98 dodge 4x4 1500 when heater fan is on high it blows colder air didnot do this till had heater core replaced and in ac mode when giving a lot of gas like to pass ac air jumps up to defoster let off of gas go back to dash vents why

heaters are called heat exchangers, from hot engine coolant 180f to air in the cab box, called AC/heater air distribution box.
so if scans show the ETC engine temps are good, then most times
the cause is in the cab, heater box
top cause of fails in the cab (tests for all) ask
are (add word heater to all) blower works so not that.

• heater damper stuck, closed or nearly so, at core heater
• core is packed full of rust and gunk inside corfe.
• core air side fins packed in, leaves, or insect nets or ?
• controller failure to heater damper failing.

if both fire wall heater core tubes are both 180f then core is no bad in the water side, if one is way cooler the core is packed, up
in that case we back flush it hard. HOses off)

Could be low on antifreeze. The heater core is the highest spot in the cooling system and it may develop an air pocket. Another possibility is the thermostat is stuck partially open and doesn't let the engine warm correctly, watch your temp gauge on the dash and see if it comes up to the middle like it should while idling. If it doesn't, it may need replacement.

fix coolant leak where hose or tube goes from overflow jug to the radiator cap and replace radiator or overflow jug pressure cap see what happens.if problem still exist your blend air door stuck or air door vacuum or electrical actuator not working not receiving vacuum could have broken vacuum hose or faulty mode switch, could be many things.could have plugged heater core check both heater core coolant hoses both hoses should be hot.if heater core inlet hose hot and out let hose cold heater core could be clogged need flushing out.

the heater core does not have a fuse, it is passive. most likely it needs to be replaced. a good fan blowing over a cold core will not give you warm air. you need a hot core.

the fact that after you changed the part from the junk yard it blew warm air indicates that the heater core is not the problem
could be the heater control is not fully opening or the junk yard replacement part is faulty

OK, most cars have heater cores that get hot but the blender door directs air away from this core in the a/c mode when you want cool air from the evaporator.

I'm not sure if there is a control valve which stops the water from going through the heater valve.

I think the part is called a 'heater blend door actuator' that you are after.
http://www.autoanything.com/air-conditioning-65-861/201A1060.aspx?kc=AFFCJ&mr:referralID=c9ca506d-9f22-11df-bf8d-0026b95a1738

Sounds like the flapper valve for the rear heater/AC core is not doing its job to send the air through the AC core when called for.

Another clue of this is the fact that the air is coming out on the lower vents when it should be up high fro AC.

Have someone listen back by the fan inlet in the back usually on the right while you switch between AC and heat to see if they hear any noise or change in air flow.

Worst case is that the interior panel there has to be removed to service the assembly behind it.

It seems to me if your heater core is hot, and the fan blows cold air, then your ducting is closed off. Try and see if your duct door is opening up. You say you changed out your controls, but it sounds as though it wasn't your controls that were bad, but what they control is not opening up.
It's all well and good that the heater core is getting hot, but if the fan cannot deliver that hot air passing through it, then walah.
It has to be an air duct obstruction. Either the ducting is collapsed, the duct door is not opening, or you have some obstruction blocking the ducting. My guess, because that is all I can give you without seeing for myself, is that the linkeage or solonoid that opens the duct door is not working. Older cars had a cable, but I believe newer use a solonoid to operate the opening and closing of the door. The door is there so you don't get the heat from the core when you are using AC.
Hope this helps.

the blend door is roughly in the middle of air distribution Plenum assembly between the heater core and the AC evap core, it either directs air through the heater core or the AC evap core or mixes air from both.

heater core may be clogged. disconnect the hoses. CUT THEM OFF THE HEATER CORE!!!! If you pull them you may damage the solder joints at the tubes causing a leak. Them connect a garden hose and flush one way then another till water runs clear.