Bill...there isn't a difference in side to side flow from the posters. There is only a temperature difference. That'* what has me confused. There is only 1 heater core...one passage the air flows through and then maybe is pushed into driver and passenger sides. That'* the crazy thing.

Right, the fact that there is no difference in flow, makes it unlikely to be a flap problem. That'* why I am thinking about the heater core itself. Imagine if the core were internally blocked so that half is hot and half is not, but coolant can still pass through the core. If air flow through the core were laminar, and remained so, that would explain the problem.

Not sure I agree, maybe I'm not fully understanding. However it'* a whatever we can do to help situation.

When you say blocked, do you mean airbox internally or inside the piping of the core itself? I am thinking you mean the latter.

How can we have him test or check that w/o pulling the heater core not knowing if it will fix the problem?

95 and 98 can you blocl off the passenger side vents partially and see if you get the warm air out the driver side when the passenger is blocked? I assume you should get more flow and warm air.

I am thinking the latter. That internally, the coolant passages of the heater core become blocked in such a way that only one part of the core warms up. then when air passes through the core it is only partially heated. (Now here is the hard to believe part) - that the air remains largely stratified as a separate layer of hot and cold air as it moves through the ductwork and because of the arrangement of the outlets, the hot air exits on the passenger side. That'* why I thought the flow would have to be laminar, especially after it passes through the core. If it was turbulent and mixed, the temperature would be more uniform as it exited the ductwork.

I admit, this is just a guess. But it might not be that far-fetched. Consider the function of the screen in the throttle body. Not unlike the air passages in the heater core, its purpose is to produce laminar flow for the MAF sensor.

If this guess turns out to be correct, the only real fix will be to replace the heater core. [edit: EXCEPT...

Bill W may be right about a portion of the ductwork undone. This might produce a similar effect, allowing cold air to be sucked in to the air stream after the heater core. In that case, the core might be OK, and you would still have the problem. What we need is a warm junkyard not under a foot of snow :( where we can study the actual system and figure out what is happening. The diagrams in the FSM and pix of the exterior are helpful but don't provide the internal pathways we need to examine.]

And we couldn't be sure until the heater core was changed and magically things were better.

It'* a far stretch in my thought process.

Yeah, I agree, it is a far stretch. See above edit.

there'* a yard w/in a 1/2 hour of my house. But I can't get there until Saturday. Dash is out of the car.

OK forget my far-fetched idea about a half-plugged heater core and stratified laminar flow. It is not at all likely because the way the temperature is regulated is that air flow from the evaporator is partially or totally diverted through the heater core. If the air flow did not mix after the heater core, we would not get uniform heat when the heater core is functioning normally.

So, that just leaves a ducting fault as a possible cause.

Maybe sometimes its not so good to think aloud.

One'* thoughts might lead to the solution. It'* always good to get them out here as a possible.

You openly stated it was not very probable. The one thing we all try to do is say when we are not sure of something.

Can anyone think of a more scientific way to test the air flow coming out of each vent? I bet we'd find that the ariflow is highest through the left passenger'* vent, slightly lower in the right passenger'* vent, even less at the driver'* right vent and least in the drivers left. How about a spare MAF and a volt meter? You'd need to have a 5 volt source to run the MAF though.