If it has a carburetor:
The idle speed is usually adjusted using a screw on the carburetor itself. This screw is often labeled "idle speed" or "idle adjustment." Be cautious! Incorrect adjustment can lead to poor running or damage.
1. Locate the idle speed screw: It's usually on the side of the carburetor. It's often a small screw with a slot for a screwdriver.
2. Warm up the engine: Let the engine run until it reaches normal operating temperature.
3. Locate the fast idle cam: This is a mechanism on the carburetor that increases the idle speed when the engine is cold. Make sure it's fully retracted (not engaging).
4. Adjust the idle speed screw: Turn the screw slightly (1/8th of a turn at a time) to increase or decrease the idle speed. Consult your owner's manual for the specified idle RPM (revolutions per minute) for your engine. A typical idle speed is between 650 and 850 RPM.
5. Check for smooth running: Listen for any rough running, stumbling, or hesitation. If you hear these, back off the adjustment slightly.
6. Re-check the idle speed: Use a tachometer to accurately measure the idle speed RPM. Adjust the screw as needed until the idle is at the correct RPM and the engine runs smoothly.
If it has electronic fuel injection (EFI):
Adjusting the idle on an EFI system is usually not done manually with a screw. The idle speed is controlled by the Engine Control Module (ECM) and is usually self-adjusting. You generally should not attempt to adjust it yourself unless you have a diagnostic scanner and experience working with EFI systems. Incorrect adjustments can potentially damage the ECM or other engine components.
Instead of adjusting the idle speed directly, if you're having idle problems with an EFI system, focus on these:
* Check for vacuum leaks: These can significantly affect idle. Inspect all vacuum hoses for cracks, holes, or loose connections.
* Clean the throttle body: A dirty throttle body can interfere with proper idle operation. Use throttle body cleaner and follow the instructions carefully.
* Check the air filter: A clogged air filter can restrict airflow and cause idling issues. Replace it if necessary.
* Inspect the IAC (Idle Air Control) valve: This valve regulates airflow at idle. A faulty IAC valve will likely require replacement. This is often a dealer or specialist level repair.
* Get a diagnostic check: Use an OBD-I scanner (for your year Bronco) to retrieve any diagnostic trouble codes (DTCs) that the ECM may have stored. This can help pinpoint the problem.
Before attempting any adjustment, always consult your owner's manual. It will provide the correct idle RPM and may offer troubleshooting advice specific to your Bronco II. If you're uncomfortable working on your engine, it's best to take it to a qualified mechanic.
