What are the main reasons for unstable idle speed in electronic fuel injection engines?

[Nigel]

Fuel system failures mainly include gasoline quality, fuel pump, fuel filter, fuel pressure regulator and its pipelines, as well as sludge and carbon deposits adhering to the throttle and fuel injectors. Electrical circuit failures primarily involve electronic control components, relays, fuses, and wiring issues. Reasons for unstable idle speed: 1. Intake system: The first reason is the intake system, including leaks in the intake pipe and various valves. Air entering from the intake pipe, gasoline vapor from the charcoal canister valve, and exhaust gases entering the intake pipe can cause the mixture to be too rich or too lean, leading to abnormal engine combustion. Excessive carbon buildup in the throttle and intake passages. ③ Inaccurate idle intake air volume. 2. Fuel system: Fuel injector malfunction. If the fuel injector has a drip or blockage, it cannot inject fuel according to the ECU's instructions, resulting in a mixture that is too rich or too lean, causing poor operation in individual cylinders and leading to unstable engine idle speed. Fuel pressure issues. If the fuel pressure is too low, the fuel sprayed from the injector will have poor atomization or form a line, and in severe cases, only droplets will be sprayed, reducing the fuel injection amount and making the mixture too lean; if the fuel pressure is too high, the actual fuel injection amount increases, making the mixture too rich. Inaccurate fuel injection amount. Faults in various sensors or wiring can cause the control unit to issue incorrect commands, resulting in incorrect fuel injection amounts and causing the mixture to be too rich or too lean, which is an indirect cause of unstable idle speed. 3. Ignition system: Most modern cars integrate the ignition module with the ignition coil or use dual ignition coils or direct ignition. First, the ignition module: if it malfunctions, it can cause weak sparks or no spark at all. For testing, you can only measure the secondary voltage in some models; the primary voltage is not visible, but you can test the ignition trigger signal. Issues with the ignition coil and ignition module may be due to missing ignition signals from the computer or problems with the ignition module's power supply lines. In actual repairs, this situation occurs frequently. New cars have a higher chance of experiencing this issue. When repairing new cars, some mechanics still spend a lot of time searching for various problems. For new cars, it's essential to ensure their power supply, both positive and negative terminals, operates reliably.