My Ranger has a broken blend door. One of the previous owners installed a shut off in the heater core lines, which worked fine, but I noticed that the truck has never really warmed up all the way, and the temperature seems to fluctuate some. If you idle around at low speed, it will stay pretty warm, but on the highway the gauge barely rises above cold.

When I bought the truck the factory heater valve was still installed, but I broke it while replacing the 02 sensor and removed it. The problem was there with and without it.

This winter when I opened the shut off I noticed that the heating up problem went away, the temp gauge goes right to the middle and stays there like it should.

My conclusion was that there needs to be coolant circulating through the heater hoses for the thermostat housing (and sensors) to stay hot. As long as that heater valve is open, it heats up perfectly.

So my question is, does the factory heater valve allow coolant to flow through it while closed? If not, what needs to be done to remedy the problem?

I noticed that one heater hose has an inline tee and a small line that runs to the intake. I tried switching it from one line to the other and it didn't seem to make a difference, the gauge still doesn't read right.

I have also replaced the thermostat and temperature sensors early on thinking these were the culprit.

 

Heater hose setups changed over the years and engine models

So need your year and engine size

Yes the heater is the by-pass for water pump on most years, so those two hoses need to be connected regardless of heat setting in the cab.

On some years the heater core was the by-pass, which yours sounds like, so coolant was always flowing thru the core, and needed to be.
This has nothing to do with the core itself, just the two heater hoses needed the flow.

So on other years they used a 4 hose by-pass
2 hoses in, one from from intake and one from water pump
2 hoses out to heater core
With a valve inside

With heat OFF in cab the by-pass valve connects the 2 hoses from intake and water pump so coolant can flow between them
As you turn ON the heat in the cab, coolant flows thru core
But at all times the 2 hoses from intake and water pump have maximum flow

"T'ed" heater coolant hoses were often found on the 3.0l engines, this was a pre-heater for the upper intake manifold which had a tendency to freeze up in northern climates
Some remove these smaller hoses if they leak, and block off the intake ports, this would not effect cooling system at all.
As said this is just a pre-heater for very cold climates

 

Sorry, it's a 98 2.5

The heater valve looks just like this



​​​What puzzles me is how coolant is supposed to circulate when the heater valve would be closed, like when the selector is set to off or Max A/C. That makes me think that there has to be a bypass built into the valve.

I guess the only way to know for sure is to grab a heater valve from the junkyard and mess with it some more.

 

Looks correct if valve is working

The hose at the far left in the picture, kind of on its own connector, that needs to go to the intake manifold connector

The raised dot on that fitting is the pivot point of the valve inside
Draw an X on it and that is the valves positions

So like this \ no coolant is flowing thru the heater core but is flowing to water pump
Like this / coolant is flowing to heater core then back out to water pump

And just as heads up, swap heater hose positions at the firewall when you change coolant, every 2 years or so
That will make heater core last longer

heater cores fail because debris flows in and can't get out, this debris sits on the metal and it no longer gets the anti-corrosion protection it should from the coolant flowing past
So it gets eaten away and you get a leak
When you swap hose positions you reverse the flow so push out any debris the way it came in, keeps the core cleaner

That T'ed coolant line IS part of the cooling system so shouldn't be removed