Why does the car engine idle speed fluctuate up and down?
2 Answers
Idle speed switch not closing is the cause. More information about the car engine idle speed fluctuating up and down is as follows: 1. Idle speed switch not closing: Diagnostic method: When idling, turn on the air conditioner or turn the steering wheel. If the engine speed does not increase, this fault can be confirmed. Fault resolution: Adjust, repair, or replace the throttle position sensor. 2. Idle speed control valve (ISC) failure: Diagnostic method: Check the operation sound of the idle speed control valve. If there is no operation sound, the idle speed control valve is faulty. Fault resolution: Clean or replace the idle speed control valve and use a special decoder to perform a basic setting of the idle speed. 3. Intake pipe leakage: Diagnostic method: If a leakage sound is heard in the intake pipe, it proves that the intake system is leaking. Fault resolution: Find the leakage point and reseal or replace the corresponding components.
I have extensive experience in car repair, and fluctuating idle speeds are quite common, mainly due to excessive carbon buildup in the throttle body. When carbon deposits accumulate, the valve's opening and closing become sluggish, and the engine control unit (ECU) can't precisely adjust the RPM, much like a person struggling to breathe. There could be other issues as well, such as a dirty or contaminated mass airflow sensor, which can misreport the air intake and lead to inaccurate fuel delivery. Worn spark plugs or faulty ignition coils can also cause unstable combustion, resulting in RPM fluctuations. If the idle control valve gets stuck, it directly affects stability. A vacuum hose leak can allow air to seep in, disrupting the air-fuel mixture ratio. Over time, this can increase fuel consumption, cause engine shaking, or even stalling, posing significant safety risks. I recommend starting with a throttle body cleaning, as it resolves most issues. If that doesn’t work, consult a professional mechanic for a thorough diagnosis to prevent minor problems from turning into major repairs. Remember to schedule regular maintenance and inspect related components every few tens of thousands of kilometers to prevent this issue.
