While the fine SN-95 platform was hardly a living death for Ford's seminal pony car, the 2005 model year marked a serious rebirth for everything that made a Mustang a Mustang. While enthusiasts might view the car's history through rose-tinted V-8 lenses, Ford knew going in that most of its pony cars left the factory with six cylinders or less. While the Mustang's 210-horsepower, overhead-cam Cologne V-6 produced more power than many of its V-8 predecessors, it's also proven no less susceptible to coolant leaks and other minor failures.
Cooling System Basics
The 4.0-liter Mustang's cooling system is much like any other designed in that it has a water pump, a thermostat to control fluid flow out of the engine block, hoses to carry the coolant to the heat exchangers, and heat exchangers to remove heat from the coolant and transfer it to the air. The Mustang typically has two heat exchangers: the primary cooling radiator and the heater core inside the HVAC system. The cooling system itself operates under continuous pressure, generally around 15 pounds per square inch at operating temperature.
Typical Failure Points
Anywhere coolant system components meet is a potential failure point where leaks are concerned, but you're most likely to encounter them where system pressures are the highest. The thermostat housing probably is the single highest-pressure part of the cooling system -- aside from inside the engine block -- since that's where the hot coolant meets its greatest point of restriction. Apart from that, your most likely source of leaking is at the water pump. In this application, about half of the water pump case itself is incorporated into the timing cover; the "case" splits in half in the middle where the pump itself bolts on. This is a prime location for leaks.
The Thermostat Housing
While the thermostat housing is, by nature of being a point of restriction, more susceptible to leaks than many other parts of the system, the 4.0-liter's thermostat housing is a particular example of what not to do to prevent leaks. This engine uses a two-part thermostat housing; the bottom half of the housing sits on the engine and holds the thermostat, and the top half serves as an attachment point for the radiator hose. The Mustang also uses an external bypass hose to return coolant from the housing to the pump. Between its two-piece design and external bypass, the thermostat housing offers plenty of opportunity for leaks.
The Mustang's cooling system gets rid of excess pressure through the coolant overflow tank; high pressures resulting from overheating easily can cause the overflow tank to dribble or spew coolant into the engine bay. Obviously, a damaged radiator, heater core or hoses cause a persistent coolant leak, as does leaking O-rings around the temperature sensor and coolant crossover tube. Very few coolant leaks on the V-6 are particularly life-threatening to the engine, but one at least could indicate something very serious. Fluid leaking from the head gasket, either outside the engine or into it, indicate a head gasket in need of immediate replacement -- a problem with earlier generations of the Cologne V-6.