What are the reasons for unstable idle speed fluctuating between high and low?
2 Answers
The reasons for unstable idle speed fluctuating between high and low are: 1. Excessive carbon buildup in the throttle body; 2. Leakage in the intake manifold or various valves; 3. Engine misfire (cylinder not firing); 4. Fuel system malfunctions, including clogged fuel injectors, clogged fuel filter, clogged fuel pump screen, faulty fuel pressure regulator, and clogged fuel return line; 5. Ignition system failures, including faulty ignition module, incorrect spark plug gap, eroded or damaged spark plug electrodes, and excessive resistance in high-voltage wires. If a car exhibits unstable idle speed fluctuating between high and low, it should be addressed promptly; otherwise, both fuel economy and environmental performance will deteriorate.
I've noticed that unstable idle speed fluctuating up and down can have many causes over the years. The most common is excessive carbon buildup in the throttle body blocking the intake passage, causing the engine to have difficulty breathing and resulting in erratic RPM jumps. If the fuel injectors are dirty or clogged with debris, it can also lead to uneven fuel spray, making the idle speed go haywire during acceleration and deceleration. Ignition system problems are troublesome - after several years of use, spark plug electrode wear can cause ignition failures, or aging ignition coils may malfunction when hot. If there are air leaks in the intake manifold or hoses, the air leaking in and out disrupts pressure balance, causing unstable idle. Faulty sensors like the idle control valve or incorrect feedback signals can make the ECU lose control, causing the idle speed to jump. Low battery voltage can affect the overall system operation. Once when I was repairing a car with this symptom - idle speed fluctuating wildly - inspection revealed a cracked vacuum hose, and replacing it immediately smoothed things out. Regular maintenance while driving, like cleaning the throttle body every 10,000-20,000 km and replacing air filters, can help prevent this. If left untreated, fuel consumption will skyrocket and stalling becomes a dangerous risk, so it's best to have a professional diagnose it promptly.
