i have a 1998 4.6, the problem is the tempeture guage was running all the way up to red with in a few mile ride, at first i was thinking it was overheating so i changed the thermostat which actually seemed to work. but then the gauge would start to climb again but then it would drop down. but after a little thinking i checked the degas tank and the water is not hot and the cap never opened as in an over heat condition. i removed the upper rad hose and water was moving so i no the water pump is working. i am thinking its the sending unit, the auto parts store list them as sensor's. there are two on the intake i believe its the one with red/y and y/r wire. i plan on changing the coolant and checking the radiator to make sure it's not cloged to bad. so im wondering if thats the right sensor and any input if iam going in the right direction.

 

First off I'd start with a rad flush. It sounds like something is blocking the water flow.

 

problem solved, the water pump impeller had completly broken free from the shaft

 

Was the water pump an aftermarket or a factory unit??

 

im not sure, but the truck has 185000 miles and i have not owned it from new i would say its been changed and the only reason i say that is becuase the fan clutch nut came off with out a fight and when i checked inside the threads it looked like someone had put a little oil on them, on the flip side to that the bolts holding the pulley on did'nt look like they had ever been touched, also the radiater was full of rust due to the coolant being so old,so i replaced it with a new one. aftermarket pumps should be fine as long as you use a new one and not a reman.