I have a 1997 Ford F-150 with the 4.6L V8 engine. Heat has been an issue since I got the truck last fall. There is some heat in the cab, but very little. It is enough to defrost the windshield, but that's about it. The AC works great.

What I have done;

  1. Changed the thermostat (replaces with OEM specs)
  2. Flushed the cooling system (air removed, both heater hoses get hot)
  3. Checked the blend door (big pain to get to, but it is not broken like a lot of posts I found)
  4. Checked the blend door actuator (working well, not stripped)
  5. Checked heater controls - Fan, temp and selector all work

While I was in the heater box, I ran the engine. The core is heating up, but there was no air flowing thru it. It felt like all the air was coming thru the AC evaporator side. I had limited view inside the box because I cut a 4" x 4" hole in the box behind the glove box to access the blend door without taking the dash out.

I have not been able to find a diagram of this heater box / system to see how the airflow works with this system.

What is keeping air from passing thru the heater core?

  • Did you measure the temp of the core? How warm is it really getting? What pushes the air through the core when the heat is on is the cabin air fan yes? And that fan is shared with the AC system and that works so it's not that. If the fan is blowing but air is not going through the core, then it must be coming out somewhere else, or the path through the core is obstructed? Is there a cabin air filter? What condition is it in? Any chance a critter made a nest in the heater core air path?
    – cdunn
    Commented Feb 20, 2017 at 14:20
  • I didn't let the engine get to full temp, so I don't know how hot it is getting. The air is blowing hard in the box. Like I said, it appears to be coming thru the AC evaporator but not the heater core. If it were a blocked filter, I would think it would restrict the air flow on the AC side also. Correct? I do not know how to get to the back side of the heater core to see if there is an obstruction. As stated, haven't been able to find diagram or pictures of the box. As far as a filter, I do not know if there is one installed. Considering all the dust in the vents, my guess is not.
    – CharlieRB
    Commented Feb 20, 2017 at 14:31
  • Cabin air filters didn't seem to happen for other brands till later, (I'm only familiar with Toyota for that history) so I wouldn't be surprised if Ford didn't have them till later. You're right about the filter doing both so it was a bad thought on my part anyway. Seems like it all comes down to tracing back the heater path, and that means you might just have to disassemble the dash. Trust me, it's a job I hate too.. Can you find the Ford Factory Service Manual online somewhere? I would think that would have the diagram you need.
    – cdunn
    Commented Feb 20, 2017 at 15:06
  • I have not been through it, but here's a link to an online version of the F150 shop manual: manuals.co/workshop/ford/f-150/…
    – cdunn
    Commented Feb 20, 2017 at 15:12
  • Thank you for that link. It was a bit painful to navigate, so I gave up. I did happen to find a complete service manual elsewhere. Looks like the air flows thru the heater core in the opposite direction I was expecting. This leads me to believe, it is just not getting hot enough, i.e. not enough flow. That leaves me with the only thing I haven't changed yet...the water pump. Could be it is worn, but just working enough to prevent overheating.
    – CharlieRB
    Commented Feb 20, 2017 at 16:15

1 Answer 1


Air flow in the heater box is not in the direction I originally thought. Having found a diagram, I was able to determine the air is passed thru the AC evaporator core all the time.

Then, depending on the blend door position, the air either goes thru the heater core or bypasses it. After that, it is sent wherever the selector is set to send it in the cabin.

enter image description here

My misconception was that the heat and AC were separate sources to the heater box. I now know the heater core is not blocked. It is time to look for another reason for no heat.

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