hi any help would be greatly appreciated.
I have a 93 f150 and the heat blows warm at best. a/c works fine, all vents blow but only warm. I have checked the hoses coming in and they are both hot. The heater core is hot as well. when I take the cover off the heater core there was a mixture of cold and hot air. Cold air coming from the left. The heater core itself is hot. The ***** all seem to function ok. When I switch from floor to vent I notice there is a split second of really hot air then back to warm??? Any ideas???

 

find where the mix or blend door is and check that

 

Check your vacuum hose going to your heater controls too. Don't forget the one by itself over on the far passenger side on the firewall. The plastic lines are very prone to break and can be hard to find when they are under the plastic conduit.

 

welcome to FTE.
as already said, check the vacuum lines.
but i think you are going to find it a little harder than that to fix.
my bet is a stuck blend door.
open the heater box, and remove the core. reach inside the box and there are two doors. one to let air into the heater core or to the AC vents that is controlled by the temp ****, and one that switches from floor/vents/defrost.

the temp blend door gets jammed by junk that falls down the defroster ducts in the dash.
it is tight, but you can reach in there to pull all the crap out. i have pulled pens, pencils, change, petrified french fries. you name it, i think i have pulled it out of there.

one truck had a door that was stuck after a full cup of coffee with way too much sugar in it spilled down there and the sugar turned into super glue after it dried.

 

ok there is a broken white hose to the control valve that butts up to the firewall. Now I manually moved that control arm and nothing seemed to change. I just adjusted the cable adjustment clip for the thermostat control. It seemed to help but not sure yet as I have not drove the truck. It was running for about a half hour.... Do I have to take the heater core itself out to access the blend door that you mentioned? Again thanks for the help. It is greatly appreciated.

 

no, you do not. but it makes it a lot easier to remove it, and it only takes an extra 10 minutes to get it out of the way.

i forget what exactly that vacuum control does, but it has nothing to do with your lack of heat.
you should still fix it. even though it is a very small line, it is still a leak that can cause other problems.

 

A great diagram of the HVAC system from Subford:


Some more photos from Subford:


And one more: