I have a 1987 300E. When I am driving the engine temperature is normal. But if I sit in traffic or let it idle the engine temp starts to climb. The auxilary fan does not turn on until the temp is almost in the red. Is this normal or should the auxilary fan kick on before it gets that hot? The radiator has been replaced in the last year. Could this be a bad thermastat or is there something else. Any imput would be appreceated.