I've got a 99 EXP 4.6 with 160k on it. This winter it won't blow heat very well. AC still works fine, blower moter is fine. I flushed the cooling system as well as flushed water through both the front and rear heater cores.

Once the engine is warm if I leave the fan on low there will be heat coming out, but the moment you turn the fan up (regardless of whether on panel or floor) the heat dies out and it blows cold. Turn the fan back down and you can feel the heat start to recover slowly.

When I feel the heater hoses, the main radiator hose is hot and one of the heater hoses is hot, the other cool. But when I unhook them and run water through, it seems to flow fine with no restriction.

The cooling system was VERY dirty when I did clean it out.

Looking to move to a newer EXP soon and want to minimize the repair bill.

Does this sound like a blend door issue, heater core, water pump?

I've also replaced the thermostat althought I didn't expect it to fix it.

 

Your heater core might have to be replaced for blockages. Even though water will still flow the blockages are whats causing the lack of heating. One thing you can check on is I know GM has a strong cleaner for heater cores, might check with a dealer and read the instructions.

 

I'd consider a weak water pump (impeller eaten away) as a strong possibility, especially if both front and rear systems are exhibiting the weak output symptom.

Steve