Interesting and stupefying heater question.

[TheWuster]

This is a stumper as this is the first vehicle I've ever had with rear heat.
Getting heat in the back and not in the front. Figured it was a blend door actuator but after removing the ungodly center console this fall and realizing I'd have to remove the dash to change the actuator I started looking at alternatives that could at least hold me over until warmer months when I could drop the dash. I got a low mileage climate control unit from a salvage yard yesterday and tried a swap in case the ic in the original unit was fouling and not telling the blend door to move.
No luck.
I then cut into the blend box to manually move the door over to heat figuring I could wedge it and leave it that way until I drop the dash and change the actuator.
The door was in its proper position!
I cycled the control unit down to the coolest and up to the warmest and the blend actuator is doing fine.
I have no heat coming from the front core??!!
Rear is doing fine and hoses are hot.
Is there some servo up in the dash or something amazingly weird or is the core just clogged up somehow? That is my first guess but seeing how replacing it will be a major deal I wanted to ask you all first and see what we can come up with.
Thanks for any help you all can provide. I promise to be more active and help where I can as my work slow season is starting. I know a lot but this one is either glaringly obvious (I hope not) or it's a feature I'm not aware of.

Edit: I've never had this done but do you all think before replacing the heater core I try a coolant system flush?

 

[Boostedbus]

If you unhook the heater hoses going to the core then you can flush just that core by itself with a product like CLR. Pour it in and let it sit flush it out backflush it pour in let it sit repeat process a couple of times. I did this to an old Jeep I bought with no heat and now have plenty of heat and no leaks as of yet 5 years later. CLR stands for Calcium,Lime,and Rust….. worked for me.

 

[TheWuster]

Interesting. Thanks. Now the question is how the hell to get those off??? They are so far back up on the firewall I cannot even see how they have them clamped on. Is this something I will need to remove the dash for or do you have a quick fix?
Edit: I know the product well. We had some pretty horrible well water for a lot of years at the house before this. Stained everything orange with time even after a heavy softening and filtering.

 

[TheWuster]

Found my answer. I was seriously questioning how they could get clamps up in there unless the core was installed before the couling. Other than getting my hand up in there and clipping those off without losing the o-rings it looks pretty simple. My problem is due to an accident my right hand is useless for stuff like that. Thumb and forefinger are paralyzed. Have to crawl up on the engine and use my left... again.

 

[Hamfisted]

Have you verified that hot water is getting through to the front heater core ? Are the hoses hot right at the firewall where they enter the heater core ?

 

[MesaGuy]

Usually one of three common problems on these older models.
1. Heater core is totally clogged.
2. Blend Door base has broken
3. Blend Door Servo has failed
----
That presumes that air is blowing, e.g. that you do not have an issue with the blower. (If the blower, almost always the blower motor resistor, not the blower motor itself.)

4. The Heater core has actually failed, and is leaking. (You can usually tell this because you get a mist of ethylene glycol on the inside of the windshield if you use the defrost function. (Or you can look for the ethylene glycol leak at the bottom of the vehicle, where the "water usually drips" when the A/C is on. (Its behind the front passenger wheel.)

In your case, you replaced the blend door servo. (Bad news, I have replaced with a brand new one before, and got a dud, so a new servo is no guarantee of anything.)

Bad news 2... If you replaced the servo, and the door is frozen, then it will break the original door. The original doors are all plastic at the bottom, not reinforced. Inside the heater box, they connect with the servo actuator with two offset cube teeth plate wheels. One on the servo, one on the blend door base. The blend door base connection plate wheel teeth often crack and break, rendering the door useless, even if it is not "stuck". Then the servo turns, but the door does not. The only fix for that is either cutting into the box on the left side through the glove compartment (see a youtube video), or pulling the whole dash and accessing the box properly.

Finally, if the heater core is clogged or leaking. The official fix, pulling the dash, opening the bolt on top, and replacing the core is labor intensive. (I've done it 3 times, I don't recommend it, but if your core is actually LEAKING, you either do that, or you cut a large cut on the box to remove the blend door, and then the core.

If you core is "just clogged". You can just disconnect the quick-connects. They are a royal hassle on an old vehicle. Nothing "quick" about them in the engine compartment. Connect up some temp rubber hose put a funnel on it, and pour in CLR or LimeAway, and let it sit a good long while (overnight). Then backflush with water, and repeat.

If you are too frustrated with the quick disconnects (many, many are) then just leave them on, and cut the hose between the T and the core. (Go get hose clamps and the appropriate nylon or brass double nipples first at AutoParts or HomeDepot. Then hookup a funnel, CLR.

