Hi ashmwest. Welcome to the forum. It sounds like you have a leak that is still developing. From your description, I would look closely at the water pump. A leaking hose will be more obvious and will show up during a pressure test. The front seal on the water pump can be a little trickier to spot. Often the added pressure from a pressure tester, (or from normal engine operation), will make the seal tighten up and seal better. They will commonly leak after the engine cools down and when it is running but still cold. As the seal deteriorates even more, it will leak all the time.
The good news is you see the coolant on the engine. If you had seen white smoke coming from the tail pipe, that would indicate a leaking head gasket and big bucks to repair.
If the source of the leak still can't be pinpointed, your mechanic can add a small bottle of dye to the coolant. After running the engine for a few minutes, the dye will show up as bright yellow under a black light. Follow the trail of dye to the leak.
Monday, May 10th, 2010 AT 6:30 PM