What are the causes of unstable idle speed and stalling in cars?

[DeGabriela]

The causes of unstable idle speed and stalling in cars include: air intake system leaks, low fuel pressure, clogged air filters, fuel injector atomization leaks, improper idle speed adjustment, faulty spark plugs, and air flow meter malfunctions. Under normal circumstances, the intake air volume must be strictly controlled. If there is a leak in the intake pipe, the air flow sensor cannot accurately measure the actual intake air volume, causing the electronic control unit to inaccurately control the air intake and resulting in unstable idle speed. Fuel injector dripping, clogging, or circuit malfunctions can affect the quantity and quality of fuel injection, preventing it from following the electronic control unit's instructions. This leads to an excessively rich or lean air-fuel mixture, causing poor performance in individual cylinders and unstable idle speed.

[MacHarper]

I've also encountered issues with unstable car idling or even stalling, let me share the common causes. Unstable idling mostly stems from carbon buildup inside the engine, especially when the throttle body gets dirty and affects air intake, leading to improper air-fuel mixture ratio and causing stalling. Another common point is ignition system failure, such as aging spark plugs or faulty ignition coils - unstable ignition can easily make the idle speed fluctuate. There are also intake system problems, like a clogged air filter preventing proper air intake which affects fuel combustion and causes stalling. Vacuum hose leaks frequently occur too, as old rubber hoses are prone to cracking and air leakage. Finally, don't forget about engine sensors - a faulty crankshaft position sensor causing control inaccuracy can also affect idling. I recommend regular throttle body cleaning and air filter replacement, which can prevent most problems, being both cost-effective and safe.

[Marilyn]

I frequently deal with abnormal vehicle conditions, and the main causes of unstable idle speed and stalling include several key components. Issues with the ignition system are the top culprit; worn spark plugs or aging ignition coils can lead to weak sparks, causing the engine to stall if it fails to ignite. Fuel system malfunctions are also common, such as clogged fuel injectors or an underperforming fuel pump failing to supply adequate fuel, resulting in an overly lean air-fuel mixture. On the intake side, a clogged air filter or a stuck idle control valve can disrupt air supply. Abnormalities in the electronic control unit or related sensors, like a faulty temperature sensor, can cause the computer to misjudge the idle state. Additionally, vacuum leaks or ruptured hoses can allow the engine to draw in excess air. In such cases, I recommend checking the spark plugs and air filter as simple initial steps. If the problem persists, promptly visit a professional shop for a diagnostic scan instead of attempting random repairs.

[OJuliet]

I've driven quite a few older cars, and unstable idle or stalling is often caused by minor issues. Carbon buildup inside the engine cylinders blocks the air passages, making the idle rough and prone to stalling. In the ignition system, worn spark plugs with excessive gaps fail to ignite the fuel properly, disrupting idle. Clogged fuel injectors cause uneven fuel spray, leading to stalling. Also, a dirty air filter that hasn't been replaced for too long accumulates dust and affects air intake, making idle unstable. Common faults like loose wiring connections or vacuum hose leaks are also worth noting. I usually clean the throttle body or replace spark plugs myself to try fixing it, saving a lot on repair costs.