* Coolant Temperature Sensor (CTS): This is the most likely culprit. A failing CTS provides inaccurate temperature readings to the Engine Control Unit (ECU). When cold, the reading might still be within a range allowing for a decent cold idle. However, as the engine warms and the sensor malfunctions, the ECU receives incorrect information, leading to a poor fuel mixture and rough idle.
* Throttle Position Sensor (TPS): While less likely to be the sole cause of this specific issue (as TPS problems often affect idle at all temperatures), a failing TPS can cause erratic behavior. The change in behavior *only* at operating temperature suggests another culprit is more likely, but a faulty TPS could contribute.
* Intake Air Temperature (IAT) Sensor: Similar to the CTS, an inaccurate IAT sensor reading can throw off the fuel mixture. Again, this is less likely to be the *sole* cause of a problem *only* at operating temp, but could be a contributing factor.
* Vacuum Leaks: A vacuum leak can cause idling problems, but it's more likely to affect the idle at all temperatures. However, a leak might worsen as the engine components expand with heat, making it more noticeable when hot. Check all vacuum hoses for cracks or looseness.
* EGR Valve: The Exhaust Gas Recirculation (EGR) valve helps control emissions and can affect idle. A sticking or malfunctioning EGR valve could cause rough idling, especially when hot.
* Mass Air Flow Sensor (MAF) Sensor (If equipped): While less likely on an '89 (they might have used a simpler system), a malfunctioning MAF sensor will drastically impact the air/fuel mixture, possibly only showing symptoms at operating temperature.
* Ignition System Components: Worn distributor cap, rotor, spark plug wires, or failing ignition coil could cause misfires, worsened by the heat. While misfires could happen cold, the heat often exacerbates the problem.
Troubleshooting Steps:
1. Check the Coolant Temperature Sensor (CTS): This should be your first priority. A simple test involves checking the resistance of the sensor with a multimeter at different temperatures (you'll need a reference chart for your specific sensor). Replacement is relatively inexpensive and easy.
2. Inspect Vacuum Lines: Carefully examine all vacuum lines for cracks, leaks, or loose connections.
3. Check for Error Codes (if equipped): Your '89 S10 might have a rudimentary OBD system; checking for any stored diagnostic trouble codes can give clues. This requires a code reader.
4. Check the other sensors (TPS, IAT): If the CTS is okay, test the other sensors using a multimeter and following manufacturer's specifications.
5. Inspect ignition system components: Look for wear and tear on the distributor cap, rotor, spark plug wires, and plugs themselves. Replace as needed.
6. Consider professional help: If you're not comfortable performing these checks yourself, take it to a mechanic specializing in older vehicles. They have experience diagnosing these types of problems.
Remember to always disconnect the negative battery cable before working on any electrical components. Good luck!
