My corsica has been running great for a while but i noticed the other day it was smoking under the hood, while not being overheated. I poped my hood and noticed a large amount of oil had spewed from the under the dipstick cap all over the engine. What can be the cause?

The car is missing at low RPMs, and has smoke coming out from the dipstick tube if i take the dipstick out, once i get going in the car, around 20mph the car runs fine.

What should i check first?

 

I'd suggest changing the PCV valve

 

i checked the pcv valve, it's still rattling, that means its still good right? should i change it anyways?

 

What you describe is high crankcase pressure. I'd suggest make sure the pcv valve is working. With engine running, pull the valve with the hose hooked to it, place your thumb on the engine side, does it stop?

 

Something might be wrong with one of the pistons/rings...
or a bad head gasket... or bad heads...