How do you know when it's time to replace the Heater core? I have a '98 with about 120,000 miles on it. I'm not leaking any coolant or anything. It's always full when I check and the coolant color still looks fine. My current question, at idle or at low speeds, the air blows out fairly warm. Only at higher, above 45mph, does it blow out hot air? Any help please?