Just because you have a 3D printer doesn't mean you should start 3D printing plastic replacement parts for everything. Especially for your car.
There's a purpose for 3D printing and using those parts on your car: test fitting parts you'd eventually buy (like brake calipers), or designing parts you'd eventually custom make, or replacing other plastic components that would otherwise cost a ton of money just because they stopped making them for your car. However, replacing your engine's cylinder heads should be the last thing you do with your car that involves a 3D printer.
To showcase just how terrible of an idea using plastic parts for critical engine components, the Project Farm YouTube channel just tried this asinine idea on their lawn mower. They've replaced the cylinder head with two different types of plastics you'd normally find for 3D printers: PLA (Polylactic Acid) and ABS plastic.
Not only did it cause a lot of smoke (most likely toxic) but the heat from the combustion caused the plastic cylinder head to slowly melt away. However, that's not before the engine failed to run, both cases due to spark plug failing, one from fouling and the other failing at the threads.
So the next time you're tempted to 3D print parts to replace engine or structural components, please film it, so we can watch the destruction.