Diagnosis
The stock gasket around the lid can compress, harden, or peel off over time, especially with high-heat cooks. A bad seal kills temperature stability and smoke retention.
How to Fix It
You need to replace the lid gasket. It's a standard self-adhesive high-temperature gasket material. Carefully peel off the old, crusty gasket, clean the surface with a degreaser and then rubbing alcohol, and apply the new gasket in one continuous piece, pressing firmly as you go.
If That Didn't Work
If you've replaced the gasket and still have a gap, the lid itself might have warped slightly. Check the hinge bolts to ensure they're tight and the lid is sitting square. In rare cases, the hinge assembly can be adjusted.
Heads Up
Don't use a gasket with a foam core—it'll just melt. Get a nomex or fiberglass-based high-temp gasket designed for grills. A good seal makes a huge difference in fuel efficiency and temp control.