Crumbling plastics with cars of our age is something to expect, worse in the hotter climates because of the exposure to UV. The plastics in the engine bay also suffer from repeated heat cycles that degrade the plastic. Sorted a mates air leaks which revealed a cam cover was crumbling around the No4 spark plug well and letting air in which affected the fuel trims. Replacement cover sorted that ok once we found it by smoke testing the vaccum system.