What Causes Unstable Idle Speed When the Engine is Cold?

[LeLandon]

Car unstable idle speed during cold start may be caused by air intake system leaks, low fuel pressure, clogged air filter, injector atomization leakage, improper idle adjustment, faulty spark plugs, air flow meter malfunction, etc. Here is a detailed explanation: 1. In cold conditions, the car needs to bring the engine into normal operating conditions as quickly as possible after starting. Therefore, during cold starts, the engine control unit automatically injects more fuel to rapidly increase the engine's water temperature by burning more fuel than normal idle speed, allowing the engine to quickly reach the appropriate temperature. 2. The main manifestation is that the engine idle speed will be relatively high when cold, generally between 950-1300 RPM. When the engine temperature rises to a certain value, the idle speed will decrease to the normal level. 3. Under normal circumstances, the engine's air intake must be strictly limited. If there are abnormalities or leaks in the intake system, throttle valve, air flow meter, or idle control valve, it may cause unstable engine idle speed. 4. The same applies to the fuel injection system. If the electronic control unit receives incorrect signals and misjudges the car's operating conditions, it may lead to abnormal fuel injection. 5. Additionally, clogged fuel injectors can also cause a decrease in engine power and unstable idle speed.

[OliviaRose]

I've been driving and repairing cars for years, and unstable idle speed when cold is quite common, mostly caused by carbon buildup. Carbon deposits on valves or injectors, especially if you frequently drive short distances without fully warming up the engine, accumulate more and more, leading to uneven fuel-air mixture and unstable combustion during cold starts; aging spark plugs or ignition coils cause inaccurate ignition; faulty air flow or temperature sensors mislead the ECU into incorrect fuel supply; intake manifold leaks cause unstable air intake; thick engine oil in cold weather leads to poor lubrication and increased friction, causing idle vibration; weak battery increases starting pressure. Regularly cleaning carbon deposits and replacing old spark plugs can prevent issues. Once, after repairing an old car, the owner reported much smoother idling and a worry-free drive. Preventive measures include avoiding short trips to let the engine warm up fully, ensuring accurate ECU adjustments, and not waiting until a breakdown to fix problems.

[StJulian]

I often drive the early shift, and the engine's cold start idle jump and shake is really annoying. Carbon buildup clogging the injectors or valves is the most common issue, causing uneven fuel combustion; dirty injectors or fuel pump supply problems; air intake system leaks disrupting the air-fuel mixture ratio; sensors like the coolant temperature sensor being inaccurate, misleading the ECU; worn spark plugs failing to ignite; insufficient lubrication when cold increasing resistance. It's recommended to first clean the carbon deposits, check sensors and wiring, and ensure proper fuel system maintenance. Routine maintenance like using high-quality engine oil and warming up the car for a minute or two before driving can stabilize the idle. Last time my car had this issue was in winter, and timely handling reduced problems. Safety is paramount, don't ignore small shakes that could lead to bigger troubles.

[VonPreston]

I've driven several cars and always found unstable idle when cold-starting quite annoying. Excessive carbon deposits affecting combustion are the most common cause, with clogged injectors leading to uneven fuel-air mixture; aging fuel injection or ignition components; faulty intake sensors giving incorrect readings; poor lubrication during cold starts increasing resistance. Maintenance like carbon cleaning and spark plug replacement can solve it. I do these regularly, and the idle becomes smoother, making driving more stable.