For Carburetor Models:
* Vacuum Leaks: Leaks in the vacuum lines can cause incorrect readings that lead to a rich fuel mixture and a high idle. Check all vacuum hoses for cracks, splits, or loose connections. Pay particular attention to the hoses connected to the brake booster, distributor advance, and the carburetor itself.
* Fast Idle Cam: This cam is designed to hold the throttle open slightly when the engine is cold. If it's sticking or not returning to its normal position, it will cause a high idle even when the engine is warm. Inspect for binding or damage.
* Throttle Linkage: If the throttle linkage is sticking or binding, the throttle plate may not be closing completely, resulting in a high idle. Lubricate linkage components and check for any binding.
* Idle Speed Adjustment Screw: This screw controls the idle speed. If it's been inadvertently adjusted too high, the idle will be elevated. Refer to your service manual for proper adjustment procedure.
* Choke: A sticking choke can keep the throttle partially open, resulting in a high idle. Ensure the choke is functioning properly and fully releasing as the engine warms up.
* Dirty or Clogged Carburetor: A dirty carburetor can lead to inconsistent fuel delivery and a high idle. Cleaning or rebuilding the carburetor might be necessary.
For Fuel-Injected Models:
* Throttle Position Sensor (TPS): A faulty TPS sends incorrect signals to the engine control unit (ECU), which can lead to a high idle. The TPS should be tested with a multimeter to ensure it's providing accurate readings.
* Idle Air Control (IAC) Valve: The IAC valve regulates airflow at idle. A dirty or malfunctioning IAC valve can result in a high idle. It may need cleaning or replacement.
* Mass Airflow Sensor (MAF) Sensor: A dirty or faulty MAF sensor can also cause a high idle. Clean the sensor (carefully!) or have it tested.
* Vacuum Leaks: Just as with carburetors, vacuum leaks can significantly affect idle speed. Check all vacuum lines.
* ECU Issues: While less common, a faulty ECU can contribute to high idle. This is a more advanced diagnostic problem and usually requires professional help.
* Air intake system: Any restriction or leak in the air intake system can cause a high idle.
Troubleshooting Steps:
1. Start with the basics: Check for obvious vacuum leaks. Listen for unusual sounds near the engine.
2. Check the engine temperature: Is the engine cold or hot? A high idle is normal when cold but should decrease as it warms up.
3. Inspect throttle linkage: Ensure it moves freely.
4. Consult a repair manual: A factory service manual specific to your 1987 Chevy S-10 will provide detailed diagrams and troubleshooting steps.
5. Professional help: If you're not comfortable working on your vehicle, it's best to take it to a qualified mechanic.
Remember safety first! Always disconnect the negative battery terminal before working on any electrical components. If you're unsure about any repair, seek professional assistance.
