I have a 2006 Grand Prix that I've been having problems with the heat on for the entire winter. Back in early December, I discovered that I was only getting barely warm air from the heater. It was about this time that I noticed the engine temperature not getting up to where it needed to be. I replaced the thermostat. This took care of the low engine temperature, but the heater problem persisted.
The next thing I noticed was that the hose going into the heater core was hot, but the one coming out was only warm. I knew I needed a coolant change anyway, so I had a mechanic change the coolant and back-flush the heater core. After this procedure, my heat was excellent...for about two weeks. After this time, it was back to only slightly warm air coming from the heater. Another thing I noticed after the change was that I couldn't keep coolant in it. This was a problem I had a couple of summers ago. It had stopped leaking, and hadn't given me any problems since. I'm guessing the combination of a flush and new coolant reopened the leak. After searching for a leak, I finally found that one of the coolant elbows was cracked. I replaced both elbows with aluminum replacements. No leaks, I haven't had to add coolant since, but I was still just getting slightly warm air coming from the heater.
Fast forward about a month, and I am down to NO heat. It's much warmer to turn it off than to leave it blowing on heat. I've tried the knob, and you can hear the blend door opening and closing. You also can feel the difference between freezing cold air, and just cold air as you switch between cool and hot. The air coming out when it is set to hot is not remotely warm. It's not even room temperature. It's cold. What I've discovered is now, both the hoses going into the heater core and coming out of the heater core are barely warm. They feel like the only warmth on them is due to the engine heat. They aren't remotely as warm as they are supposed to be.
Along with that, I'm also seeing swings on my temperature gauge. It will swing from the "quarter" mark up to halfway, where it normally would run, and back down to the quarter mark. I say it like that because I'm not certain what temperature the marks represent. Normally, shouldn't it warm up to operating temperature, the thermostat open, and just hold that temperature?
Does anyone have a suggestion of what may be going on? I'm about out of ideas.