02' Sunfire high rpm's in neutral??

[chickenstrips]

Ok i have an 02 sunfire gt manual trans.
Has 112000 original km but had engine replaced that has only 89000 km on it.
It was working great for two weeks but all of a sudden everytime i pushed the clutch into neutral the engine would rev up to 6000rpm and then it was leaking rad fluid from the back of the engine. i think the rad hose ended up blowing on it or the water pump went, still haven't found that out yet.

But i'm wondering what would cause the engine to rev so high in neutral?

If anyone could help me out there it would be greatly appreciated!!

[chickenstrips]

O and its the 2.4L twin cam engine if that makes a difference.