Why does the car sometimes have unstable idling speed that fluctuates up and down?

[LeRebecca]

The reasons for the car's idling speed fluctuating up and down may include: 1. Excessive carbon buildup in the throttle body. 2. Leakage in the intake manifold or various valves. This fault directly affects the quality of the fuel-air mixture and combustion efficiency. Possible leak points include the intake manifold, vacuum hoses, charcoal canister valve, exhaust gas recirculation (EGR) valve, positive crankcase ventilation (PCV) valve, etc., which need to be inspected one by one. 3. Valve timing fault, incorrect valve timing, misaligned timing belt installation, excessive carbon buildup on intake and exhaust valves, broken valve springs, etc. If the intake air volume of each cylinder is inconsistent, it will cause unstable idling. 4. Blocked catalytic converter. A blocked catalytic converter not only causes unstable idling but also leads to insufficient power. 5. Fuel injector faults, such as poor atomization or clogged injectors, which require cleaning. 6. Low fuel pressure. Mainly check whether components like the fuel filter and fuel pump are clogged, or if the fuel pipe is deformed, etc. 7. Engine misfire (missing cylinder).