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my heat works good then it turns cold then back to heat what couls it be? on 1999 Mercury Grand Marquis

could it be the actuator blend door motor

Asked by for the 1999 Mercury Grand Marquis
I do agree with the other posts but make sure you also check the vacuum supply to your hot water control valve, if the vacuum line has a leak in it the valve will open and close as you accelerate and decelerate causing the heat to come and go. This leak will also cause your A/C to come and go with load differentials.
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REPLACE THERMOSTAT AND BE SURE IT IS "FULL" OF COOLANT!
could be a blend door but goodguy is correct about the fords and the doors breaking
fords are bad for the heater box door breaking
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