'99 Honda CR-V EX. 120K miles. Stock radiator and thermostat. Car warms up and runs fine. But, whenever I turn on the heater, the coolant temp needle takes a dump all the way to the bottom. The car still runs fine, has good power and has been getting better gas mileage than ever. The heater air blows warm, but it seems slightly cooler than before, but maybe that's just my imagination. Obviously, waste heat is a byproduct of burning gasoline, so the engine MUST warm up. If I keep on driving, the needle usually SLOWLY rises up to near normal, but sometimes it doesn't. So, anyone know why the coolant needle would rapidly to drop to near bottom when turning the heater on with the car fully warmed up? It did this slightly before, but it's really been noticeable after the last coolant flush/fill service a few thousand miles ago. Zero other noted problems with the vehicle.