2005 F-150 Heater Issue
 

New to the forum, but I've searched and searched and can't seem to find this exact issue. I have a 2005 F-150 XLT 4.6L with 206,000 miles on it. A few months ago it developed an issue with the heater. With the temperature control set to hot, the heater would blow hot for about 15 seconds and then the air will become cool. Per what I was seeing here, I checked temperature of heater hoses, radiator hose, etc. and all feel hot. Coolant level is correct and the truck has no issues with overheating. You can hear the blend door "shut" when you turn the heater on and the air is blowing hot, with no noises coming from that area as the air become cooler. I talked to a mechanic friend who suggested that it sounded more like an actuator problem to him. After taking a contortionist class and growing and extra finger, I put in a new blend door actuator hoping this would fix the issue. If anything, now the heater only blows hot for closer to 10 seconds. The next step would be the PITA change of the blend door, but in my mind that's not the issue since the heater does blow hot when first turned on and I don't want to go through that unless I'm somewhat positive that it will fix the problem. Does anyone have any experience with this issue, or have any suggestions? Thanks in advance...

What condition is the coolant in? How well was the cooling system maintained? You could possible have a partially blocked heater core. I would try back flushing the core to see if anything comes out like rust, sludge, or sediment.

I checked the temperature of the input and output hoses on the heater core with a laser thermometer and there was less than 5 degrees difference between the two. I wasn't thinking it was the heater core due to the fact that I do get hot air when I first turn the heater on, and it is hot again after leaving the heater off for less than a minute. I did notice something last night that seems to point back to the blend door. The lower the blower speed, the longer the hot air seems to last.