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sgtfleming
New User
Jun 13, 2009, 9:13 AM
Post #1 of 2
(3297 views)
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1999 F-150 AC problem
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I have a 1999 F-150 4.2L 6cyl. The AC blows cold, but when I accelerate, I hear a "thump" and it blows hot until I turn it off and then back on. I've charged the freon, replaced the low pressure switch and changed the AC clutch. What should my next step be? Thanks, Mike is hot in Houston.
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Hammer Time
Ultimate Carjunky
/ Moderator
Jun 13, 2009, 12:23 PM
Post #2 of 2
(3293 views)
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Re: 1999 F-150 AC problem
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It sounds like you have a vacuum leak in the door control system.
The mode selection is done through a series of air doors that are operated by vacuum. When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.
Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.
Note that the F150's are famous for a white hose in the passenger side rear of the engine compartment rotting out. It goes to the recirculation door.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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