Is there a way to tell if you have a waterpump problem. My XJS is still overheating after changing the 2 themostats and the water temperature sending unit. I am pulling my hair out. I don't know if its the rad or water pump. I did flush the rad but it still runs really hot HELP
It is unlikely to be the water pump if the car is overheating. The usual symptom for a bad water pump is a coolant drip out the water pump shaft and/or a noisy water pump bearing. I have not heard of the impeller ever breaking apart, or corroding away sufficiently to cause the engine to overheat through lack of coolant flow.
The most likely cause is a radiator that has silted up, or leaves blocking the radiator between the oil cooler and radiator. The silt found in the radiator can also be found in the engine itself, which can cause over heating issues if it is bad enough.
Another couple things to check before taking the rad out, are to pressure check the cooling system, AND the two rad caps. This was a problem with our VDP. Rad was good, rad cap was bad, and thus the system never pressurized.
Finally, check the mechanical advance on the distributor. If it is seized, as they tend to on the V12's, then the engine runs with the spark fully retarded, less power, and hot.