1. Identify Fuel System:
* Carbureted: Look for a carburetor on top of the engine. It'll be a fairly large metal casting with various linkages and a throttle plate.
* Fuel Injected: If you don't see a carburetor, it's fuel-injected. This is less likely on an '89 but possible.
2. Adjusting Idle (Carbureted):
This is more involved and requires more hands-on mechanical ability. Proceed with caution.
* Locate the Idle Speed Screw: On the carburetor, you'll find a screw usually on the side or front. It's often a small screw with a spring on it. This is your idle speed adjustment screw.
* Warm-up the Engine: Let the engine run until it reaches normal operating temperature.
* Locate the Fast Idle Cam: There's a mechanism (often a cam or lever) that increases idle speed when the engine is cold. Make sure it's fully retracted – the engine should be at its normal operating idle speed.
* Adjust the Idle Speed Screw: Very slightly turn the idle speed screw. A small turn makes a significant difference. Increase the idle speed by slowly turning the screw clockwise. Check the RPMs with a tachometer (highly recommended) or by ear. The factory spec is usually around 700-800 RPM but consult your owner's manual for the exact specification.
* Fine-tune: Make small adjustments, checking the RPM after each turn. Avoid making large adjustments at once. Aim for a smooth, consistent idle.
* Idle Mixture Screw (Optional, Advanced): You may also find an idle mixture screw (often marked with an "A" or "M") on your carburetor. Adjusting this will fine-tune the air/fuel mixture at idle. This is much more complex and should only be done if you have experience and a vacuum gauge. Incorrect adjustments here will negatively impact engine performance, fuel economy and emissions.
3. Adjusting Idle (Fuel Injected):
Fuel-injected systems are generally controlled by the computer (ECM). Adjusting the idle manually is typically not possible or recommended without specialized diagnostic tools. If your idle is incorrect, the problem likely lies with:
* Throttle Position Sensor (TPS): A faulty TPS can cause erratic idle.
* Idle Air Control (IAC) Valve: This valve controls the amount of air entering the engine at idle. A dirty or malfunctioning IAC valve is a common cause of idle problems.
* Mass Airflow Sensor (MAF): A faulty MAF sensor will also cause incorrect idle speeds.
* ECM Issues: Problems with the engine control module itself can affect idle.
For Fuel Injection: Troubleshooting usually involves:
* Diagnostics: Using an OBD-II scanner (or a specialized tool for older systems) to read trouble codes and check sensor readings.
* Cleaning: Cleaning the IAC valve and MAF sensor.
* Professional Help: Fuel injection systems are complex. If you're uncomfortable troubleshooting, take it to a mechanic.
Important Notes:
* Consult your owner's manual: The manual will have specific idle speed specifications for your engine.
* Safety First: Always disconnect the negative battery terminal before working on your vehicle's electrical system.
* Professional Help: If you're unsure about any part of this process, it's best to take your Comanche to a qualified mechanic. Incorrect adjustments can damage your engine.
Remember that a rough idle might also be caused by other problems like vacuum leaks, ignition issues, or other sensor problems. Addressing the underlying cause is vital for long-term reliability.
