Hey all, not a technical guy here at all but I'm trying to learn (basically because I don't have the money to pay other people to fix my stuff lol).
My truck stopped blowing hot air awhile back when I had the heater on. Didn't think anything about it but one day I drove it, parked it, got out, and there was steam billowing from under the hood. Opened it up and there was a crack in the radiator. At first, I fixed the crack (with JB weld) and added coolant but it was overheating within a few minutes driving it. After learning a little about the coolant system I decided to change the thermostat and see if that helped. It did. Overheating problem was gone and I had heat... for a few days. Since then, I only have heat sometimes. A friend of mine just changed the radiator for me and while I was driving it this morning to work I had a little heat for a few miles and then it started blowing cold air again. Heat gauge never moved above half-way.
Any ideas?

 

Need vehicle specs (Year+Model+Engine+Trans+...) in thread+signature; makes asking+asnwering questions much easier+quicker.
Your profile = 2004 Ranger V6~4.0L~SOHC, an 18+ yo vehicle?
If you've not already replaced any FactoryOE CoolingSystem components, it's time, they've more than done their duty.
Don't trust InstrumentCluster TempGauge; by the time you see it move high, engine has already overheated, get an OBD2~BlueTooth~SmartPhon setup.
JBWeld on a cracked\leaking Radiator is just asking to burn up your engine; install a new Radiator.
* FanBlade+FanClutch If Equipped
* Pump
* AllBelts+Hoses+Clamps
* PlasticThermoStatHousing Swap To Aluminum Housing
* ThermoStat
* Radiator\PressureCap
* HeaterControlValve
* New Coolant
* ...

 

Reads like heater core is plugged or your by-pass valve is sticking closed

After engine is warmed up, 5-8min, open the hood
You will see the 2 heater hoses at the Firewall(cab)
Feel both, both should be hot
Feel upper radiator hose, that's how hot they should be

If one is much hotter than the other then heater core is plugged up
If both hoses are just barely warm then follow them to the By-pass Valve, it has a vacuum hose, unhook it
See if the hoses at firewall are getting hotter, if so someone goofed and hooked the wrong vacuum hose to the By-Pass Valve, plug the hose you removed with a screw or bolt, as its a vacuum leak


On a 1995 and up Ranger you have to pull out the whole dash to change the heater core, A LOT of work
So you will want to try and Flush the heater core FIRST
Cold engine
Remove both heater hoses at the firewall, there is no left or right or IN and OUT, so no "order", but in future you should Reverse these hoses every 2 years to reverse the flow thru the core to keep it cleaner
Use a garden hose and flush the core in BOTH direction, try to get better flow
Remove a hose from By-Pass Valve and get a funnel
Put hose on either firewall heater core port and blow into it to get the water out
Mix hot water and CLR or Vinegar, pour it into the core
Let it sit 20min then Flush it out
Repeat until flow is better



In 1995 Ford also change to an Electric Blend Door, very very common failure
Google: Ford Ranger blend door actuator

If you have a **** on the dash for COLD and HOT then you have a blend door actuator
These fail at an alarming rate, lol, in Ford Cars and Trucks
You lose air temp control when they fail
I doubt that's your issue from your description, but it will be down the road