There is no "heat control valve" its a Heater Core by-pass valve, its an on/off valve no Temp control
The engine bay valve either circulates coolant thru the heater core in the cab or by-passes the heater core and sends coolant back to water pump, so on or off
A leak in a heater core will cause wet carpet on passenger side and a musty/coolant smell in the cab
Temp control in 1995-2011 Rangers is done with an electric motor in the cab, the Blend Door Actuator
Air flow strength is from the blower/fan speed
Air flow direction, defrost, panel, floor, is set by vacuum "motors" that open and close vent doors in the cab
None of these would be effected by having, or not having, the by-pass valve installed and working
In summer months its not uncommon to get less and less air flow from the fan because AC's Evaporator is TOO COLD, its freezing moisture in the air on its surfaces which blocks air passing thru it
Blower/fan>>>>>Evaporator>>>>>>>>>Cab>>>>>Vents
All the air from the fan must pass thru the Evaporator before it can flow into the cab and then to the vents
The Evaporator "Dries the air" going into the cab, moisture in the air condenses on the colder evaporator fins/passages it then drips off onto a tray under the evaporator which has a drain hose in the firewall, and the water will drip out onto the ground, you see that on all vehicles with AC, water dripping out on warm days
And if AC Evaporator's surfaces are below 32degF then that water can Freeze in the Evaporator and block air passage
(Freezers run with temps well below 32degF to freeze foods, but they also have a heater circuit to prevent icing, automotive AC has no such heater)
AC systems that are low on fluid often run too cold on Hot days, this has to do with higher pressure in an AC system when outside temps exceed 90deg
And this type of blockage would be more common in humid and hot areas
Just check AC lines at evaporator(under the hood) to see if ice is forming on the outside, if so then evaporator, inside the box, has even more ice forming, blocking air flow thru it
The grey Vacuum hose for the by-pass valve only has vacuum present when selector in cab is in AC MAX or OFF
If Grey line is unhooked and not "Capped", then in OFF position there would be vacuum loss but it would not effect anything and its not a "vacuum leak" that could effect engine
In MAX AC setting the uncapped Grey hose could effect vent controls, defrost, panel, floor, but vacuum at the control panel would already have to be marginal, i.e. a crack or leak in vacuum reservoir or hose from it to cab
If your air flow from the fan changes to Defrost Vents when accelerating, that means you have a vacuum leak in the reservoir or in the hose from reservoir to cab