Got the truck in October and heat has worked fine in the past. The other morning it was 27 degrees out and heat hasent worked since. I checked coolant and it was low so I topped it off. Radiator hoses hot, heater core hoses are cold, both of them. Truck does not run hot at all but I have always heard water running through the core before, and in my old GMC, and it no longer does that. When I turn it from hot to cold I can feel a difference so I don' believe it' the blend door. I' leaning towards a clogged core, bad water pump or thermostat or all of the above. BTW, 4.6 v8 295k miles. One thing that was weird the other night was after I topped off the coolant the temp gauge wouldnt read. As it was running I slowly cracked the overflow cap and got a lot of pressure and then the guage started working again. Any suggestions before I start throwing parts at it?

 

You might have a blockage in the system.

I would flush it and then refill it.

Prestone has a flush and fill kit that you attach to a garden hose.

 

Coolant has to be going somewhere - you may have a blown head gasket.

 

I thought about that. Oil is clean, no water. Doesn't use that much coolant but I have noticed a small puddle under the truck right about where the heater hoses connect to the core. Wondering if it' leak8ng from there because it isn' much.

 

Possible. Wouldn't hurt to do a hydrocarbon test on the coolant to make sure.