98 Ranger, 2.5L, 5 speed Manual, 347,000 miles - I've read some of the threads on this issue already just like to confirm suspicions. I was out photographing the Aurora on October 10 and used the heater then. On October 17 I was doing a model shoot and on the way home went to turn the heat on and it was blowing cold air. The coolant is at normal levels in the overflow and drops normally when the engine is running. Engine heats up normally and maintains a good temperature. Radiator and heater core hoses are hot to the touch when engine is operating. But I get no change in temperature with the temperature control **** in any position from all the way cool to all the way hot. It's not blowing air conditioned air and the AC is not turning on, it's just blowing cool air. I can't hear the blend door opening or closing when I go from cool to heat on the control and it's the same cool air in whatever position the vent selector is in. So is this a blend door issue or is it a temperature control issue?

 

Check the temperature of the coolant hoses feeding the heater core. BOTH hoses muse be too hot to hold onto or you have less flow
than needed to produce heat. It will get tepid or luke warm, but not give out the expected heat if one of the hoses is not hot.
If one is hot, the other warm or cool, you have blockage in the heater core. You may be able to back flush the core by using
a 'flushing tee' or connecting a garden hose to one of the inlets, and pushing water through. I would figure the HOTTER hose
would be the inlet, so would put the hose onto the other pipe, and make the fluid flwo in reverse.
When done, you may be able to get the plain water out using compressed air with fingers to force the air through the heater,
at least to some degree. You should run a 50:50 mix of coolant and fresh clean water year round.
You may have a valve the opens & closes to control coolant flow, especially in summer when the A/C may be in use. I do
not know if you do, but it could be sticking or not opening fully. That also would cause the hoses to have different temps.
tom

 

Update - Thanks for the reply by the way. Both hoses are hot. There isn't a temperature control valve in the engine compartment. On Saturday I drove about 75 miles to pick up some antique furniture. The streets were very rough. On the way down the heater was still blowing cold air. After bumping down the streets, all of a sudden I started getting normal hot air. So I think what must have originally happened was while bumping over uneven ground, i knocked a connection loose in the temperature control and bumping over uneven streets I knocked the connection back together. Anyway it's blowing hot enough to force you out of the cab now. So I'll take control panel apart soon and see if I can't find a loose wire or connection. Thanks for the advice!

 

There is a flapper or 'blend door' that routes air across the heater core or lets it bypass to regulate temperature. If stuck, you may get
HOT or COLD air or whatever the mix was when it got stuck.
The flap may be moved by a stepper motor, and may also have failed pivot points, a post stuck into a hold, causing it to bind. The
actual cause is year dependent. There are kits on the web to fix sticking blend doors without taking the whole dash out and
replacing the plenum($$$ for time & material).
One of the originals 'zeroed' the stepper at each key ON. Wham bam bzzz... and finally wore something out. Another had
hinge failure, and so on.
I do not remember which vintage had what problem.
tom

 

Thanks! Possibility that the blend door hinge is binding and got stuck closed and now it is stuck open. In any even I have heat for now. I'll know what to explore if it goes out again.