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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. –  user2610 Jan 14 '13 at 14:52
    
Do not think its the water pump..had mine replaced also the thermostat...problem still there. Did not notice till now, with the cold weather. Will try the top up of coolant if needed. –  giomarch Dec 3 '13 at 21:00
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3 Answers

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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up vote 3 down vote accepted

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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I have this same problem, and I can tell you exactly what it is. GM vehicles use the coolant DEX-COOL, which is the purple color coolant. In the impala's if the coolant hasn't been changed for a while the dex-cool will leave deposits, or get thick is easier way to think about it. When it runs through your heater core, that silt like deposit will clog the heater core. If it acts like mine did, when the car is idling it will not produce any heat but when you drive there is no problem with the heat. This is because the higher RPM's will allow for the fluid to move through the heater core and produce heat. To fix this you need to flush the heater core backwards and forwards to unclog the heater core. There are two hoses on the right side of the engine, which connect from the top right under the plastic cover to the fire wall. The hose on the left is the inlet and the hose on the right is the OUTLET. Disconnect the hoses from the engine and use a garden hose with a spray tip on it. Put the tip in the right hose and spray water through it. You will see a thick dark purple fluid come out of the other hose. When you see the water come through clear, then switch to the left hose and repeat. do this several times, and make sure your letting the water run through it for at least 30sec to 1min. reconnect the hoses, and replace the lost coolant then bleed the air out of the system. ON THESE CARS IT IS VERY EASY TO LEAVE AIR IN THE SYSTEM BECAUSE WHERE THE HOSES ATTACH TO THE ENGIN IS THE HIGHEST POINT OF THE COOLING SYSTEM AND NOT AT THE CAP. You may have to do this several times, and if it continues, then there has probably been too much damage to the heater core and will need to be replaced.

ALSO, if the coolant is bad enough the whole engine will need to be flushed, and if you don't have the knowhow to do that, I would recommend to take it to a mechanic to have this completed as all components of the cooling system including the reservoir need to be cleaned, flushed and refilled.

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