*Possible causes:*
1. Idle Air Control (IAC) valve: The IAC valve controls the amount of air that bypasses the throttle body, helping to regulate the idle speed. A faulty IAC valve can cause the engine to idle too high or stay running after the key is turned off.
2. Throttle Position Sensor (TPS): The TPS tells the engine control module (ECM) the position of the throttle valve. A faulty TPS can send incorrect signals to the ECM, causing the engine to idle too high or stay running after the key is turned off.
3. Vacuum leaks: Vacuum leaks can cause the engine to run lean, which can lead to high idle speeds or stalling. Check for any loose or disconnected vacuum hoses.
4. Fuel injector issues: A stuck-open fuel injector can continue to spray fuel into the engine after the key is turned off, causing it to run on.
5. Electrical issues: A short in the ignition switch or wiring can cause the engine to stay running after the key is turned off.
*Troubleshooting steps:*
1. Check the IAC valve: Disconnect the IAC valve and start the engine. If the engine idle speed drops significantly or the engine stalls, the IAC valve is likely faulty and needs to be replaced.
2. Check the TPS: Check the TPS for proper voltage output. The TPS should output a signal between 0 and 5 volts as the throttle valve is opened and closed.
3. Check for vacuum leaks: Inspect all vacuum hoses for cracks, tears, or loose connections. Replace any damaged or disconnected vacuum hoses.
4. Check fuel injector pulse: Use a fuel pressure gauge and observe the fuel pressure while cranking and running the engine. If the fuel pressure does not drop when the key is turned off, there may be a stuck-open fuel injector.
5. Check electrical connections: Inspect the ignition switch, wiring harness, and relays for any signs of damage or corrosion. Repair or replace any faulty electrical components.
*Note:* If you are not comfortable performing these troubleshooting steps yourself, it is recommended that you take your vehicle to a qualified mechanic for diagnosis and repair.
