How do you adjust the idle on a 1999 Ford F150?

You can't directly adjust the idle on a 1999 Ford F-150 like you could on older carbureted vehicles. The idle speed is controlled by the Powertrain Control Module (PCM), a computer. The PCM uses various sensors (like the throttle position sensor, air flow sensor, and coolant temperature sensor) to determine the appropriate idle speed. Attempting to manually adjust anything will likely damage the system or cause erratic operation.

If your 1999 F-150 has an idle problem, the issue is almost certainly *not* a simple adjustment. Instead, you should:

1. Check for trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the PCM. These codes can pinpoint the source of the problem, such as a faulty sensor or other malfunction.

2. Address any codes found: Based on the trouble codes, you can diagnose and fix the underlying problem. This might involve replacing a faulty sensor (like the IAC valve - Idle Air Control Valve), cleaning the mass airflow sensor (MAF), or addressing a vacuum leak.

3. Inspect the throttle body: A dirty throttle body can affect idle. Cleaning it with a throttle body cleaner is often a simple and effective solution. Be extremely careful when working around the throttle body.

4. Check for vacuum leaks: Vacuum leaks can drastically affect idle. Inspect all vacuum hoses and connections for cracks, loose fittings, or damage.

5. Consider a professional mechanic: If you're not comfortable diagnosing and repairing automotive electrical and engine systems, take your truck to a qualified mechanic. Incorrectly diagnosing or repairing the idle system could cause further problems.

In short: Don't try to manually adjust the idle. Diagnosing the underlying cause is the only effective way to fix an idle issue on a 1999 Ford F-150.