When I was a teen, I rebuilt the engine on my first car. When done, there was a spring left over. I had no idea what it was for, the engine ran fine.

So I drove it around. The oil pressure was very low, but I figured it was just a broken gauge.

Then, the engine got way, way too hot.

It turns out, the spring was for the pressure regulator from the oil pump. The oil was pumped out of the pan right back into the pan. The engine needed to be rebuilt again.