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I had the heat on high and the fan on high.
When i would stop at each light..the hot air would start blowing some what cold...then when i started driving for a minute..the heat would come back.
I noticed my F-150 is starting to do that a little bit, but what do you do about a heater core that is partially plugged. I believe thats the problem I'm having on my Bronco, but I'm also noticing that the temp gauge barely gets up off the "C" even after its been running all day. I just changed the thermostat on sunday but that didnt do anything. Any ideas??
Low water level. At idle, not enough warm water is pumped back to the heater core. When you start moving and raise the rpm's, there is sufficient flow to get the warm water back to the core.