the cleaner shouldn't hurt plastic or anything like that. when i had the GT i never cleaned the UIM, but it couldn't hurt. more specifically, use the brake clean to clean up the lower intake, the heads, the valve covers, that kinda thing. anywhere gaskets are going you want them oil free. i would always rub brake clean over the gasket surface of the UIM, but i'm sure it wouldn't hurt to get up in there and get any dirt out.