No heat... not sure what to check next.
 

Car is a 1987 auto.

I've been having no (or VERY little) heat in the car for the past few months, and I can't diagnose why. Here's what I've checked:

Heater valve opens/closes normally
Water temp is 160F in and 140F out, so heat is being exchanged and there is flow through the system
I can see the mixing flap arm moving down as I move the temp slider from cold to hot
The comb flap opens on full cold, and closes on full hot
I've tried two HVAC controllers, and neither solves the issue, including a Greg Brown rebuild unit.

I noticed that if I leave the car parked after driving, and come back to it after a few minutes (like going into a store) the heat will be noticeable warmer on startup and get colder over time.

Are there some arms or levers I can manually move to force air over the heater core? What could be preventing air from bypassing the heater, but still come through the vents?

Seriously running out of options. Doesn't look like the core is bad because there's flow and heat being exchanged. Something is preventing the air from passing over the core. Does anyone have pictures of the inner-workings of the heater core? Anyone with similar symptoms? Maybe something dropped into the vent preventing a flap from opening or closing??

Very frustrating when it's cold out, and cold air is blowing at you.

Check the orientation of the hot water valve.

It's correct. Like I said, hot water is flowing in/out of the core.

if your HCV is older than 5 years replace the HCV and short hose,

push the heat selector all to way to hot and see if you now have full heat,
if so then the temp sensor strand is probably broken,
usually the place is the connector under the left fender,
cut the temp sensor connector out and resolder the wires.
Pay attention to the wires IIRC they can be done backwards so cut one wire at a time and resolder it
Also check the HVAC relay and the blower relay and the X bus relay

I messed around with this a bit last night after doing a load of work on this car before he picked it up. HCV is open (didn't even think about orientation but know it's open when I looked in it. Blew the core with low pressure air and it's not clogged up. When the engine is warm getting 160 degree water in, 150 out. Did not check flow but the hoses are warm going in and out. Odd thing is, th is car had an '80-83 head unit in it and I swapped it out with a newly rebuilt, proper year one and got a little bit more heat but not what it should be getting. Stepper motor is working but honestly, I've not had to dig in to these too deep so consider myself a novice on them. Also didn't have much time to go through the WSM test procedures on it.

Sounds as if your heater core is partially glommed up with something. You've got hot water in/out of the core, but you don't know how much of the core is actually working. Can you back flush just the heater core? It could be crap that is created when PO's change coolant types without correctly flushing everything and new alien organisms/materials are made from the combination of the different types of coolant.

If there is plenty of air going through the heater core, the coolant is circulating and the flaps are working s they should then the only thing left is that the core isn't pulling the heat from the coolant and the core. Also, if the in and out temperature difference is low, like it is in this case, this also points to the core not pulling the heat. I would first make 100% sure sufficient air is passing through the core, then make sure the mixing flap is working and sealing as it should. If both these check out then the problem has to be the core not exchanging the heat.

https://cimg9.ibsrv.net/gimg/rennlis...55d0f422ee.jpg
https://cimg6.ibsrv.net/gimg/rennlis...2d33a4f192.jpg
1st pic shows the core is the inner most part in the box, followed be the mixing assembly and then the outer Firewall side AC

Car runs up to temp and holds pressure, correct?
Is all air is out of the system, that would be my first guess of a no heat, if all else is in working order, if the HCV is not blocked from opening and closing as you work the Temp lever.

Not much to these Heater cores, however blowing air through may not tell you how much of the core is blocked, right?
So when was the last time the coolant was serviced and how long has heat been a no-op ?

The coolant flow path through the heater cores starts at the heater valve at the rear of the right-side cylinder head. Hose to the heater core, then the heater core itself. From the heater core, a hose passes across the firewall area and connects to one leg of a steel "Y" pipe that runs forward along the right (passenger) side fender wall. The coolant reservoir connects to the other leg via a short section of hose. From the front end of that "Y" pipe a hose connects to the water bridge. All the hoses are 15 or 19mm.

Check the temperature immediately in and out of the heater control valve using your IR thermometer. Then look at the temperatures in the other hose sections.

The heater system depends on a certain pressure drop through the thermostat. It also depends on correct coolant system level. You can test flow through the system by disconnecting the heater valve from the stub hose at the head, and the return hose from the Y pipe where it connects at the water bridge. Put the water bridge end of the hose in a big bucket, and connect your garden hose to the heater control valve. With engine off and/or vacuum hose lifted, the valve will default to "open" position. Garden hose water should readily flow through the system, exiting at the end of the y-pipe hose into the bucket. Note: Drain the cooling system down a gallon or so via the radiator drain plug before disconnecting hoses. Loosen the reservoir cap to allow the reservoir to drain at the same time. Then configure the hoses for the flow test.

On the airflow side, there's a diverter flap/"blend door", position managed by the little blend door servo motor. Check that the motor is actually moving the door by moving the temp level in the control head to hot and cold extremes. You should be able to hear and see the motor move the door. If the servo motor is not moving the door, it's also not releasing the vacuum on the heater control valve from the little solenoid in the console. SeanR confirms that he's observed the HCV moving, so possible blockages happen when the little sliding door in the valve breaks off and stays in the closed position. Folks seem to notice heater valve failures where the valve disc/door fails to close, when heat added bleeds into the AC airflow in summer. Not so many report the valve sticking closed. Regardless, the valve actuator is moving, and the water flow test described above with the garden hose requires that the valve actually pass water.

Possible causes of restricted heater system liquid flow include, in order, kinked rubber hoses, blocked heater core, corrosion and related blockage in the steel "Y" pipe on the fenderwall, and heater control valve internal failure.

And if you look closely at my pic of the Heater core, both in and out pipes are attached to the same side tank (so the core could be blocked), the coolant should flow into the bottom pipe and Fill the tank and exit it, self bleeding.
Like the way we plumb the Inter-coolers, fill from the bottom exit the top,

If that core is getting hot than the front (inside car) side of the HVAC box should be hot too, as the core is right there behind that plastic box.

Pretty sure it is a double pass heat exchanger where the header tank is divided into two chambers so coolant is forced to flow over to the other header then back to the exit. Other wise it would never opt to go through the core as it would simply follow the path of least resistance.

Assuming the heater control is actually turning on the hot water and flaps and such, It seems there are only two major choices: Blocked water flow or blocked airflow

Waterflow:
air embolus in system
degraded hose
heater valve
clogged heat exchanger

Airflow:
leaves clogging air intake
heat exchager filled with dirt/dust
(like a leaf-and-moth filed radiator in some cars)
improperly flapping flaps
interior air passages clogged or misrouted (?how?)

Work through them all methodically and the answer will become clear.

Then, bc it is a 928, the answer will be wrong, you will need some strange part, and it will heal itself after you curse and donate blood.

Engine temps never got above 165 at the T-stat via my IR gun. With some of the other stuff installed on the car, I'd venture a guess that it's got a lower temp T-stat from one of our vendors.

That's the piece of the puzzle we needed. It actually sounds more like a stuck open t-stat at that temp. My 79 was like that when I bought it, would never get much above 160F. Even at the track it took several laps at speed to see any movement on the temp gauge.

One test, disconnect the fans and see if the temps start to rise.