It doesn't hold vac like you said above ( Reminds me of my old Lincoln that had headlights that would stay closed as long as the vac was there.) The pump pulses for a few seconds after you dial to 4x4 and then again when you are ready to disengage. It will also pulse every time you shut down to alleviate any sticking hub problem. Pumps; not a vacuum.

 

OK I'm new to this whole forum thing, so if I i'm doing this wrong please forgive me. I've been reading you forum concerning the intermitent air flow through the vents. I'm having the same problem. I have bought a check valve and canister. The local Ford garage can't tell me the location of this check valve and canister. Can you help me or am I going to have to start tearing my dashboard apart and simply start looking?

 

The cannister is actually under the hood, right behind the battery. If you are standing at the passenger side head light with the head light at your chest, hood open, you see the battery, then this black oval cannister. replace it. worked fine from then on for me.

 

I just bought an 06 Lariat, having similar symptoms, but not intermittent. My vent selection will stay on defrost, even after selecting vent or floor. I can here something click when I select it, if I turn the fan speed down to one, it will cycle to vent, and voila, problem solved. I was thinking there was some type of solenoid that directs air to the selected vent piping. Looks like that unit would be right under the dash, has a little metal arm on a pivot. I have done no TS as of yet, but I will post up what I find.

 

Check the small electric solienoid valve that controls vacuum to the auto hubs. It is located on the pass. side fender under the hood. Sometimes a change in engine rpm will give you enough vac even with a leak to operate the dampers inside the dash. A lack of vac caused mine to go to defrost. I bypassed the solienoid valve under the hood and problem was solved. Its just a cheap plastic valve. I would check this first it is simple to do. The replacement cost is 74 dollars from Ford. Good luck.

 

Well, the truck had to go in for the turbo sticking and a whine anyway, so I talked to the tech. Told him bout the AC. There is a motor driven vent selection device that is used in the models with auto climate control (which mine has) But it took about 5 seconds to fix mine. There was a hose that had come off giving me a vaccum leak, (mine is 2wd, so no hubs to worry about). The hose was right up against the firewall, he said they come off all the time. Now my AC works great! No charge. Verdict on turbo whine should be in on Monday, hopefully they have to replace it all, only have 12k miles left on the warranty

 

Glad you lucked out on the vacuum leak. good luck on your turbo.

 

Turbo is good, they told me I need to "get on it" more often and run some cetane booster in it if I do a lot of around town driving.
As for the AC fix, when I picked it up, they charged me 50 bucks for the "repair" that I stood there and watched him fix. I was pissed, I got a call back apologizing for it, they even offered me a free oil change. I told them it was too late for that, there are 5 ford dealerships withing 25 miles of me, they just lost all future business from me.
Its back in the shop again for a complete rear diff rebuild. Metal shards in rear diff oil, whine, at least now I know why the previous owner traded it in. Wouldn't surprise me if they found sawdust in there as well. The dealer I bought it from offered to do the repair for cost of parts only...that was still more than I am having it done out in town with parts and all. We wonder why American car companies have been struggling........
over
Back to the actual topic

 

Could be the vacuum 'switch' that engages 4x4 front hubs. Mine was cracked on our 2003 diesel. I don't think you have a vacuum pump, but the net effect would be the same.

Look for small fist-sized black box with both vacuum lines and electrical connector on passenger-side inner fender. Betcha that is cracked and is leaking vacuum.

Oh...sorry. I did not read all posts. You found it in the loose hose.

 

Rather than starta new topic I thought I woudl try my luck with this thread first. I have an intermitted vacuum leak also.

2003 EX 7.3L

My ESOF hub lines are cut and capped, due to the hubs not working properly, so there is no leak there. It seems as if once the temperature outside drops below 40 degrees the vacuum pump runs as soon as I turn the key on. It does not do this when the temp is warmer. I have tried to diagnose the system previously with no luck. I hooked a pump upto to the vac line going to the HVAC system and it held vac. The lines under the hood and the reservoir seemed to be good also.

I am leaning towards a slow leak that wouldn't be noticable unless I left it on test over night. Being that this issue arises with cold temps, I am thinking that a line is contracting with the cold temp and slowly leaking. Has anyone else experienced something like this before?

 

Look also on the passenger side inner fender for a black vacuum switch about fist-sized. This can crack, leading to a big leak. This is "upstream" of your hubs.

 

Had this problem on my work truck f350sd withb v10. Was the check valve in the vacuum canister. Would switch to defrost everytime I pulled a hill or started of at a light. Not too expencive, two bolts and I vacuum line connection. No problem. Had it happen twce on same truck.

 

I am curious, how many miles did you have on the vacuum pump?

The aircraft I have been involved with normally get about 700 hours on a dry vacuum pump.

 

I know that this is an old thread but....
Wouldnt it just be easier to add a vacuum pump from a diesel truck and eliminate the canister. Then just wire it so it powers up with ignition. It would be a sure fire way to fix the problem wouldnt it? The reason im saying this is that I am having the same problem.

 

Oops double post.