I can help with the pics for the timing belt and water pump procedure. I just replaced my uncle with timing belt, water pump, idler, cam & crank seals while you are at it. If that is what you want to tackle yourself. I believe there two cooling sensor on that GS300, one is on the engine and the other is under the radiator tank. I presume that you changed the one under the tank, right? Are you due for a timing belt change soon or later? I am not a mechanic so I can't recommend to change the cooling senor on the engine. I would go to clublexus.com and ask folks in that forum for more advise for your issue. If you know that is the water pump is the cause, then it's simple to swap it out. I can help with that and timing belt. Good luck.