The reasons for engine shaking when braking at idle speed are: the idle switch not closing, a faulty idle control valve, and intake manifold air leaks. Idle Switch Not Closing: When the idle contact opens, the ECU determines that the engine is under partial load. At this time, the ECU calculates the fuel injection amount and timing based on signals from the air flow sensor, crankshaft position, and engine speed. However, the engine is actually operating at idle, where the air intake is lower, resulting in an overly rich air-fuel mixture and an increase in engine speed. When the ECU receives feedback from the oxygen sensor indicating an overly rich mixture, it reduces the fuel injection amount and increases the opening of the idle control valve, leading to an overly lean mixture and a drop in engine speed. Conversely, when the ECU receives feedback indicating an overly lean mixture, it increases the fuel injection amount and decreases the idle control valve opening, causing the mixture to become overly rich again and the speed to rise. This cycle repeats, causing unstable engine idle. The solution is to adjust or replace the throttle position sensor. Faulty Idle Control Valve: The normal idle speed of an electronically fuel-injected engine is maintained by the idle control valve. The ECU processes signals such as engine speed, temperature, throttle switch, and air conditioning switch to adjust the idle control valve, either by opening the intake bypass passage or directly increasing the throttle opening to boost air intake and raise the idle speed. When the idle speed exceeds the set value, the ECU commands the idle control valve to reduce the intake bypass passage, decreasing the air intake and lowering the engine speed. Issues like sticking due to oil residue or carbon buildup, or incomplete throttle closure, can prevent the ECU from correctly regulating the idle speed, leading to instability. The solution is to clean or replace the idle control valve and perform a basic idle speed setting using a dedicated diagnostic tool. Intake Manifold Air Leaks: According to the engine idle control principle, under normal conditions, the opening of the idle control valve and the air intake strictly follow a specific functional relationship—that is, as the idle control valve opening increases, the air intake increases accordingly. An air leak in the intake manifold disrupts this relationship, making it impossible for the air flow sensor to measure the actual air intake accurately. This results in the ECU inaccurately controlling the air intake, causing unstable engine idle. The solution is to locate the leak, reseal it, or replace the affected components.
