What are the causes of unstable engine idle speed?
3 Answers
There are several reasons for unstable engine idle speed: 1. Intake system failure; 2. Fuel system failure; 3. Ignition system failure; 4. Electronic control system failure. Unstable engine idle speed is a common issue encountered in engine maintenance, which refers to changes in gas force within the cylinders causing an imbalance in power among the cylinders. This results in inconsistent horizontal component forces on the pistons during the power stroke, creating an unbalanced torque that tilts the engine laterally and causes engine vibration. Common troubleshooting methods include cleaning the throttle body and intake passages, cleaning and inspecting fuel injectors, replacing electrical components, checking wiring harness fault points, cleaning grounding points, and repairing the engine's mechanical structure.
I've been driving a taxi for over a decade, and unstable idle is a common issue. Carbon buildup is definitely a major headache - when the throttle body gets dirty, it's like having a stuffy nose during a cold, making air intake difficult. Spark plug aging is another problem, making ignition as unreliable as an old lighter. If the engine mounts crack, the vibration makes the steering wheel dance and the seats shake. My old taxi once had a faulty oxygen sensor - fuel consumption suddenly skyrocketed and the engine ran rough. Also, when the AC compressor kicks in and steals power, older cars are particularly prone to shuddering. Never delay fixing these issues. Get them diagnosed early - last time I spent just over 100 yuan cleaning the throttle body and solved the shaking problem.
As a frequent visitor to repair shops, I've suffered from unstable idle speeds. Vacuum line leaks are the most troublesome—just a crack in the rubber hose makes the engine gasp like a slow tire leak. Dirty fuel injectors cause uneven fuel spray, and incorrect spark plug gaps lead to ignition failure. During my last checkup, the intake pressure sensor was clogged with sludge, making the idle speed feel like a rollercoaster. The scariest was a stretched timing chain, which almost caused valve damage. Many beginners don’t know that filling a car designed for 92-octane with 95-octane fuel can trigger a warning light. I recommend regularly checking intake seals—if the rubber hose is aging, it’ll creak when squeezed.
