I have a 13 Silverado 4.8. When my truck idles for a few minutes either in drive or park with the heater on the temp starts to rise. Its never over heated. Once I start moving or I hit the AC button and the other fan kicks on the temp will do down. Never climbs past the half way mark any other time. It's done this since I bought it new, I don't know if this is a normal thing or if I need to get it fixed. Could it be the second fan doesn't kick on with the heater on?