1990 Mercury Sable heater is not working properly

1990 MERCURY SABLE
89,000 MILES • 4 CYL • FWD • AUTOMATIC
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1990 Mercury Sable mileage: 89,000. The heater is not working properly. It starts out blowing hot then gets cold. The head gaskets were replaced recently, a new thermostat installed and the heater core was flushed. The radiator is only six months old. The temperature gauge is also erratic, intermittently goes to hot then comes down to normal range. I cannot figure this out and neither can my mechanic. I need your help desperately.
Nov 18, 2010 at 10:14 PM
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We think your problem is a low coolant level. With the engine cold, and we mean cold, open the radiator cap (not the overflow cap) and it should be all the way full. If not, fill it to the top. Over the next couple of days, check for coolant leaks under the car when parked.
Nov 18, 2010 at 10:16 PM
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.