Heater blowing cold air
 

I own a 2006 F350 6.0 Powerstroke, the heater will blow hot air for 2 minutes then go to cold, turn off for a minute and will blow hot again then go cold. Probably coincidence but the truck started blowing out more than usual black smoke at acceleration and bluish smoke when decelerating. Any ideas?

Truck has 4 inch exhaust, no cat, airraid cold air filter and Hypertech stage III. It has 117k miles, I bought new. Before it turned 100k had the EGR valve replaced twice.

Thanks,

Rocky

i have a 97 silverado, my heater blows cold air, a/c works fine and air flow is also ok, it just doesnt blow hot air when i switched to heat