Possible blend door problem? Need advice

[97exploder19]

99 5.0 Limited with EATC.

Problem Ive had since buying the truck is weak heat when its under 50 outside. The truck has never had good heat unless I play around with the temp settings (going back and fourth from cold to hot) and even then, it doesnt last long. For instance, I had hot heat when I was idling while waiting for someone and playing around with the temp. Woke up the next morning and the same settings produced weak heat but that could be due to driving around and the roads moving the blend door.

The truck is getting up to operating temperature (verified with OBD2 bluetooth dongle and Torque app). Doesn't overheat in the summer months.

What Ive replaced or done over the last year or so:

New Ford blend door actuator (old one still worked after removing and testing)
New Ford heater control valve (old one was actually faulty and not moving inside, despite the vacuum actuator moving outside the valve)
Reverse flushed heater core with garden hose. Not really dirty at all and plenty of flow from one side to another.
New Motorcraft thermostat (installed in 2013 by myself).
Verified flow to the heater core after bypassing heater control valve and replacing small section of hose with clear hose.

This all leads me to the blend door. I can get cold air coming out of it when I turn the temp to 60. If I move it around 75, ill get warm air, moving it to 90 and there isn't much of a change in temperature.

Does this sound like a blend door problem? I really don't know what else could be the problem now.

 

[97exploder19]

So I figured it out.

I used a cheap wifi bore scope I bought off eBay and stuck it inside the vent to verify if the blend door was broken or not. To my delight, it was still working just fine.

Last weekend, I added some Prestone super flush to the coolant, that was brown and very rusty looking. Probably the original coolant besides what I added when I replaced the radiator hoses in 2013.

Drove around all weekend, heater stopped working entirely during that time. Sunday morning, back flushed the heater core again and got some **** out of it. Flushed the opposite direction, then filled it with CLR and let it sit for no more than 10 minutes. Flushed all that out, both directions as well. Then used the Prestone Flush N' Fill kit with the garden hose to flush out all the nasty coolant in the car. Did that till the water was pretty clear. Filled up with new 50/50 and my heat is great again! Lets hope it lasts. I bought a new heater core but Im trying to avoid replacing it, if I don't have to.

 

[410Fortune]

EXCELLENT!!!!

Heater core replacement is quite a chore, removing the entire dash just to get at it.
I ALWAYS try to backflush these gen II heater cores, as they are aluminum and rarely get holes in them but clog easily...
You did an excellent job, thanks for sharing!
Dealer charges $1200+ to replace heater core in Gen II (R&R dash) we do it at my shop for $600 including the new heater core....takes about 2 hours to drop dash and change core, another 2 to re install dash...but we are Gen II experts here the dash is not terrible if you have done many of them.........

 

[97exploder19]

I wasn't looking forward to doing the heater core at all lol.

Ive done one in my 2002 Mustang, so I know the pain of pulling a dash and dealing with all of that stuff.