I'm really hoping someone can throw a few ideas my way on this one.

The truck is a GMC Yukon XL 2500 with 6.0L engine. When idling it will not blow heat out of the front or rear. Even if I let the engine warm up to normal operating temps, it just blows cold.

When driving, it will eventually start to blow warm air, but takes quite a while (10 min of driving). This morning, when it started blowing out warm air, it was also blowing considerably warmer in the front, and cooler in the rear.

If I let it idle, it starts blowing cold again.

Water pump? or are there other things that could cause this?

Also, the engine is essentially stock.

 

Check the coolant level. If its just a little low, it may be letting an air pocket in the heater core.

 

Yes the coolant level doesn't have to be very low for no heat. Check water pump and pump gaskets if coolant is missing.

 

Also check to see if your functions are cycling properly at the control head. Mine has done this several times, and I have corrected it by unplugging the control head for a minute and resetting it.

 

good ideas i will check when I get home. thanks!