I've had this car for over a year now, and I went through last winter with this issue and I don't feel like doing that again. The issue is, my heat is barely warm when on defrost, or bi-level. But on AC its toasty hot. So I have to play a game when driving, defrost for a bit to clear the windshield then AC to warm the wife.

The can is an 87 gl-10. I've done a couple coolant flushes, putting the hose to the radiator and heater core to make sure they're clear. The heater core was somewhat clogged, and after I cleared it it was a tad warmer, not enough to please the wife. My question is, would this more likely be a heater core issue or flapper issue? Does the AC pull air by a different part of the core then the defrost, defrost part could be a cold spot on the core? Or is a flapper probably hung up or out of adjustment?

Pic cause everyone likes pictures.

Well, is there as substantial a temperature difference between the settings as you imply?

 

The a/c button is a little misleading. It doesn't draw from anywhere else that I'm aware of, what it does so is direct all air flow through the front dash vents only.

 

What I'm thinking is that your heater core is still clogged. Try running the heat on the number 2 fan setting for a little while with the vents on ac. If it's coming out nice and toasty warm on 2 switch it up to 4. If it starts cooling off after a short amount of time your heater core is not heating up efficiently.

ya, definately a substaincial temp difference between the AC setting and all the other settings.   i'll try running it at 2 and then switch it up to 4 to see if it cools down.

The fan speed change is a good way to spot inefficiency in the heater core. You could be on to something with a flapper door being out of adjustment though. If the heat stays hot with the blower on full I'd start checking that route. I don't have much experience with the door adjustments though, I'm sure somebody else will chime in.

Also one hint, the best way to ensure there is no air in your cooling system when you flush and fill is to have the nose of the car up in the air while filling and burping it.

with the AC button pushed, blower on 2 or 4, it doesnt matter, it stays hot forever.   turn it to bilevel/defrost/heat, barely stays warm.  stays just warm enough to keep the windshield clear but the rest of the car cools right off.   i'm also noticing that on defrost there isn't much air being pushed up at the top of the dash.   with the blower on 4, i can barely feel any air passing at all.  this is whats making me think its a flapper issue, or maybe even a rodents nest.  although i figure i'd smell that.

Having the same issues with my 93 loyale spfi ea82

Some things that might help - EA82 GL & Loyale wagon based:

 

[i am not sure if there may be any differences for a GL-10]

 

Coolant is always pumped through the heater core, even when the engine thermostat is closed - it's the "bypass" so the pump isn't dead headed.  If you are so low on coolant that you have no / low heat, you're in for blown head gaskets.

 

The temperature of the air from the vents is adjusted by the lever - it moves a door that directs air through the heater core or around it.  It is mechanically moved by a cable. 

 

Vacuum powered actuators move the doors for the vent choices. 

 

The only difference between A/C and A/C MAX button position is that in max, the blower draws air from the interior of the car, as opposed to fresh air for all other settings.

If air flow doesn't change, the problem could be vacuum, actuators door linkage, or stuff in the ducts.

I've seen mice eat the sealing material of of the doors - possibly interfering with the air routing?