I haven't seen an answer to this question after a quick scan so I'm posting a new thread.

On the way into work this morning I noticed the heater would blow warm than cold than hot and all over again. It seemed like RPM would affect it but not 100% sure.
Truck was fully warmed up and did not overheat.
I have a new thermostat and housing but the rubber hose coming off the housing (about 8" long with that stupid 90 degree bend) is in need of replacing and has a small leak. I have a replacement but I couldn't get it on within the 30 minutes of trying and just used the old one.
I top off coolant every couple of weeks (usually about a quart). The last top off was about a week ago.
Last September I replaced the radiator, fan, clutch and belt.
Last February was the thermostat and housing.
The controls are working for direction (defrost/face/toes) but the fan speed has always been on or off. Works on High but shuts off on all other speeds. That doesn't bother me, I'm used to it.

I'm hoping its something simple like the water pump but I'm worried its the heater core.

Any ideas/advice?

Thanks in advance!

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'02 Super Cab, Auto, 4x4, 4.0L, 195K+ miles
Glass delivery truck

After replacing all those parts, it might be the temperature control. Turn your key on and cycle it, you should hear the door both ways. Possibly shorting and cycling while your driving. Maybe you need a new/used heater control unit. Get your coolant leak fixed.

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2005 FX4 II
Dark Shadow Gray
Black leather buckets and interior
L/S 8.8" 4.10
5 speed Auto

What am I listening for when cycling (I'm assuming you mean turning the temp control knob back and forth with key on but not started)?
I haven't checked temp of heater core. Wouldn't that be difficult if it's all covered up (or am I missing something obvious)?

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'02 Super Cab, Auto, 4x4, 4.0L, 195K+ miles
Glass delivery truck

Yes, not started. Listen for the door to cycle. Heater core is the two hoses going to it.
But you said you get good heat but it goes warm, cold, hot.

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2005 FX4 II
Dark Shadow Gray
Black leather buckets and interior
L/S 8.8" 4.10
5 speed Auto

Near perfect description of heater behavior when there is air in the system. Despite the leak, you can verify this by......properly purging and filling the cooling system. This will answer the question unless that leak "isn't so small", and worse yet, if the cooling system replaces lost coolant with air every time the engine cools down. This could happen because drawing air through that leak might be easier than drawing coolant from the burp tank, and thus, air is introduced into the cooling system and...."the problem" of wandering heater temp output. Because of this, that leak really needs to be fixed to eliminate a potential cause of the prob.

Recommendation based on experience......that hose with the 90* bend....it needs to be Motorcraft thanks to the 90* bend being pretty much a right angle rather than having a bend radius (look how tight it fits into that space). Good luck with the non-Motorcraft hoses (Dayco, for example), been there tried that.







The most common cause for this is the resistor pack has failed. It's located in the airstream in and on the evap case, simple to replace and inexpensive. Removal of the part can confirm failure, as will usage of a DMM and with the part in place (faster).

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2004 Ranger FX4 Level II, 5R55E, Sonic Blue Pearl

Proudly JAP/RICE free since March 2000

Great advice - both of you!
I'll look for the Motorcraft brand and the resistor.
How does one "properly" purge the system?
If there is a video on YouTube I can look it up when I get a chance tonight.

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'02 Super Cab, Auto, 4x4, 4.0L, 195K+ miles
Glass delivery truck