What Causes Unstable Idle Speed After Starting?
2 Answers
After starting the car, unstable idle speed can be caused by errors in the engine control logic or faults in the air conditioning control circuit. The details are as follows: Engine Control Logic Error: Unable to receive the signal indicating the air conditioning is turned on. When it cannot determine whether the air conditioning is activated, the engine control unit (ECU) mistakenly interprets this as a temporary high load and attempts to compensate for the idle resistance by increasing the engine speed. When the engine speed decreases, it cannot handle the load from the air conditioning compressor, leading to another increase in speed. This back-and-forth adjustment results in unstable idle speed. Air Conditioning Control Circuit Error: Unable to send the air conditioning activation signal. Similar to the first scenario, because the ECU cannot determine the air conditioning's operational state, it continuously adjusts the idle speed, causing unstable idle behavior.
I recently encountered the issue of unstable idle speed after startup, which is quite common, especially in cold weather. I tried checking it myself and found carbon deposits accumulated on the throttle body, blocking airflow and causing the RPM to fluctuate. Dirty fuel injectors can also affect fuel supply, making the engine struggle. Sometimes, an aging ignition system, worn spark plugs, or faulty coils can cause poor ignition, making the idle speed shake erratically like dancing. Sensor failures, such as the throttle position sensor sending incorrect signals to the ECU, are also common causes. This problem shouldn’t be taken lightly, as it increases fuel consumption and emissions, and in severe cases, the engine may stall in traffic. After cleaning the throttle body and replacing the spark plugs, the issue improved. I recommend starting with simple steps like cleaning the throttle body and checking the air filter—it’s both hassle-free and safe.
