Question about 1997 Mercury Grand Marquis

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A/C and/or heater only blows very hot air

Heard what sounded like a vacuum valve failing several weeks ago. Since then, whether car is set for A/C or heat, it only blows very hot air. A/C unit is charged. Is the valve that I suspect is the problem up in the dash board or in the engine compartment? Seems like it would be a valve to close off air flow thru the heater core and open access to the A/C evaporator.

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  • gsmith2894 Jun 24, 2009

    Yes, the line is frosty cold. The valve that I suspect is at fault, made some funny thumping sounds for a couple of days then stopped, and since it stopped I only get the hot air. It sounded like a vacuum valve that couldn't move the damper and finally failed. Could it have ruptured the diaphram? Appreciate this help, thank you.

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When the a/c is running you need to check if the lines coming out of the evaporator are cold. if they are cold the blend door operation could be an issuse. if they aren't cold the a/c is not working and you have to start diagnosing that.

Posted on Jun 24, 2009

  • Aaron Murchison
    Aaron Murchison Jun 24, 2009

    I belive the vent controls on you car are vacuum actuated. so either the blend door is jammed or there is a leak in the vacuum line. you may be able to see the controls under the dash. playing with them you might be able to figure out what controls the blend door by checking for temp change. you'll be able to tell is the door itself is stuck or the actuator broke.

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