Most Likely Causes:
* Faulty Idle Air Control (IAC) Valve: This valve controls the amount of air entering the engine at idle. A sticky, dirty, or malfunctioning IAC valve will often cause a high idle. Cleaning it is often the first step (though replacement might be necessary).
* Vacuum Leaks: A vacuum leak anywhere in the intake system can cause the engine to run lean and increase the idle speed. Check all vacuum hoses for cracks, holes, or loose connections. Pay close attention to the hoses connected to the brake booster, PCV valve, and the manifold.
* Throttle Position Sensor (TPS): A faulty TPS can give the computer incorrect information about the throttle position, leading to a high idle. Testing this sensor is usually required, often using a multimeter.
* Dirty Throttle Body: A dirty throttle body can restrict airflow, causing the computer to compensate by increasing the idle speed. Cleaning the throttle body with throttle body cleaner is a common fix.
Less Likely, But Still Possible Causes:
* Faulty EGR Valve: The Exhaust Gas Recirculation (EGR) valve helps reduce emissions. A malfunctioning EGR valve could contribute to a high idle, but this is less frequent than the above issues.
* Computer (ECM) Issues: While less common, a problem with the engine control module (ECM) could be at fault. This is a more difficult problem to diagnose and usually requires professional help.
* Restricted Catalytic Converter: A clogged catalytic converter can cause backpressure, increasing idle speed. This is less likely to cause *only* a high idle, though.
* Mass Airflow Sensor (MAF) Issues (if equipped): If your 1987 S-10 has a MAF sensor (less common in that year), a problem with it could affect the idle.
Troubleshooting Steps:
1. Check all vacuum lines: Inspect all vacuum hoses for cracks, leaks, or loose connections.
2. Clean the throttle body: Spray throttle body cleaner into the throttle body while lightly working the throttle plate.
3. Clean or replace the IAC valve: You can often clean the IAC valve with throttle body cleaner and then carefully re-install it. If cleaning doesn't work, replacement is the next step.
4. Check the TPS: This requires a multimeter to test the voltage output at various throttle positions. A repair manual will show you how to do this properly.
5. Check the EGR valve: Inspect for sticking or blockage.
Important Note: Before starting any work, disconnect the negative battery terminal to prevent accidental short circuits. If you're not comfortable working on your vehicle's systems, it's best to take it to a qualified mechanic. They have the tools and expertise to properly diagnose and repair the problem.
