could it be the actuator blend door motor
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1999 Mercury Grand Marquis Question: my heat works good then it turns cold then back to heat what couls it be?
Answer #1goodguy October 23, 2012, 14:25Master
fords are bad for the heater box door breaking
Answer #2Greg's Orange Auto (15216 Answers) , Orange, CA - (714) 361-9386globalhelper October 23, 2012, 16:29Master
could be a blend door but goodguy is correct about the fords and the doors breaking
Answer #3pushrod October 23, 2012, 21:08Master
REPLACE THERMOSTAT AND BE SURE IT IS "FULL" OF COOLANT!
Herb Ackside October 24, 2012, 19:35Journeyman
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.