Question about 1999 Cadillac DeVille

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Can a bad thermostat cause heater to blow cold?

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  • torossi Nov 08, 2010

    thank you, it's a 99 deville concours. and yes the temp gauge reads normal, and even a little hot sometimes, but goes back to normal. mi had the water pump replaced in april and never had a reason to use the heater until now. also, i dont get heat in the back seat vents when i do get heat in the front. i only get heat when the car is moving, but not when stopped and idling. wonder ing if the shop messed something up when they changed the water pump.


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Yes, but only until the engine is warm. If your engine temp is normal, it is not the thermostat.

Please see my tip at . These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me with model, year and engine info.

Posted on Nov 08, 2010

  • Jeff Turcotte
    Jeff Turcotte Nov 08, 2010

    possibly 2 different problems. There is an actuator in the console that runs the back vents. Take the panel off on driver's side to see if that actuator works when you change the controls. I have pasted a pic below of that actuator.

    On the problem of no heat when idling, there are a couple things that might cause it. Check for the presence of a control valve on the heater hose. I don't see it in the manual, but you may have one. If so, make sure it is opening at idle. The temperature of the hose should be the same on both sides of the valve. The other thing could be just a clogged heater core that is allowing flow at higher RPMs. Have you tried racing the engine in park to make the heat come out? If that works, try backflushing the core.

    let me know if you're still stuck after this.


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