The heater in my car works fine one day then blows cold air the next for parts of the day then works fine for a week?Have replaced thermostat and everything else checks out.

Might help but probably not. In my considerable experience, when scale and sludge blocks a heater only high pressure flushing has a chance of shifting it and a blocked heater is blocked - not blocked one day and ok the next.

More likely an airlock is preventing circulation, maybe the degassing system is blocked...
Try checking both heater pipes are hot when the heater is blowing cold - if they are the trouble could be with the climate control or a stuck air mixing flap or actuator unit that works sometimes...

heater blows cold air idle chrysler 300 Questions & Answers ...

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Find solutions to your heater blows cold air idle chrysler 300 question. ... blow warmair for a few mins then back to luke warm. i replaced thermostat ... HEAT BLOWSCOLD AIR WHEN IDLING ONLY GETS HOT WHEN START DRIVING ... About 15 minutes after driving the diver side vents start blowing straight cold air but the ...

Heater only works when pressing the gas? - DoItYourself.com ...

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Nov 3, 2011 - When I press the gas, the air from the vents is nice and hot. .... So am Idriving a car with a ton of problems, or could the power steering ... Just anotherquestion, when warmed up does the engine temperature ... When the heater is on and I'm not pressing the gas pedal, luke warm air blows from the vents.

Heater blows cool air when stopped and warm air while ...

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RedditFeb 12, 2014 - Heater blows cool air when stopped and warm air while driving 2008 Kia Spectra (self. ... just fine but appears to blow cool air when I was stopped at a stoplight. ... My temp gauge would also be pegged high when at idle / low RPM. ...thermostat only blocks coolant going to the radiator, and the heater core, ...Missing: 300m ‎luke

Why is My Car AC Blowing Hot Air? - BlueDevil Products

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What is happening here is that the faster the heater blows air through the fan, the more it cools the liquid in the heater core, and so heat output is reduced. The root of the problem is either the engine is not warming up enough, or the heater core, and/or heater core lines, are plugged up, not allowing the water to circulate properly in the heater core. Here is what I would try.

1. Top off the radiator and overflow.
2. Is the engine warming up properly? Is your temperature gauge showing normal temperatures? If it is not, then replace the thermostat.
3. If steps 1 and 2 do not solve the problem, get the vehicle in a shop, locate the heater core lines, take them apart and flush out the heater core. Look for blockages in the fittings. Do a flow test on the core. It may be clogged and need to be replaced.

The control valve or the cables out of adjustment or thermostat is stuck open.

If the car overheats also, replace thermostat.Could be heater core needs to be replaced, Any water on passenger side floor?, or smell of coolant?or are windows misty on inside of car?when you slide control from cool to heat it is supposed to close a door that lets in cold air, make sure thats working, usually you can hear it shut. lastly, check water pump. You do have enough coolant in system right?

Could be the thermostat

Sounds like the water pump is cavitating . Replace water pump. Check coolant level also. Open the bleeder valve usually located on thermostat housing to remove air from system. close valve when coolant is present while filling. close radiator cap and top off recovery bottle to correct level. These engines are prone to overheating, do not let temp gauge get in the red.

You need to get the engine full of coolant. Air is trapped in system. Use bleed valve

the thermostat is either stuck open or the heater core is clogged

squeeze the hoses to get any air bubbles out and refill coolant as necesary. are you sure you put thermostat in the right way? may be backwards. also may be your sensor.