Warm HEAT - Weird plenum issue

[homermandy]

1998 Expedition 5.4. Here is what has been done:

Replaced radiator
New water pump
Blend door verified to work
Recirc door behind glovebox closed.
New thermostat

Engine gets to operating temp, all hoses hot,both heaterc core hoses hot.

When I began looking at the blend door, after I cut the opening behind the glovebox, I noticed a lot of COLD air in the plenum. Given that everything else checks out, any idea? The recirc door to the outside is closed. I have no idea where this cold air is coming from or if it is normal?

My gues is that this cold air is mixing with the heater core air and giving me warm.

Any ideas?

 

[stamp11127]

From what I remember the air inlet door is always open unless on max a/c.

From the service manual:

When MAX A/C is selected:

The air inlet duct door is at full vacuum, closing off outside air and admitting only recirculated air.
The panel/defrost door is at full vacuum and the floor/panel door is in the no vacuum position, directing airflow to the instrument panel A/C register (19893).
The temperature is usually set for maximum cold but may be heated if desired.
Air will be picked up at the recirc opening by the blower motor (18527). Within the A/C control set for maximum cold, airflow across the A/C evaporator core (19860) will be diverted past the heater core (18476) and then directed into the passenger compartment through the instrument panel A/C register. There is also some airflow to the AC side window demisters.
The A/C compressor (19703) will be enabled when MAX A/C is selected.
The blower motor is on.


When the FLOOR/DEFROST is selected:

The air inlet duct door is in the no vacuum position blocking recirculated air and admitting outside air.
The panel/defrost door is in the no vacuum position directing airflow to the windshield defroster hose nozzle
The floor/panel door is in the partial vacuum position, allowing airflow to both the windshield defroster hose nozzle and the heater outlet floor duct. There is also some air flow to the A/C side window demisters.
The A/C compressor will be enabled when FLOOR/DEFROST is selected to dehumidify the air and reduce windshield fogging.
The blower motor is on

Check the heater core temp with a IR temp gun since you have access.

 

[homermandy]

Just checked temps

Ok just verified recirc door to outside air is closed on heat mode. Also checked temps.

Upper radiator about 170ish and lower radiator about 165ish. Heater core inlet 170ish and return heater core hose 165ish.

Inside with the cutout behind glovebix heater core temps were exactly like hose temps.

Heat out the vents might get to 115 if sitting and fan on lowish.

I don't believe I have a cooling system problem. What kind of temps are you guys getting out your vents?

 

[1955moose]

I just checked heat temp in my 2000 expy, at vent. I put control on vent, shut off all vents except one next to radio. With SUV warmed up, heat at 90, fan on high, my gun about 1 inch from vent. Highest reading I got was 140 degrees, and that was increasing rpm to 2000 rpm. Hope that helps. What degree thermostat do you have? Should be 195 degree.


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[homermandy]

Thanks for the temp reading. I did get the 195 thermostat. I may permanently close the fresh air door behind glovebox to increase heat. Someone posted similar issues with cold air in that area and also did that. Will keep you posted. Thanks

 

[AbbadonTD]

Question:

the same or worse when driving vs parked?

 

[homermandy]

Well a $2 fix gave me hot heat. I tool a threaded rod and closed the fresh air door. I will remove in the spring. I now have 135 out the vents at 20deg. Before I closed the door I was getting about 110 but this did the trick. Hope it helps someone

 

[1955moose]

So it was the blender door. The one with the vacuum diaphragm on it?


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[homermandy]

The one behind the glovebox. Not the one controlled by the warm cold selector.

 

[1955moose]

Glad to hear you got your heat back. This is going to be a cold winter.


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[homermandy]

Quick update

I bit the bullet and replaced the heater core myself. Believe it or not it fixed the issue. Not sure why as flow was good through the cors, but if you have exhausted all options change the core. The heat is so hot I now have to turn it down. also removed the rod that I put in to close outdoor air. Unbelievable. Hope this helps someone

 

[1955moose]

It must have been partially blocked. Glad to hear you got your heat back.


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[braddaddy]

did you have same heat issue with both front and rear heat??

If so...did you change just the front core?