The full dash job is a BIG one. You have to disconnect the steering column mount and drop the column, remove numerous screws in the base and in the dash. If you drop the column wrong (or just unlucky, but "right"), you often break your shifter cable at the shifter end (plastic that snaps and shatters) There are video on how to get it done. I suggest watching them many times before beginning. And then still, I still always have problem with the disconnects, even with special tools, and/or long reach 90 degree needle nose. They are a horrible idea. (Good only for Ford, to put the truck together cheap, not for anyone to ever service it.)

 

[Bilsto]

Hoo boy, you're singin' to the choir here. I had the exact same problem. What I've learned is that my engine is just chock full of rust. I'd backflush those cores and the problem would come right back. What I ended up doing was this: With the engine cold, just for safety, I pulled the heater hose off where it comes out of the block on the top of the engine. I connected a wet vac to that hose, topped off the system, and sucked everything backwards out of that hose until it sucked air, then started again. I did this for 3 hours. Then I cut that same hose and installed a heater hose filter in it (reusable), *before* where it splits off to the back heater. That way all the coolant is getting filtered before it goes into the cores. I went for months having to clean out the filter 1-2 times a week, there was so much rust! That was a couple years ago, now I only have to clean it once a month or even less than that. The front heater is still anemic but it's enough to defrost the windshield. The rear heater does the real work of heating the passenger compartment. I'm counting it as an epic win for me.

 

[Ellison Brown III]

Did you check the heater control valve under the hood? I have an ‘04 that had the valve fail twice. It actually cracked and allowed coolant to leak. However, in your case it may not be actuating; allowing the hot coolant to travel to the heater core. It’s vacuum actuated. Located on the passenger side firewall on my ‘04. See attached pics for location reference…

Just an FYI… myself, and a fellow coworker, fixed a 2009 Titan heating issue. The young lady’s truck that worked with us was suffering from two issues: busted blend mode actuator, and heater control valve. I was able to rotate the arm with my hand. This allowed heated coolant to travel in the cabin. Good luck.

 

[Bilsto]

Did you check the heater control valve under the hood? I have an ‘04 that had the valve fail twice. It actually cracked and allowed coolant to leak. However, in your case it may not be actuating; allowing the hot coolant to travel to the heater core. It’s vacuum actuated. Located on the passenger side firewall on my ‘04. See attached pics for location reference…

Just an FYI… myself, and a fellow coworker, fixed a 2009 Titan heating issue. The young lady’s truck that worked with us was suffering from two issues: busted blend mode actuator, and heater control valve. I was able to rotate the arm with my hand. This allowed heated coolant to travel in the cabin. Good luck.

The first-gens don't have that valve, at least none of the ones I've ever seen have had it.

 

[MesaGuy]

Blisto is correct. The 1st Gen's (97-2002) do not have the heater control valve. They are full time flow on the front core, as well as the back core. But the '03 and later have the front control valve. So the original poster (who did not specify his model year) could have either a frozen heater control valve (if its an '03 a later), or just a clogged front heater core unit (not at all uncommon if a 1st Gen, given that is 18-25 years old now).

Disconnecting the quick-disconnects with a "bad hand" is basically impossible. But you can just cut the hose about 2 inches past the T. (Or if you have no T's in the hose (some F150's), back about 6-8 inches from the firewall (or as much as you can get, while still leaving 2 inches to the T.) Cut both hoses. Then you can flush all day long there. When done, you just nipple + hose clamp it back together.

The quick disconnects are miserable even with two good hands. Its a mis-normer. They are quick connects (the first time, brand new, in the factory line), and slow disconnects and slow reconnects any future time, home shop, dealer, or independent.

I'd honest just cut the hose, do the deed of the flush with CLR (or Limeaway), rinse and repeat. The soak time is usually 3 hours, then rinse and repeat. You can soak longer, but after about 3 hours, you have gotten most of the work out of the chemicals. Try that for 5 rinses the first time you do it. Then you should be able to see some flow through the core with a hose nippled to one line, and the other hose nippled to a bucket. Try to flow it it in both direction. e.g. Turn off the hose, and swap in and out at least once, perhaps a few times.

As much work as all that is. It is still easier than tearing the dash apart.

And after you nipple and hose clamp it back together, as someone else noted, you can also add a heater-core filter inline at your cut, instead of the nipple for the input side.

I no longer use FixLeak head gasket leak treatments in my coolant. While these are great for head gasket leak prevention, they seem to shorten heater-core life quite a bit. (They just clog a lot faster.)

Good luck.