Hello, I have a 1998 f150 4.6 that has a problem with the heating system. It's blowing out a little warm, but "far" from what it should be. ( on a scale from 1-10, i'd rate it about a 3) I am puzzled because I flushed the radiator, changed the thermostat, and there a no signs of heater core leaks inside,hose leaks on firewall, or radiator/water smell present. I don't know if this has anything to do with it, but I did notice the top radiator hose was hot, but not the bottom. I don't know what else to look for.

 

This is the 2004-2008 forum.

In answer to your question, assuming that your temp gauge is reading the correct temp after warm-up, The most likely causes would be a blend door issue or a clogged heater core.

 

Some vehicles have a valve that opens to admit coolant to the heater core when you select for heat. This is usually at the firewall in front of the heater core. You can find out a few things without any disassembly. Drive the truck until the thermostat opens, like a mile or two ending up where you started. Leave it running with the controls set for heat. Feel the supply and return hoses for the heater at the firewall - they should be hot. If there's a valve you should see it at this time. If the supply hose is not hot, there is no flow through the heater core, for whatever reasons, like a bad valve or a clogged core. If you think the core is getting hot, the indication would be the blend door in the heater plenum assembly. Sometimes you can find a lever with a cable attached that moves the door, and just set it one way or the other to get heat, but this is a stop-gap measure until you can restore proper function. There could be vacuum lines that control the baffles, and on older vehicles they tend to have leaks.

 

There's nothing on the firewall other than the hose connections. One hose is hot, but the other is not. I'm not sure which one is is going in or out. One is slightly higher than the other, and that's the one that get's hot. If that one is the "in-flow" hose, shouldn't it be putting out heat being that the fan blows good?

 

If the core is not clogged one hose will be hot and the other pretty warm. If one is cold you have little or no flow through the core. The hot one is in, cold is out.

This happened to my daughters Taurus a couple of years ago and I was able to take off the heater hoses and use a garden hose to force water through it in the opposite direction of the normal flow. All sorts of rusty looking sludge came out, and the heater worked fine afterward.

 

Yep, try that. If I had to guess, I'd say that if that works, the core will start leaking soon so I would plan on replacing it soon. I mean, if you get a lot of rust, etc. out of it with a back flush. It will be interesting to see what does come out of it.
Reminder to self: its time to change my coolant!