I have a JDM FD (RHD) bought at auction that has never had heat to the cabin. So that I could operate manually to rule out, I removed the actuators for the heater blend valve and the other that appears to be for the A/C and blows air through the front vents. I flushed the heater core. I then ran the car with the core outlet hose (the upper one) removed and draining into a bucket. At idle, no coolant flows. At higher revs, it flows out well and confirmed that there was heat in the cabin. I did it only for a second because the engine would otherwise be drained of coolant. I reattached the outlet hose, filled the coolant, and there was still no heat at any revs.

The Big Fat Cooling Thread (https://www.rx7club.com/3rd-generation-specific-1993-2002-16/big-fat-fd3s-cooling-thread-571088/) says:
"Regardless of thermostat position, the heater core is fed by a large coolant line below the oil filter, and is pulled back into the waterpump housing inlet. It always has substantial flow, and always bypasses the radiator."

If it's always supposed to have substantial flow, then there must not be a coolant flow control valve. If there was, I thought a malfunctioning valve could think the coolant is cold and prevent flow to the heater core. Other than confirm flow at the core outlet hose where it joins the coolant from the lower radiator hose and feeds back into the waterpump, I don't know what else to check that would be impeding flow. Any ideas?

 

I know mine wouldn't work without the rear wiper fuse even though my car didn't have a rear wiper.

 

Just so there's no misunderstanding, is this about a Type X/Touring X with the Auto A/C System or no? They have quite a few additional parts compared to the standard HVAC setup akin to North American FDs. One of which is a Water Temperature Sensor.

 

Few things -

The FD's heat is really only designed to work when blowing to the floor vents. You don't get heat on face vents.

There is no valve in the heater core or anything; coolant is always flowing through the core.

There is an air mix motor that moves the flapper from cold to heat, I have seen those fail. Key on, engine not running, you should be able to hear it move; too hard to hear with the car running.

Dale

 

So I finally have heat in the cabin, and it turned out to be a plugged return tube from the heater core back to the engine.

I knew that coolant was flowing through the heater core, so I disconnected the hose returning coolant back to the engine, and there was no flow. I stuck a plumbing snake and it wouldn't go all the way through because of a tight turn, but I was able to poke it from both sides and clear out some black stuff in the line. I never saw anything like it. I've seen some cooling system cleaners that are supposed to remove gunk, but I'm afraid to use them and hurt seals and whatnot.