air type, which controls the air flow of the throttle bypass channel. The actuator adjusts the air intake of the bypass air passage by changing the position of the bypass control valve, thus realizing idle speed control. Both of these forms control the airflow by adjusting the cross-sectional area of the inlet, so as to achieve the purpose of idle speed adjustment.

Idle speed control system consists of three parts: sensor, ECU and actuator. The actuator of the idle speed control system is the idle speed control valve (ISCV).

Basic principle of idle speed control system: Engine control system has the function of maintaining normal idle speed, that is, idle speed control function.

The traditional idle speed control is the original manual control or mechanical control. At present, the idle speed of modern new cars is generally controlled by ECU. The ROM of the engine control module ECU stores the optimal idle speed (target speed) under various idle conditions. When the engine is idling, the ECU determines the target speed according to the engine state parameters detected by the sensor, calculates the difference between the target speed and the actual speed, determines the control quantity, drives the idle speed control device, and changes the intake air to make the actual speed close to the target speed.

Idle speed control principle

The essence of engine idle speed control is the control of idle air intake. Idle speed control principle is as follows:

Setting of initial starting position: When the engine stops and no engine speed signal is transmitted to ECM, the idle speed control valve is fully opened to improve the starting performance when the engine starts again.

Start-up control: During start-up, ECM adjusts the opening of idle speed control valve according to the signal of coolant temperature sensor, so as to achieve the best position of preheating control after start-up.

Warm-up and fast idle control: ECM controls the opening of idle control valve according to the signal of coolant temperature sensor and the control characteristics of ECM internal program. With the increase of temperature, the opening gradually decreases. When the temperature of the coolant reaches 70℃, the preheating control process ends.

Predictive control: In idle state, such as the change of transmission gear, power steering and air conditioning working state, the engine speed will change predictably. After receiving the signal, ECM will adjust the opening of idle speed control valve in advance to improve idle speed.

Fault analysis

The causes of engine idling instability may be various, multi-system and multi-factor, and it is difficult to diagnose and detect. It is not recommended to replace parts easily, and correct judgment should be made according to test results, data analysis and maintenance experience. The main reasons for unstable idle speed are: engine electronic control system, air intake system, fuel system, ignition system, engine mechanical failure, etc.

Idle instability caused by electronic control system: the faults of electronic control system are mainly the faults of sensors and actuators, and the failure probability of ECU is very small, so it is not considered.

Sensor failure. Sensor failures mainly include coolant temperature sensor, air flow sensor, air temperature sensor and oxygen sensor. The ECU receives the wrong sensor signal and outputs the wrong command to the actuator, resulting in unstable idle speed.

When judging whether the sensor is faulty, observe whether the fault indicator light on the instrument panel is on. If the fault indicator is on, it is necessary to read the fault code and detect the fault with a scan tool.

Actuator failure. The actuator mainly includes idle speed control device, EGR valve, activated carbon canister solenoid valve, etc. Dirty valves lead to inaccurate idle air control, unstable idle speed and even flameout.

For actuator failure, generally check whether there is air leakage or dirty blockage first, and replace it if necessary.

Idle instability caused by supply system: the main faults of supply system are oil circuit and gas circuit. The fuel system is mainly a fuel pressure fault. The air intake system is mainly the pipeline leakage of the air intake system.

Fuel system pressure fault. Common causes are: fuel leakage, poor fuel pressure regulator, blocked fuel filter, poor atomization of fuel injector, etc.

Air intake system pipeline failure. The pipeline leakage of intake system mainly refers to dirty leakage of throttle body, poor exhaust, damage and leakage of EGR valve, vacuum hose and PCV hose, joint falling off and wrong connection, damage or leakage of intake manifold or manifold pad, and blockage of air filter.

Idle instability caused by ignition system: In the ignition system, single cylinder fire or poor ignition will cause engine idle instability. Common causes are: spark plug electrode ablation or carbon deposition.

Idle instability caused by mechanical parts: the failure of mechanical parts is mainly due to poor cylinder seal, resulting in low cylinder pressure. Failure of timing chain can also lead to unstable engine idle speed.