What is the function of the PCV valve in an engine, and what happens if it becomes blocked?

1. It filters the fuel; a blockage stops the engine completely
2. It vents crankcase pressure; a blockage builds pressure that can blow out oil seals and cause engine leaks
3. It regulates engine idle speed; a blockage causes the engine to stall at red lights
4. It cools the cylinder block; a blockage causes immediate engine overheating