I have a 1994 Mercury Grand Marquis LS and was having variances in the tempurature. I changed the thermostat but it didn't help. I took it to a mechanic and had him flush the heater core for $50 which seemed to help. They also suggested a whole system flush in the future for $89. Wouldn't the whole system flush have included flushing the heater core or are these two separate procedures. Also, by not getting the entire system flushed, am I risking recontaminating the heater core?

Thanks so much :-)

Change the radiator first. Sometimes radiators gel old and corroded and cannot dissipate heat all that well. Also i dont suggest a whole system flush because air bubbles could be left in the system and that can make your car over heat. Hope it helps. Heres a link to one of Scotty videos https://youtu.be/evpaTW2WJ5Y