My 92 SC400 seems to retain coolent in the resivior pretty good, but there is a small leak under the car which i cant find, it doesnt seem to be the resivior. When I drive the car it stays cold for about 5 minutes then begins to heat up and after 15/20 min. its at the red line. I'm guessing its the water pump and I wanna replace it but i dont have any instructions on how to do it, so if anyone can help me ill appriciate it, thanks.