Plugged Core
 

First off I apologize since I'm sure its already been addressed by someone before, but I can't locate any threads... but, I have a plugged heater core (no cabin heat, inlet hose is hot while outlet hose from core is cold). I've been hopping around the web looking for some reliable advice as to how I should go about doing it, but I wanted the word from someone as passionate about these cars as myself. I've read use/don't use air, use/don't use the garden hose to flush, use vinegar, use cooling system flush, so on and so on. Bottom line, I want things back in working order, and to preserve the integrity of the car... preferably without pulling the heater core if at all possible. If it's relevant, I've got an '87 NA that's 100% Factory. It's got 77,xxx miles, and it's been meticulously maintained before, and during my ownership. It's garaged in the winter, and I did a coolant flush last summer, I'll do another this year for the sake of the backflush. Thank you for any advice!
https://cimg3.ibsrv.net/gimg/www.rx7...d3cd2e03af.jpg

Well, we know your core could withstand at least 13psi of air pressure (because your rad cap is 13psi), so I don't see any danger there.
The question would be: "Is the core plugged or is the valve?"

Unfortunately, it may be time to perform some of that meticulous maintenance you're (rightfully) so proud of.
I've pulled my core but the dash was already out, which makes life much simpler.
I've read the core can be removed with the dash in situ but can't speak from experience.
In either case, getting the core out is going to be your best longterm move.
I have never seen an old core that wasn't covered in dirt/leaves/general crap from the blower motor.
Even if the water flows freely, cleaning all the grunge from the matrix will improve efficiency.
The RX7 has one of the best heaters I've ever had, it's worth the effort.

There is no valve controlling water flow to the heater core on the FC, so if it is indeed plugged and not flowing water from the core's from inlet to outlet, it's got to be clogged with corrosion. Quick & easy test for testing the core in car is to just remove the 2 hoses at inlet & outlet, attach a piece of substitute hose to one of those ports & rig it up to your garden hose to see if you can flush water through the core. If you can't, you'll need to remove the core to replace it with a new/good one, or possibly have it professionally flushed/rodded out to remove the blockages (not too sure that's even possible though).

As for removing the heater core, can't help you there as I've never had to do that. FWIW, the AC evaporator core was pretty easy to remove & reinstall - had to do that to do a full AC system restoration on my FC. To get to the evaporator, I just needed to remove lower dash trim, glove box and the HVAC blower motor unit, and then the AC evaporator unit can be removed (once AC lines were disconnected).

its actually more typical that the motor that actuates the valve stopped working. i've been able to take it apart, clean it and then it works again.

I read above that there is no valve in the heater core, so I am not sure of the situation I have.

• I attached a water supply to the driver side of the heater core but could not get water to flow. It was so "clogged" that it would blow the hose off (no clamp being used).
• I then attached the same hose to the passenger side and water flowed out as easy as in under low or high pressure.
• So, I tried back-flushing again at the driver side. Still no water would move through the core.

Does anyone have a suggestion as to what is going on here?

When you did try the flush
1. Was the car in the on ignition position?
2. Was the temp slider all the way to hot?

there is a valve in the heater, needs to be open. the temp motor fails a lot, i've had good luck cleaning them (they guy who greased them at the factory may have gone overboard)
https://cimg2.ibsrv.net/gimg/www.rx7...29a011a18f.png

Sorry I missed your reply. To answer your questions, the car was not running so I don't believe I had the key in the ignition, but before I pulled the hoses to try the flush, I felt both the in and out hoses with the car up to temp and the heater set to full hot and blowing. I could feel no heat on one of the hoses.

I have seen that diagram before, and I have looked under the center stack when I removed the trim piece, but I cannot see that lever at all. I think I will have to remove the driver seat so I can actually lay on the floor.

Thanks