What Causes Poor Idle in EFI Engines?

[MacStella]

Here are the specific reasons for poor idle in EFI engines: 1. Air intake system leakage or blockage. 2. Insufficient cylinder pressure. Use a cylinder pressure gauge to check. If the pressure is not within the 800-1300 kPa range, exceeds standard differential pressure, or if individual cylinders aren't working, inspect whether the spark plugs are tightened, valve timing, cylinder gasket, timing belt position, piston ring sealing, and valve sealing. 3. Weak spark intensity. Any performance issues in the ignition system components can cause insufficient spark intensity. Check the resistance of ignition wires and distributor rotor, spark plug gap and condition, distributor operation and conductivity. Also check if the ignition coil or ignition module overheats. If necessary, use an oscilloscope to observe primary and secondary ignition waveforms to identify faults. 4. Malfunctioning idle control valve. Determine whether the issue lies with the idle control valve, sensors, or ECU. 5. Faulty throttle position sensor. Through self-diagnosis, instrument diagnosis, or manual diagnosis, if the sensor is identified as faulty, verify whether the idle switch signal reaches the ECU. 6. Malfunctioning auxiliary air valve. This valve controls cold engine fast idle. Observe if there's fast idle during cold starts.

[MelanieFitz]

I've encountered many cases of poor idle performance in fuel-injected engines, which is really annoying. Common causes include sensor failures, such as a dirty or faulty mass airflow sensor, causing the engine to misjudge the intake air volume and resulting in erratic idling; a malfunctioning throttle position sensor, which monitors throttle position, can also lead to unstable idle if inaccurate. Carbon buildup is another major culprit—excessive deposits on the throttle body and fuel injectors obstruct air and fuel flow, causing severe engine shaking at idle. Vacuum leaks often introduce excess air through damaged hoses, disrupting the air-fuel mixture ratio and causing noticeable RPM fluctuations. Additionally, worn spark plugs or faulty ignition coils can affect ignition timing, leading to tremors. The ECU controls the overall idle speed, and if sensor signals are incorrect, the entire system becomes chaotic. I recommend regularly cleaning the throttle body, checking hose seals, and having the car scanned for trouble codes—these small maintenance steps can prevent idle issues from worsening, saving money and ensuring safety.