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Mercury Sable Heater Is Not Working

Q. My wife has a 1999 Mercury Sable. It’s a 6 cylinder engine with A/C and approximately 77,000 miles. She was saying that the heater would not blow hot air. I instantly thought to myself that the thermostat was bad even though the dealer we purchased it from back in September was supposed to replace it.

Mercury Sable Heater Is Not Working

The heater worked great this winter then suddenly stopped blowing hot air just a couple weeks ago. This weekend I replaced the thermostat and checked the coolant level and the heater still won’t blow hot air. When the blower is running after the car warms up it blows cool air.

When you accelerate while driving the air gets a little warmer but not much and cools down completely when you are back at idle. While checking the new thermostat for any leaks I noticed a "squealing" noise from under the hood.

I didn’t see any leakage from the water pump weep hole so I assumed that the problem was not with the pump. Also, the car does not over heat and runs at normal temperature so I used that as an indicator that the pump was still good. Any ideas on this whole situation ? I’ve never seen anything like this before.

Thanks for your help,
Regan

A. It really sounds like the vacuum lines which control your air ducts under the dash may have developed a leak. The fact that it heats up a little when you accelerate is very telling. You should check under the hood and notice a vacuum hose routing diagram which will tell you where the source of the climate control hose is. From there, thoroughly inspect the lines going to the firewall for cracks leaks or bad fit.

Continue your inspection under the dash where you will notice small vacuum diaphragms from which you can trace the lines back. You should find a leak fairly easily in one of these lines, they are usually hard plastic and very susceptible to cracking and pulling out of the rubber fittings on the ends.

Additional Information provided courtesy of AllDATA

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