I have a coolant leak

The first thing you need to check is for visible leaks, once you turn your vehicle on, there have to be some kind of leaks, pay attention since if the leak drop on top of the manifold, it will evaporate and you won't notice it.
Another thing to look for is excessive heat, with the vehicle on, put yourself in front of the vehicle, if you feel an excessive amount of heat, then probably the head gasket needs replacing, and it evaporate the coolant.
So as you can see, if there are no visible leaks, it is probably a problem from the inside of the engine.

Intake manifold gasket probably leaking