Whether the grill lid should be open or closed depends on what you're cooking. Use an open lid for direct heat grilling (searing steaks, burgers) and a closed lid for indirect heat grilling (roasting chicken, baking).
The grill lid acts as an oven when closed. When grilling with the lid open, you're primarily using direct heat from the flames to cook the food. This is ideal for foods that cook quickly and benefit from a sear, like steaks, burgers, and vegetables. The open lid allows moisture to escape, promoting browning and crisping.
When grilling with the lid closed, you're creating a convection oven effect. The heat circulates around the food, cooking it more evenly. This is best for larger cuts of meat, like roasts or whole chickens, that require longer cooking times. Closing the lid also helps to maintain a consistent temperature, which is crucial for even cooking. Think of it like baking; you wouldn't bake a cake with the oven door open.
In summary:
Open Lid: Direct heat, high temperature, quick cooking, searing. Closed Lid: Indirect heat, lower temperature, slower cooking, roasting/baking.
Avoid constantly opening and closing the grill lid, especially when cooking with indirect heat. Each time you open the lid, you lose heat, which can significantly increase cooking time and affect the final result.