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I have 2006 impala that randomly will blow cold air at idle. Sometimes if I stop at redlight the heater will blow cold air. If I get the engine above idle by reving the engine to about 1K rpm it starts to blow warm again. The temp gauge remains at the normal spot on the gauge. The radiator and expansion tank are at the correct levels.

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I had this problem with my 2007 Impala. The dealership service dept. told me I needed a new head gasket ($1800). I took it to a private mechanic who fixed it for $30 by bleeding the cooling system and filling the recovery bottle. I think that if I check the coolant levels more frequently and keep that overflow jug full, (like the guy above said) I can even save the $30 charge next time. – Mike K Jan 14 at 14:52

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Possible stuck thermostat. The "normal" indication on factory temperature gauges cover a huge range. My Eclipse has a factory gauge and an aftermarket gauge. The factory gauge settles in at "normal" for a 160-200 degree range (normal temp is 185). At 160 the car barely makes any heat in the cabin, at 185 it's got decent heat, and at 200 it can light your feet on fire. :-)

Could also be a failing water pump, but that mode of failure is not one I've heard of very often.

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The water pump crossed my mind also. I have ruled out the thermostat (in my head anyway) because the gauge never waivers once the engine is warmed up and the heat returns almost instantly once the rpms increase. – mikes Nov 27 '12 at 21:46
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It turns out the problem is pretty common with 3.5L Impalas. For some reason these engines loose coolant. Some blame is placed on headgaskets, some blame Dex-Cool. What ever the cause, refilling via the overflow jug can leave an air pocket in the cooling system. The cure is to fill via the pressure cap right to the top. Recheck the level after several heat/cool cycles. Then never let the overflow jug get empty.

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