henryfordwasagrandol

Hello! So I have a 2002 F150 Lariat with automatic climate control. Last summer i noticed it would stop blowing from vents as I accelerated (I tow lawn equipment) but as soon as I let off the gas would come right back out the vents. I have a 91 Ford that has done that for years on max AC so I didn’t think much of it. But it gradually got worse, taking longer and longer to come back also would go back and forth from defrost to panel vents even as I drove constant, even just a slight load would make it do that. Finally it stopped blowing out vents altogether unless I drove for 10 minutes or longer and then it was very sporadic. I took it to my mechanic and he diagnosed it as the control unit itself. Well, I didn’t want to spend $600 right then so I put up with it and then summer was soon over so I put it on the shelf till now. I found a refurbished one that they claimed had been tested, etc and it didn’t fix my problem. I do have vacuum going to the control unit but I don’t know if it’s enough. With it being such a gradual thing, it does seem like a vacuum leak but I have no check engine light on and the engine runs smoothly. I had it shut off for several minutes the other night and it still had vacuum in the system so if the tank has a leak wouldn’t it all bleed away? Could the mode door itself act this way if it was bad? Thanks for any help!

PapaWilly

Hi and welcome to the forum.
Follow this link: https://www.f150forum.com/f6/help-ai...7/#post7299689
Look at answer #3 where projectSHO89 explains how the vacuum system works for switching vents.
You can run a test vacuum line from the vacuum motor above the gas pedal, up thru the firewall and over behind the battery to the other vacuum connector to see if the line is bad.
I performed this exact step and just bypassed the bad line when I had a degraded vacuum line buried in the dash.
There are many links on this subject on here, so I'm attempting to save you some time.
Good Luck.

manicmechanic007

Your vacuum canister has a leak or the one way valve is no good IMO

mbb

About 10 years ago I had a similar type problem. Sometimes in the afternoon especially leaving work, hot day, my blend door would change, my cold air would quit and it would even blow hot air out.. you noticed that when it's 95° outside and 125 in your vehicle...... So there was something wrong in the vacuum controls. Losing vacuum when accelerate. I did absolutely nothing and the problem eventually went away..... Haven't had it happen again in years. Go figure.

Not suggesting you do nothing but there is a chance.....