Mercury Repair Questions? Ask a Mechanic for Answers ASAP
A leak doesn't necessarily cause overheating, it's the being low on coolant/anti-freeze that might cause is being caused by the leak. There are a few other thing, too though that might cause overheating that has nothing to do with either a leak or low coolant/anti-freeze and that is a thermostat stuck closed, a cooling fan not working, the ignition timing being too far retarded, to name a few, so let me begin by asking what engine is in this so I can figure out what to check, but chances are it's going to start with removing the radiator cap, making sure the level is full, starting the engine and watching for when the engine gets hot if the thermostat is indeed opening, witnessed by flow through the radiator tubes nd last but certainly not least that when it does get hot before it overheats that the cooling fan or fans is electric turn on. If it's a belt driven fan with a clutch, when the engine gets hot, shut the engine off and see how many seconds the fan spins before it stops spinning. It should be just a second or 2, if any longer than the fan clutch is bad. Also, if any fan shrouds are missing, put them back on and make sure no leaves are blocking the grille and if the transmission is overheating, that would cause the radiator to also get hot and lead to the engine overheating as well.
If the fan doesn't run, first checks it's fuse and/or relay/s.