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Recently I've come across the problem of having very little heat through the heater core.
While the blower is not running, the heater core hoses are both hot, while the blower running the inlet hose is hot while the outlet is cool. Which means that the heater core is not clogged. The upper radiator hose is hot, while the lower cool which I would expect. The thermostat has recently been changed along with the cooling system being burped properly, including the blower being on high/hot while burping the system to get rid of any possible air bubbles within the heater core.
I noticed while changing out the thermostat that the coolant temperature sensor was eroding because of the glycol mix of the antifreeze. It will be replaced. However, I haven't noticed that the fans turn on because of this, but the coolant gauge reads normal operating temperature whether idling, cruising at low RPM, or doing 65 MPH down the freeway.
The water pump works fine as I can visually see the coolant flow from the water neck on the radiator.

I would imagine that the radiator cap would cause a problem much like a clogged radiator; opening sooner rather than later by the pressure in the system. As I'm not sure when the radiator cap was last changed, this is also a common problem when concerning loss of coolant (which isn't a problem immediately).
The other problem that I would see is the coolant temperature sensor itself, however, it has nothing to do with the heater core temperature, rather the fans not turning on.
While squeezing all the hoses (upper and lower radiator, heater core hoses), when cold and hot, the coolant raises in height with the radiator cap off. This might lead me to believe that there might be a sleight blockage in the system not allowing the hot coolant to flow through the heater core.

CaffeineTripp wrote:Recently I've come across the problem of having very little heat through the heater core.
While the blower is not running, the heater core hoses are both hot, while the blower running the inlet hose is hot while the outlet is cool. Which means that the heater core is not clogged. .

I am having the exact same problem. Though I have not changed my thermostat. My temp gauge reads normal, the car never over heats and the cooling fan acts normal.

I assumed it was my heater core, but the same as you; when i leave my blower off, both hoses will get hot. With the blower on, the outlet is cold.

A little of topic, is there a cabin filter? My blower speeds work but not much air is pushed through the vents. My blower motor is clear of leaves or debris.

There is no cabin filter for these vehicles.

What I'm thinking the problem might be is one of the doors next to the heater core is not completely opening, only slightly. Makes sense that route, or even a partially blocked heater core, though as I wrote above, that doesn't seem to be the problem. This weekend I'll be tearing into the dash to check out if there's anything blocking the heater core along with removing one of the doors to allow heat to constantly pass through the HVAC.
If that doesn't work, and there's no leaks from the heater core, next up is doing a complete system clean. Might even get the water pump, along with the timing belt and tensioner, done at the same time so I don't have to worry about it later on.

Since I wasn't in the mood to tear apart the dash, I removed the kick panels from the center console side near the ECU. The cable and mechanism that operates the blend door from cold outside air to warm heater core air actuates properly. I can both hear and see the cables and actuators moving as well as the blend door.
So, that doesn't seem to be the problem.
I noticed that the engine is running rather cool; I can put my bare hand on top of the valve cover and it's quite warm, but not hot.
The upper radiator hose and the lower radiator hoses are warm, but not hot either. The radiator itself is rather cool to the touch. Stuck thermostat? I doubt it as I've just replaced it per the directions on the Chilton's manual and by taking pictures of the original thermostat in the housing. Spot on. Highly doubt I've gotten a bad thermostat given the heat gauge reads halfway, as it usually does.
Is there a sensor that could cause this problem of no heat? Some how I doubt it as it seems more of a mechanical issue rather than sensor/electrical related.
As stated before, the fans do not turn on either, but they wouldn't turn on given the temperature of the engine. Currently, the temperature outside is about 20*F. Pretty warm out.

i thought if one side of the heater core was cold it meant that it WAS clogged, and not vice versa.

my 323 had no heat, inlet was hot, outlet was cold. flushed it a few times with my garden hose and it was absolutely disgusting. looked like yoohoo. flushed it both ways until the water came out clear. unfortunately i don't know if it fixed the problem because I threw a couple of rods before i could check the heat.

Dustinlib wrote:i thought if one side of the heater core was cold it meant that it WAS clogged, and not vice versa.

my 323 had no heat, inlet was hot, outlet was cold. flushed it a few times with my garden hose and it was absolutely disgusting. looked like yoohoo. flushed it both ways until the water came out clear. unfortunately i don't know if it fixed the problem because I threw a couple of rods before i could check the heat.

That's the general consensus. Unless you had something after the heater core, blocking the passage to the motor. <-Unlikely. I am pretty convinced it's my heater core. If i let the car idle for a LONG time or take an extended trip at 70ish mph, my heater will get....warm.

I am going to try to flush it and see if it helps.Then replacing the thermostat as soon as I can find time.

CaffeineTripp wrote: Currently, the temperature outside is about 20*F. Pretty warm out.

Kiss my a**, it's -20 here. You get no sympathy from me.

Daninski wrote:

CaffeineTripp wrote: Currently, the temperature outside is about 20*F. Pretty warm out.

Kiss my a**, it's -20 here. You get no sympathy from me.

lol Fair enough. I'm from the north too, so I know how cold it can get. Last week we had a wonderful -20 as well, without windchill. No sympathy intended.

Dustinlib wrote:i thought if one side of the heater core was cold it meant that it WAS clogged, and not vice versa.

my 323 had no heat, inlet was hot, outlet was cold. flushed it a few times with my garden hose and it was absolutely disgusting. looked like yoohoo. flushed it both ways until the water came out clear. unfortunately i don't know if it fixed the problem because I threw a couple of rods before i could check the heat.

So that's pretty much the going thing is that the heater core is clogged no matter what huh? Well, that's a bit easier to deal with. Time to bust out the water hose and flush her a couple of times. Be nice to do a complete flush of the cooling system, but that will have to wait until summer to get it done properly, including a system cleaner run through, just to be safe, maybe even replace the radiator hoses.

I'll definitely get back to everyone once the flush has been completed to make sure that is the problem. Also, I'll do a full write up of how to get it done and some of the problems that would cause a no-heat situation (it's always in the winter isn't it?) just because I'm that anal retentive.

Apparently I need to drive the girlfriend's car more often. Dropped her off at work today to get the heater core flushed out, noticed that when I'm coasting down hill (a good two miles) the heat gauge falls to cold. Stuck open thermostat. Even after getting the new one put in, properly mind you (unless there's something completely different with these MX-3s than there is with every other thermostat replacement), it got stuck in the open position. Jiggle valve at top.
Also, what is the purpose of the second bypass spring? Is there another 'standard' thermostat that will work which only has one spring instead of the dual spring that the MX-3 has?

Changed out the new thermostat for an even newer one. While we were at it, we flushed out the heater core with some compressed air, did the same to the rest of the cooling system minus the radiator itself.
Installed new thermostat, burped the whole system, good to go. Guess the old thermostat was stuck open for whatever reason. Finally good to have some heat in that car.

Glad you sorted it out and it was only a defective thermostat, could've been much worse (clogged heater core, radiator, etc.)
Sometimes even "new" parts come broken straight out of the box. Reminds me when we got a starter for cousins Honda, it was bad straight out of the box.

Happens sometimes. First time it's happened to me though. Guess I've been lucking out when getting parts for cars. lol

Resolved as stuck open thermostat. Also took precautions by flushing the heater core and the cooling system with compressed air.