What causes unstable idle speed when braking?
2 Answers
Reasons for unstable idle speed when braking: 1. Oil accumulation or carbon buildup in the throttle body, or a dirty/clogged intake passage can cause idle instability; 2. Weak spark due to faulty spark plugs; 3. Clogged fuel injector passages or intake manifold issues; 4. Malfunctioning coolant temperature sensor, oxygen sensor, or throttle position sensor; 5. Faulty idle control valve. Unstable engine idle refers to changes in gas pressure within the cylinders, causing power imbalance among cylinders. This results in inconsistent horizontal forces on pistons during power strokes, creating an unbalanced torque that rocks the engine laterally, leading to engine vibration. Timely inspection and repair are necessary for unstable engine idle.
Last time I took my old car to the repair shop, we talked about this issue. When braking, the idle speed is unstable like gasping for breath, most likely because the engine and brake system are competing for air. The vacuum booster relies on the engine's suction to assist braking. If the connecting pipe is cracked or the sealing ring is aged and leaking, the engine won't get enough air, causing the idle speed to shake. Also, if the throttle body is dirty, covered with a layer of sludge, restricted air intake can cause gasping. For older cars, it's important to check the vacuum pipe connections and the brake booster's sealing. Feel the pipes yourself to see if there are any cracks or hissing sounds of air leakage. Have the mechanic read the fault codes while you're at it; it might just be the idle motor being lazy.
