What causes the B12 engine idle speed to be unstable, fluctuating between high and low?

[MaliaDella]

The unstable idle speed of the B12 engine, fluctuating between high and low, can be attributed to the following reasons: 1. Dirty intake tract: A clogged intake tract can cause idle fluctuations, and the presence of carbon deposits or oil in the throttle body is the most common cause of unstable idle speed. Carbon deposits in the throttle body can affect its opening, thereby impacting the air intake volume, leading to erratic engine speed fluctuations after a cold start, accompanied by strong vibrations. Severe oil or carbon deposits in the throttle body, including a dirty intake tract, can make it difficult to start the car in cold weather. Cleaning the throttle body is necessary and is considered a routine maintenance task. 2. Weak spark from spark plugs: Spark plugs can also accumulate carbon deposits, which can be cleaned to restore their activity. However, some spark plugs may be nearing the end of their lifespan, leading to reduced performance. Weak sparks from spark plugs in individual cylinders can affect the car's performance. Faulty spark plugs can cause the car to exhibit 'misfire' symptoms, but minor faults in spark plugs of individual cylinders can result in unstable idle speed. Therefore, when the car experiences unstable idle speed, cleaning or replacing the spark plugs is also a viable solution to the problem.

[DiJosephine]

I think the B12 engine's idle is unstable, fluctuating between high and low, possibly caused by carbon buildup issues. Excessive carbon deposits on the throttle body can hinder smooth opening and closing, causing the ECU to misjudge the air intake, leading to erratic RPM fluctuations. Another issue could be vacuum leaks in the intake system, such as aging or cracked hoses or loose fittings causing air leaks, which destabilizes the air pressure and makes the idle rough. The fuel system, like clogged fuel injectors or unstable fuel pump pressure, can also cause uneven fuel delivery. The ignition system, such as worn spark plugs, can cause RPM spikes when misfires occur as the engine tries to compensate. A faulty mass airflow sensor is another major culprit, sending incorrect signals to the ECU and messing up the air-fuel mixture. For simple checks, listen for air leaks or try cleaning the throttle body, but for more complex issues, it's best to seek professional diagnostics to avoid worsening the problem, affecting fuel efficiency and engine lifespan.

[StKinsley]

Last time my car had this issue, the engine RPM was like a roller coaster, mainly due to carbon buildup in the throttle body or the idle control valve getting stuck and failing to regulate. A failing fuel pump causing unstable fuel supply can worsen this situation, especially noticeable during cold starts. A severely clogged air filter restricts airflow, making the ECU incorrectly command the RPM to fluctuate wildly. Faulty sensors like the coolant temperature sensor can trick the engine into thinking it's always warming up, leading to erratic adjustments. Even a small leak in the vacuum system disrupts airflow, naturally causing unstable idle. If you're doing it yourself, start by replacing the spark plugs and checking the hose seals—it's cost-effective and prevents further engine damage.

[WillowAnn]

Erratic idle speed is commonly caused by excessive carbon buildup or vacuum leaks. It is recommended to regularly clean the throttle body and replace spark plugs as preventive measures. Check the intake pipe seals by spraying soapy water to locate bubbles indicating leaks. Replacing a clogged fuel filter can prevent fuel supply issues. Faulty ignition coils can also easily cause unstable RPM, and timely replacement can save trouble. Sensor issues, such as a dirty throttle position sensor affecting ECU decisions, can be resolved by cleaning. Address these issues early to avoid excessive engine wear and high fuel consumption.

[ValerieLee]

The causes are mostly attributed to control failures, where the ECU receives erroneous data from sensors like the MAF or TPS, leading to incorrect idle adjustments. Fuel pressure fluctuations due to a faulty pump or clogged filter result in inaccurate fuel injection, causing sudden RPM jumps. Aging ignition coils or spark plugs failing to ignite force the engine to self-adjust RPM. Vacuum leaks disrupt the air-fuel mixture ratio, directly causing instability. Diagnostics can involve connecting an OBD scanner to read real-time data for anomalies or sequentially unplugging sensors for testing, but be cautious not to damage electronic components. For simpler checks, clean carbon deposits and inspect for air leaks.