Back in the 90s I read an article in a magazine . . . seems they were testing plastic engine oil pans
Plastic timing case covers and plastic valve covers seem to already be pretty common
I just remembered something from pretty far back . . . I remember on some cars, when you wanted to replace a leaking headgasket, and the car had a plastic intake manifold, if you weren’t extremely careful when removing hoses from that manifold, it would break
The plastic would become somewhat brittle. The best way to prevent this was to very carefully cut off the hose, versus trying to remove and reuse it. Using those hose pick tools sometimes worked, but even if you worked that hose pick all the way around and the hose spun freely, when it came time to pull it off, sometimes the plastic hose pipe would break off of the manifold, as well
I’ve seen many guys over the years light a cigarette, inhale a lung full and then blow smoke into vacuum hoses and pipes, in hopes of finding a leaking vacuum door lock actuator, leaking hvac vacuum diaphragm, leaking intake gasket, etc. It often worked, but they were usually tired by the time they found the leak.
Back in the days when car electronics were still rather primitive, and scan tools weren’t so good, either, I saw guys spray carb cleaner at suspected vacuum leak sites, hoping the idle would change. It sometimes worked. But the results were also subject to interpretation. One guy would swear on his mother’s life that the idle went up 50rpm, while the other guy would swear that the idle didn’t change at all