* Idle Air Control (IAC) Valve: This is the most common cause. The IAC valve regulates the amount of air entering the engine at idle. If it's sticking open, dirty, or malfunctioning, it can cause a high idle, especially after the engine warms up and the computer is trying to reduce idle speed. Cleaning it (with throttle body cleaner) is often the first step in troubleshooting. If cleaning doesn't work, replacement is likely needed.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle plate's position. A faulty TPS can send incorrect signals, leading to an inaccurate idle adjustment. A failing TPS often shows symptoms of erratic idle, stalling, and poor performance across the rev range, not just high idle on warm-up.
* Vacuum Leaks: Vacuum leaks can disrupt the engine's idle control. Check all vacuum lines and connections for cracks, holes, or loose fittings. Pay close attention to lines connected to the brake booster, PCV valve, and other vacuum-operated components. A hissing sound near the engine indicates a leak.
* EGR Valve (Exhaust Gas Recirculation): While less likely to be the sole cause of a high warm idle, a sticking or faulty EGR valve can contribute to the problem. It introduces exhaust gases back into the intake to reduce emissions and combustion temperatures. A problem here might cause higher idle, especially under load or warm-up.
* Mass Airflow Sensor (MAF) Sensor (if equipped): While less likely on a '88 model (they were becoming more common but not universally standard yet), a faulty MAF sensor can provide inaccurate air flow readings to the engine control module (ECM), leading to incorrect fuel mixture and idle issues.
* Computer (ECM) Issues: While less probable, a problem with the Engine Control Module itself could be responsible. This is usually diagnosed last, as it's more involved to test and replace.
* Dirty Throttle Body: A buildup of carbon and grime on the throttle body can interfere with the smooth operation of the throttle plate and IAC valve, affecting idle. Cleaning it thoroughly is crucial.
Troubleshooting Steps:
1. Check for Vacuum Leaks: This is the easiest and often most effective first step.
2. Clean the Throttle Body and IAC Valve: Use throttle body cleaner, following the instructions carefully.
3. Inspect the TPS: A multimeter can be used to check its voltage output. Replace if faulty.
4. Check the EGR Valve (if equipped): Inspect for sticking or carbon buildup. Cleaning or replacement might be needed.
5. If still problematic: Consider professional diagnostics, as a more advanced scan tool might be needed to pinpoint ECM related issues or check sensor readings accurately.
Remember safety first. Disconnect the negative battery terminal before working on any electrical components. If you are not comfortable working on your vehicle's engine, consult a qualified mechanic.
