Here's the system , notice the outside air door from cowl vents. TR

 

IIRC when recirc. is selected no air passes through the heater core. (or in regular AC mode.)

 

If I'm reading this diagram correct, its saying that when vacuum is applied to the recirc actuator, then it closes off the cowl vent and the hvac system begins recirculating the cabin air...is that correct? If so then this shouldn't be an issue in terms of me getting cold air from outside since the recirculator actuator has been zip tied in the recirc position. I've untied and retied the zip tie holding it in position to if it had any effects on the air temp coming from the different vent settings and it didn't change a thing.

Would it be possible for the water pump to be failing slowly? The heater hoses get plenty hot, but im wondering if the pump is pushing enough volume for the heater core to come up to full volume. Additionally, with the engine temp bouncing high and low at random times i figure it can only be one of a couple things...thermostat and/or water pump. If the water pump was fluctuating in the volume it was moving I'm guessing that that would effect both engine temp and the volume of coolant making it to the heater core.

I havent noticed any specific signa of the pump going bad...no leaks, nothing out of the weep hole...but i did hear a brief squeal the other day. It lasted just a second so im not sure if it was pump bearings or just a slipped belt.

Thoughts?

 

Water pumps do not shut down heater.
A close inspection of the vacuum diagram shows recirc is only actuated in one position of the switch.(max AC)
Per 1990 shop manual page 36-61-4 tw vacuum motors are of concern regarding this problem.
The one with white hose controls outside/recirc. settings. In recirc no air will flow though heater core
Blue hose attached to the one that switches vent /heat

 

I am only here for comic relief. So let me tell you that I had recently the same symptom. And it was caused by low radiator fluid due to a slow leak from my radiator. You are undoubtedly smarter than me, but it took me a while to realize that the heater hoses come out of the top of the engine and, as a result, require a full radiator to direct coolant to the heater core. Filled the radiator with 50/50 antifreeze and problem went away. Replaced the radiator two weeks ago and heater works consistently.