In the interest of saving weight for better fuel economy, many automobile manufacturers have turned to plastics. The use of nylon and many other composites is common. The downside of this is in some applications, the plastic components don’t last as long. Jaguar 4.0L engines have a plastic coolant transfer housing that degrades over time, becoming brittle and eventually cracking. The original part is easy to identify because as it ages, the ends will become discolored and brittle. When this happens the coolant will leak or the radiator hose will come off the end of the housing. The thermostat housing on the 4.0L V8 is made out of a plastic composite and is subject to the same type failure as the coolant transfer housings. A aluminum thermostat housing that is not subject to this degradation is now available. Replacing these parts before they fail will save time, a tow bill, or even an overheated engine.