i wasnt sure which forum to post this on but my truck is a 2000 with 217000 miles on it never changed the thermostat or anything before but here is the weird part the heater will only blow warm on like 3 or 4 but when i go all the way up with it then i cools down and blows barely any heat im not sure what it could be now in the summer it was getting pretty hot so i put royal purple ice in it and that cooled it down and did a flush which helped out now im wondering could this just be a thermostat problem or the heater coil i was leaning more to the thermostat because the heat will blow warm then cool down then blow warm again any help would be great