Why won't my engine go over 2500RPM, except in park?And why is it overheating?

You need to get the air out firstly or it will continue to overheat regardless of if the system is usable or not... Fix the leak/pump, why keep a broken part on the car? And then post back if you still have an issue....

Replacing the pump should fix the leak, is it a 6 or 8 cylinder? If it's an 8 get it fixed quick before it kills the opti

the reason it probably wont go over 2500 its basicly in limp in mode. possibly from the overheating. it limits how fast the car will go to keep from further damage.

Have you checked your oil lately? How many miles are on it? Are you seeing pressure or bubbles in the overflow reservoir? It's also possible that you have a blown head gasket. If you blew a head gasket between cylinders, you will overheat and pressurize the cooling system. The motor will heat up quickly since you're pressurizing the cooling system with exhaust heat. Does the engine shake/sputter when accelerating? If yes, head gasket. Once the car warms up, is the coolant in the overflow reservoir bubbling? If yes, head gasket. You will usually see white exhaust if you blew a gasket between the cylinder/coolant passages. Does your oil look like coffee with cream? If yes, Head gasket ...You may not have any smoke with this form of a blown head gasket, but coolant will seep into the oil passages creating a thick milky sludge. This is the worst form of a blown head gasket. The motor will need to come out for a hot tank and you will have to polish the crank and thoroughly clean all lubricated parts. If no to the above, check the thermostat as it may be frozen. You can cold start the car with NO coolant and it should still pass 2500 RPMs. Some of these issues have an adverse effect. If you overheated the motor due to a blown head gasket, the excessive heat could have finished off the water pump gasket and it through you off the path of investigation.