coolant leaking when engine is running

This sounds like your water pump is leaking and IT IS SERIOUS, because you can run out of coolant and RUIN your engine in a heart beat. It runs from about $260-450 depending on who does the job, i.e Dealer or small shop with non-dealer parts. you can find it in our estimator. https://repairpal.com/estimator/mercury/mountaineer

If the coolant is comming from behind a pulley it's almost for sure your water pump and usally the pump it self isn't to pricey it's the labor that can be a bit of money as it does usally take for alot of disassembly and after wards reassembly.