When you get a chance look at the heater hoses and see if it has that shut off valve, That would explain why no smell at certain times. Please let me know when and if you get it figured out. I still have a strong suspicion about the heater core, but I have also been wrong more than once. I sure wish you well with it, if I can help further, just yell. If I don't talk to you again before, Happy Holidays to you and yours.