IMO, the biggest risk you face is a failed water pump. For most engines this would be a minor issue but for the 3.5L V6s installed in all Ford/Lincoln/Mercury vehicles, aside from the F150 and Mustang, it can lead to engine failure. The water pump is internal on these engines so when they leak coolant it is dumped into the crank case and this usually destroys the engine. Sometime they fail slowly and there is a small weep hole what drains coolant. Checking for leaks there is not easy. Instead I suggest you mark the coolant level on your reservoir when the car is warmed up. Then check the level at least once a week, or more. Any, and I mean any, drop in the coolant level requires you to stop driving the vehicle (with the possible exception to a shop) and check the oil for coolant or the weep hole for signs of leaking. A water pump replacement is a $1.5k-$2k job and most people change out the timing chains and guides while the car is apart for another $500-$700. An engine replacement is around $5k-$7k so catching the leak early is critical and you do this by diligently checking the coolant level. Even then, some pumps give no warning and just die catastrophically. Other than the water pump design flaw, the 3.5L engine is very robust and durable. If the owner really wants to hedge his bets against a water pump failure then replacing the pump as a preventative measure is an option.
www.explorerforum.com
