What Causes Fluctuating Idle Speed During Cold Start?

[LeEmilia]

Here are the specific reasons for fluctuating idle speed during cold start: 1. Intake manifold leakage: When unauthorized air, fuel vapor, or exhaust gases enter the intake manifold, causing the air-fuel mixture to become too rich or too lean, resulting in unstable engine idle. 2. Excessive carbon buildup in throttle body: Carbon deposits in the throttle body and surrounding intake passages alter the air channel cross-sectional area, preventing the control unit from precisely regulating idle air intake. This also leads to improper air-fuel mixture and unstable idle. 3. Faulty intake system components: The engine intake system contains numerous actuators such as throttle motor, idle speed control valve, duty cycle solenoid valve, mass airflow sensor, and intake pressure sensor. Malfunctions in any of these components can cause unstable idle speed.

[Addyson]

My car has unstable idle speed when starting cold, fluctuating up and down, which I often encounter. The main reason is excessive carbon buildup in the intake system, especially around the throttle body and idle control valve, where dirt blocks airflow, causing the intake volume to vary. A malfunctioning air flow sensor could also be the culprit, particularly noticeable in cold weather due to its temperature sensitivity. Carbon deposits on fuel injectors affect fuel supply uniformity, aging ignition coils lead to unstable sparks, and vacuum tube leaks all contribute to engine shuddering. It's recommended to start with simple checks, like spraying cleaner on the throttle body or getting a professional cleaning at a shop. Don't delay, as prolonged conditions can lead to greater engine wear.

[RafaelDella]

My car also had the issue of fluctuating idle speed during cold starts. At first, I thought it was due to poor fuel quality, but after checking, the problem turned out to be carbon buildup in the idle control components or sensor malfunctions. When starting in cold weather, the engine is still warming up, and unstable air intake flow can easily cause RPM fluctuations. Sometimes, it’s because the air filter hasn’t been replaced for too long, and dirt is blocking the airflow; or the throttle valve is stuck and doesn’t open and close smoothly. I later developed a habit of checking these areas every six months and cleaning them if issues are found. Ignoring it could lead to sudden stalling while driving, which is quite dangerous. Using good fuel additives regularly can help reduce carbon buildup.

[LeilaFitz]

The idle speed fluctuates when starting a cold engine, which I've encountered multiple times. It's mostly caused by severe carbon buildup in the throttle body, a dirty mass air flow sensor, or a faulty idle control valve. When the engine is cold, sensors respond slowly, and uneven airflow causes the RPM to dance. Long-term carbon deposits affect intake efficiency, especially noticeable in winter. Go to the repair shop for a thorough cleaning and check related circuits in time. Don't mess with it yourself to avoid damaging the car.