To locate it definitively:
1. Consult your owner's manual: This is the best resource for the precise location.
2. Look at a diagram: Use an online parts diagram for your specific 2001 4Runner engine (specify the engine size when searching). These diagrams show the location of all components.
3. Visual inspection: With the engine cool, carefully open the hood and examine the valve cover. Look for a small, usually black plastic valve with a hose connected to it, often near the center or towards the back of the valve cover.
Remember to always work on a cool engine to avoid burns. If you're unsure, it's best to take your vehicle to a qualified mechanic for inspection and replacement.