I have a 03 F150(5.4) and under load when the vents are being used the truck does into default mode of blowing through the front defroster. I was thinking with starting with the vaccum canister. Anyone have any ideas. Also where is it located?
I would also suspect vacuum. If you look on the passenger side in the engine compartment, most of the vacuum connections are there. Look for the line that runs in through the firewall and check it carefully.
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