Need help with heating issues

[chersman]

Gents,

I've got a head scratcher. My 2000 Suburban 2500 is displaying some strange heater/cooling system issues. Last week my heater started going cold while at idle or lower rpm. As i get past 40, the heat instantly gets hotter and then back to lukewarm at stoplights and such.

 

I haven't seen any of the typical water pump indicators. There's no antifreeze smell or puddle on the floor and the truck still maintains perfect temp while driving.

I threw in a new thermostat and topped off the dexcool. The radiator and coolant are only about 6 months old. Aside from the loss of coolant from swapping the t-stat, the system was full.

 

To add to the head scratching, while I was at a stoplight this morning and my front heat started to go cold, I felt the rear vents and they were as hot as could be. So, all signs are pointing to good flow from the water pump.

 

Since the condition appears to be rpm dependent, I'm thinking water pump. But without any of the typical indicators, it doesn't seem to be the culprit. And since I do get hot air on occasion, the air blender door doesn't make sense, as I've got good modulation from hot to cold. A broken heater core doesn't fit either, as that would have been an easy one to diagnose.

 

I'm thinking maybe a partially blocked heater core. To me it makes the most sense, as I would assume the faster the water pump spins, the more it's able to overcome whatever blockage is present. Or am I just thinking about it too much and I should just throw on another water pump and see what happens?

 

 

[txab]

Flush the heater core for starters