To address a high idle issue (too high or not engaging when it should), you'll need to troubleshoot several components:
* Fast Idle Valve/Thermostatically Controlled Air Valve (TCAV): This is the most likely culprit. It's a small valve that opens when the engine is cold, allowing extra air to bypass the throttle, raising the idle speed. If it's sticking open, the idle will remain high even when the engine is warm. You'll need to check its operation—it should close as the engine warms up. Cleaning it or replacing it may be necessary.
* Throttle Position Sensor (TPS) (if fuel-injected): A faulty TPS can give incorrect information to the ECU, leading to incorrect idle speeds. This is especially relevant if your high idle is incorrect, too low or too high.
* Idle Air Control (IAC) Valve (if fuel-injected): Similar to the fast idle valve, but for controlling idle speed under normal operating temperatures. A dirty or faulty IAC valve can cause high idle.
* Vacuum Leaks: A vacuum leak anywhere in the intake system can disrupt the proper operation of the fast idle valve or the IAC valve (if applicable), resulting in a high idle. Check all vacuum lines and connections for cracks or leaks.
* Throttle Body (if fuel-injected or carbureted): Dirt and build-up inside the throttle body can affect idle speed. Cleaning it thoroughly is often helpful. If carbureted, the carburetor itself may need attention (cleaning, rebuilding).
* ECU (if fuel-injected): In rare cases, a faulty ECU could be the cause. This is less likely than the other issues.
How to approach troubleshooting:
1. Check for obvious vacuum leaks. Listen carefully for hissing sounds near the intake manifold. Use a vacuum gauge if you have one.
2. Inspect and clean the fast idle valve/TCAV. If you have access to a wiring diagram, disconnect it to see if the idle drops.
3. Check the throttle body for cleanliness. Clean it with throttle body cleaner.
4. If fuel-injected, check the IAC and TPS. These often require a multimeter and diagnostic tools to test properly.
5. If problems persist, consult a repair manual specific to your 1987 Mazda B2600's engine. A manual will provide detailed diagrams and instructions for testing and adjusting these components.
Without knowing the specifics of your engine (e.g., whether it's carbureted or fuel-injected), it's impossible to give more precise instructions. A repair manual is crucial for accurate diagnosis and repair.
