We got our first taste of fall weather in Houston this weekend, and with it I've discovered a new problem with my truck. After the engine was up to temp, I noticed that the heater would blow cold air any time I sat at idle. If you rev up the rpm's to 1500-2000, you can feel the air start to get warm again. I'm guessing this means my heater core needs to be backflushed, but wanted to see if anyone on here has other ideas.

 

Check coolant level, operation of thermostat and water pump. Compare inlet and outlet temps of core to see what's going on.

Also check temperature blend door for proper operation

 

Blend door works great - changing setting on temperature affects the appropriate response. This seems to be a water-side issue... will check all of the above. Thanks.

 

Well, I'm embarrassed to admit it - the coolant tank didn't even have a level in it. I added what seems like a couple gallons of water, now the heater is blowing hot even at idle. To be honest, I cannot remember ever taking the cap off that tank, and I've had the truck 9 years. Obviously, I need to be paying better attention to the details. Will monitor it to see if I'm losing water or that was just depletion of the factory fill...

 

Well, as it turns out this was my early-warning sign that I had a head gasket leak. I've posted that saga in a couple other threads, but the loss of water was into a couple of cylinders when the engine cooled off. On cold start, I was getting a misfire until all the water was depleted, after which time it ran fine. Slowly over several weeks, it lost enough coolant that the heater core didn't get its water.

If anyone comes across this thread, just learn from my mistake - water doesn't normally get depleted from the system unless something is broken.