While experiencing something similar to this, I was convinced that my heater cores were fouled internally. I added flush last Tuesday and ran it until Saturday, noticing a heat increase from 85 degree air with 24 ambient outdoors, to 110 in this time span with the flush still circulating. I added one more bottle Sunday (prestone product) and wow, I had 130 degree heat! I flushed it and rinsed the system twice with plain water before refilling with 50/50 premixed prestone. To my astonishment, my ride to work in 20 degree air showed me only 60 degree air. I bled air, or tried, to, 3 times on Monday. The only time water would flow out of the bleed port for the heater was when the engine was decellerating. It never flowed as it should when all of the air is purged. The drive home Monday was terrible, 38 degree air out of my heater while ambient was up to 34! Back to the garage, I tried more purging, same results. It just got worse and worse. I ended up hoping that I was right, that the DCCV was bad. I ordered one on Amazon Prime, bumped up delivery to one day for $5 more, and received the valve Wednesday. Long story short, it fixed the problem, and the heat now responds in real time to temperature changes. My best output in 27 degree air today was 142. Setting the temp down immediately sees the output drop, increasing it gets immediate results.
The job sucked! I have a lift and must have cycled the car up and down over 30 times while trying to get the advantage I needed to remove the spring hose clamps. (I decided to take the hoses off the valve instead of the service manual's destructive recommended quick disconnect method) I had a bear of a time getting the hot inlet hose clamp released, could not get a position for the special cable operated tool to release it. I ended up using a reciprocating saw to cut off that line from the valve, since it has a right angle and lots of exposed plastic. Each clamp was a challenge. I am sure that I could do the job faster now, but I put 3-4 hours in to get the old one out. No way was I going to use those clamps it came with again, but to be honest, spiral worm gear hose clamps were not that easy to service on a few of the connections, either. This is Gen 1 V6. I spent around an hour and a half getting everything back together, it took a half hour to burp the system after starting it.
BTW the coolant that I flushed out was very clean looking, so the flush compound is pretty mild. That must be why they say you can either use it for a 10 minute flush or a 3-day flush.
Amazon's price was great, around $147 for the valve, from Motorcraft. The order was placed around midnight EST, and left New Jersey at 4 AM. It was in Massachusetts by 8:30 that same morning, and sat there for 12 hours until moving to Nashua NH for distribution to the delivery truck. You can't beat that service and price! It's a bit less money on rockauto, but that's always a 3 day process for shipping calculated at 10% of the part cost.
Note: I cannot imagine doing this job without a lift! I know everything is possible, but seriously, there is so much work topside and below back and forth, you really have to be determined to get it done on jack stands.
Props to JoeGr, he was calling out the DCCV from the get go.
