Why is the car idle unstable?

[Olga]

There are several reasons for unstable car idle, including oil accumulation and carbon deposits in the throttle body, as well as dirty or clogged intake passages, which can cause idle fluctuations. Carbon deposits or oil in the throttle body are the most common causes of unstable idle. Below are some related reasons for unstable car idle: Spark plug failure: Weak sparks from faulty spark plugs can also lead to carbon buildup. Carbon deposits can be cleaned to restore spark plug activity, but some spark plugs may be nearing the end of their lifespan, resulting in reduced performance. Fuel system blockage: Blockages in the fuel system, particularly in the fuel injectors, or faults in the intake manifold can cause insufficient fuel supply pressure, leading to poor fuel atomization. This disrupts the air-fuel mixture ratio, resulting in incomplete combustion and related issues. Sensor failure: Faulty sensors can prevent the ECU from accurately controlling the air-fuel ratio. Common sensor failures include the coolant temperature sensor, oxygen sensor, and throttle position sensor. The coolant temperature sensor is typically installed on the engine block or cylinder head water jacket, directly contacting the antifreeze to measure temperature and provide reference data to the ECU.

[Jeff]

My old Jetta has been running for almost twenty years, and I often encounter unstable idle speed, shaking violently like it has Parkinson's disease. There are too many possible causes for this issue. The most common one is sensor failure—for instance, if the little device measuring engine airflow malfunctions, the computer receives incorrect data and messes up the RPM adjustment. Secondly, severe carbon buildup in the throttle body can disrupt smooth fuel intake. Another possibility is a stuck idle control valve or excessively large spark plug gaps causing delayed ignition. If the car keeps shaking and emitting black smoke, it might be due to insufficient fuel supply from the fuel pump or clogged injectors. Unstable idle speed isn’t just annoying while driving; it also spikes fuel consumption and accelerates engine wear. Every time I notice something off, I first check the vacuum hoses for leaks, using an oil-dampened cloth to detect any issues. If that doesn’t work, I have to visit a professional repair shop to scan for trouble codes with a diagnostic tool, preventing the problem from escalating and affecting driving safety.

[VanRhett]

Man, that Fit of yours is shaking at idle like it's dancing, what a pain! After tinkering with it, we found the main culprit is the fuel system acting up. For example, the injectors are too dirty with carbon buildup, causing uneven fuel spray that makes the engine speed fluctuate. Also, a clogged fuel filter can starve the engine of fuel, leading to stuttering. Another common issue is aging spark plugs or loose coil connections, causing intermittent current and unstable ignition. And if the AC is on with the fan buzzing, the idle control might struggle, possibly due to unstable voltage. Don’t underestimate idle instability—sudden stalling on the highway is dangerous. We can DIY-check the spark plugs, regularly replace the air filter, and use fuel additives to clean the fuel system for better prevention. Remember to stick to reputable gas stations to avoid cheap, low-quality fuel, or else the same trouble might strike again.

[Tyler]

There are three main reasons for unstable idle speed: First, ignition system failure, such as worn spark plugs or faulty ignition coils causing uneven ignition; Second, fuel system issues, where impurities in the fuel line clog the injectors; Third, air system leaks, such as small holes in the intake manifold or vacuum tubes leading to an overly lean air-fuel mixture. Symptoms include fluctuating RPM and vehicle vibration. Simple solutions: Check if the spark plug electrode gap is too wide, listen for hissing sounds indicating air leaks in the pipes; In severe cases, try using higher-grade gasoline. Regular maintenance is key to preventing most of these issues.