When I give it gas in my 2001 F150, the AC blows through
the defrost vents, untill the RPMs come down then it blows
back through the AC vents. Sounds like a vacume problem
to me . Has anyone run into this?

 

A very common question that comes up every week or so.

SEARCH keywords defrost vacuum, etc...

Steve

 

I did that search and couldn't find what I need. I can't find that leak to save my life.... Any suggestions?

 

There's nothing wrong with that search. Returns 4 pages of mostly relevent posts.

OTOH, while a vacuum leak is, by far, the most common reason for this symptom, it is not the only one. Additional faults may be due to the AC control head, whether manual or auto, or the vacuum motor. Some versions of the F-series platform also have vacuum reservoirs or checkvalves in the vacuum circuit. If a 4 X4, the front hubs are vacuum operated and are fed by the same line as the climate control.

The service manual has a step-by-step dignostic procedure. The first steps involve testing the vacuum supply, it's lines, check valve, etc.....

Steve

 

I sure wish I could helpyou w.shearer, I have been fighting this problem for a month or so now. It about has me ready to pull the rest of my hair out. I had the glove box out last night along with the black plastic piece on the transmission tunnell, the cover on top of the dash, the control head out, the battery out checking all the lines under the hood and still have the same problem. I did not find any lines split or cracked, the vacuum box behnd the battery is o.k., but, if I turn the a/c on max a/c, I can reach under the drivers side dash and hold the arm down and this lets air through the dash vents. If I let it go, it will gradually move back to the defroster. I was wondering, where the little black line goes through the firewall, is this a check valve or just a fitting? If I find my problem I will definately post it on here with pics, but, any help would definately be greatly appreciated.
Michael

 

Ok. Can I pressurize the vacuum system with something, and see if I hear a leak anywhere? It's hard to find leaks with the engine running.

 

OK guys I just found out what my problem was. It was a 90* rubber boot that connects the pcv tube with the intake. It had a huge hole in it. If anybody needs some more info, just pm me.

 

Surprised you didn't have a CEL and lean codes.....\

Steve

 

Same problem as W Shearer. Any word yet W on yours? I only get air through the defroster under heavier than normal accelleration, like a big hill or a sudden passing lane change.

 

See post #2.

 

It got worse and it finally started making the truck idle crappy. That's when I finally got a code. It was loud enough for me to locate it with starting fluid. I never traced the pcv valve line,(yeah dummy me) and I didn't spray any starting fluid behind the engine. All you guys with the a/c problem need to check that hose. Turns out that it's a VERY common problem.

 

Every time I remove my A/C controls and unplug the lines that hook to it, I always have that problem, but the second time I start my truck after i hook everything back up, it corrects itself and the A/C blows out the dash vents.