Originally Posted by ctubutis

Check your voltage while you turn the vent on, I'd be suspicious of the fan motor sucking "too much" power.

EDIT: Is it the AC compressor coming on?

Originally Posted by 1986F150six

Or, referring back to the initial post, perhaps there is a leak [after the HVAC control valve] in the line going to the servo motor for vent position of doors. This would cause a lean condition when using vent.