What Causes Unstable Engine Idle?

[DiKinsley]

Here are the reasons for unstable engine idle: 1. Intake manifold leakage: When unauthorized air, fuel vapor, or exhaust gases enter the intake manifold, it causes the air-fuel mixture to become too rich or too lean, leading to unstable engine idle. A simple method for car owners to check for intake manifold leaks: Open the engine hood, remove the air filter from the throttle body. Start the engine and block the throttle with your hand (if the engine stalls, there is no leakage; if it continues running, there is a leak somewhere). If it doesn't stall, don't rush to turn it off. Check nearby pipes for any suction, which indicates a leak. 2. Excessive carbon buildup in the throttle body: Too much carbon deposits in the throttle body and surrounding intake passages change the cross-sectional area of the air channel, making it difficult for the control unit to precisely regulate idle air intake. This also leads to an overly rich or lean mixture, causing unstable idle. 3. Faulty intake system components: There are many actuators in the engine's intake system. 4. Fuel system issues: Problems such as clogged injectors, blocked fuel filters, obstructed fuel pump screens, faulty fuel pressure regulators, or clogged return lines can all result in incorrect fuel injection, leading to improper air-fuel mixture and unstable idle. 5. Ignition system failures: Issues like faulty ignition modules, incorrect spark plug gaps, eroded or damaged spark plug electrodes, or excessive resistance in high-voltage wires can reduce spark energy or cause misfires, resulting in erratic idle speed fluctuations. If your car experiences unstable idle with fluctuating RPMs, it should be addressed promptly. Remember, when this occurs, the engine is no longer operating at its ideal idle speed, leading to worsened fuel economy and reduced environmental performance.