Plastic valve covers are just a bad idea. They’re known to crack, but they also deform due to engine heat cycles and from people torqueing the mounting bolts too much. Though ours wasn’t cracked, you can see there was a steady stream of oil leaking out from under the valve cover and on to the rest of the engine. The lack of power is hard enough to deal with, but the constant puddles of oil only added insult to injury. Fortunately, later four-cylinder valve covers swap directly in place of the offending plastic units.