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This forum has been extremely helpful. I've tried all of the fixes for cabin heat except for the floor insulation:
Tunnel Insulation
Hood weatherstrip
Windshield caulk behind the fresh air vent
New fresh air vent
Checked hot/cold cable
Still get hot air through the floor vents, especially on the highway and the temperature at the front vents gets to 140 unless I turn the defroster on (without the fan). Then it goes to 120. Brrr. Anyway...for another forum. More important things now...
If you don't have A/C, then you don't have a heater shut off. A ball valve is the usual solution. If you do have A/C, the old metal vacuum controled valves suck...and the line usually breaks. Delco has a much better plastic valve as a service replacement.
Sounds like a great idea. Where is the hot air side of the vent?
I would assume the forward side. You said it happens when you're moving forward, cruising, that the heat coming from the vent gets very hot. Create a baffle that deflects that hot air like a backwards scoop.
Another idea would be to mount a small fan inside the vent, blowing outward, keeping the hot air from being sucked in.