Here's what you should do:
1. Inspect the belt: Check the belt for cracks, fraying, glazing, or significant wear. If it's damaged, it needs to be replaced.
2. Check the tensioner: The automatic tensioner uses a spring and a hydraulic or mechanical mechanism. Look for any obvious damage, leaks (if hydraulic), or unusual noises when the engine is running. A faulty tensioner can't provide enough tension, even if it's not obviously damaged.
3. Check the pulleys: Make sure all the pulleys the belt runs over are smooth and spin freely. A seized or damaged pulley will cause the belt to slip.
4. Check the idler pulleys: These support the belt and ensure proper alignment. A worn or seized idler pulley will result in a loose or slipping belt.
5. Consider the A/C compressor: If the A/C compressor is seizing up, it can put extra strain on the belt causing slippage.
If the belt needs replacing: This is usually the most likely solution. You'll need a new serpentine belt of the correct size (check your owner's manual or a parts store using your vehicle's VIN). Replacing a serpentine belt on this engine is relatively straightforward, but does require some mechanical aptitude and the right tools. Generally, it involves:
* Loosening the tensioner: You'll need a suitable wrench or socket (often a special tool) to depress the tensioner pulley enough to slip the belt off. The tensioner itself usually has a pivot point allowing you to push it.
* Removing the old belt: Gently guide the old belt off the pulleys.
* Installing the new belt: Follow the routing diagram printed on the belt, or found in your owner's manual, to install the new belt in the correct order over each pulley.
* Releasing the tensioner: Once the new belt is routed, release the tensioner and allow it to return to its normal position. The belt should be snug.
If the tensioner is faulty or pulleys are damaged: These components will need to be replaced. This is more involved and may require specialized tools. It is best left to a qualified mechanic in this case.
In short: Don't try to manually adjust the tension. Focus on identifying and fixing the *cause* of the loose belt. A new belt is often the solution, but underlying problems need attention to prevent recurrence. If you're not comfortable working on your vehicle yourself, take it to a mechanic.
