When your cooker lid is peeling back in petals, it is because of buildup of un-fully burned smoke compounds, including creosote. When you see the peeling, it is past time to scrape the lid clean. If you doubt this, take a bite of the material that is peeling back. Be prepared to spit it out quickly and to rinse your mouth. There are no compounds that accumulate on the lid that you want in your food.