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chris
chris, Ford Mechanic
Category: Ford
Satisfied Customers: 3521
Experience:  ASE Master -Ford senior master-20 years experience
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F150 5.4L: I have a 2001 F150 that when I accelerate hard the

Customer Question

I have a 2001 F150 that when I accelerate hard the vents switch from the panel to defrost. I have checked the vacuum system and replaced damaged vacuum lines and the check valve but it is still switching to the defrost vent under acceleration
Submitted: 1 year ago.
Category: Ford
Expert:  chris replied 1 year ago.

hi there

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you are on the right track , a very common place for a vacuum leak to happen for this system is in the engine compartment , next to the battery

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on the left rear corner of the battery

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its best to remove the battery and check those vacuum lines there , their is a thin black vacuum line that goes into the fire wall, use a flash light and carefully inspect this line for cracks, it only takes a small crack to make the problem, most people miss a small crack or hole in the line so take your time and inspecting the lines

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also remove the check valve and put it in your mouth and suck and blow in it , it should only let air go one way,

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also there is a vacuum tank that the check valve also supplies , make sure the line to it is ok,

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we can go back and forth on this page