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Old 09-04-2009, 07:20   #1 (permalink)
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mind of its own or vacuum leak?

On a 2007 f 250 with a 5.4 gas, does the vacuum control the settings on the heater controls? When driving it will switch to the defrost setting on its own no matter where you put the knob! You can turn it anywhere you like and it makes no difference. The only way it will change is to let your foot of the accelerator. Works every time, have tried it many many times. It seems when there is a higher vacuum it works and when you are up to speed and ease off the accelerator and loose your vacuum is when it does it. Also when you first start it I swear I can hear a little vacuum squeal for only about a second or so when going from cranking rpm to idle rpm. any one have an idea where i might look? The squeal is from under the hood I think, but I cannot find or hear it just sitting there idling
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Old 09-04-2009, 09:23   #2 (permalink)
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Re: mind of its own or vacuum leak?

It definitly sounds lik a vacuum problem. If there's no leak, the check valve is bad.
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Old 09-05-2009, 09:10   #3 (permalink)
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Re: mind of its own or vacuum leak?

I had a look at some of the vacuum lines. There seems to be lots of vacuum at them. (never put a gauge on them so I dont know what numbers its pulling) but when I disconect the hose it also flips to defrost. Where is the check valve that you might suspect? Also has anyone ever seen the actual control form a leak around the knob?
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Old 09-05-2009, 09:47   #4 (permalink)
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Re: mind of its own or vacuum leak?

I don't have access to a vacuum diagram for a 2007. On ones I've worked on, they're in a line running from the vacuum manifold to a passenger side source. Follow the lines and you should see it. The reservoir was in the passenger side fender area also. There could be a leak on one of the lines inside the cab too. If it sudenly started after something was installed or disturbed. Check that area.The control switch could leak but that would be extremely rare. You'd hear it inside with the engine running. If you get a Haynes guide it might have what you need to check it out better.
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Old 10-04-2009, 12:07   #5 (permalink)
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Re: mind of its own or vacuum leak?

I have the exact same problem in my 2006 Mercury Grand marquis. Will be interested in seeing how you get this sorted out.

Mine works fine at idle when in park or at 2000 RPM's when in park. It's only when I'm putting load on the engine does it switch to defrost and stay there until I let off the gas.
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Old 10-04-2009, 12:45   #6 (permalink)
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Re: mind of its own or vacuum leak?

It has to be a vacuum problem. The lines, reservoir, check valve or control valve. Most likely it's a line, reservoir or check valve.
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Old 10-21-2009, 21:05   #7 (permalink)
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Re: mind of its own or vacuum leak?

Thanks for the reply. I'm not a mechanical idiot, but any tips on where I should look for the lines under the hood? Also, where might the check valve be, canister/reservoir, vacuum tree...

I'd really appreciate the help. I just spent $800 getting a blend door fixed on my 98 MGM, then 6 months later, my 06 MGM started doing this. The 06 had about 50K on it when I noticed this happening.

Might start up my own thread for this, but the problem is identical to Boscat's, just a different vehicle. 2006 Mercury Grand Marquis.
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Old 10-21-2009, 21:22   #8 (permalink)
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Re: mind of its own or vacuum leak?

The tee is on the firwall about center. The check valve is too. You'll see a small plastic line running into the passenger compartment just to the passengers side. You have to follow the lines, because it could be anywhere along the route. We've had at over a hundred Crown Vics in our fleet and never had a vacuum problem or blend door motor go out so I can't say "Look here because that's where there's a weak spot."
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Old 10-22-2009, 19:55   #9 (permalink)
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Re: mind of its own or vacuum leak?

As of right now I am suspecting a vacuum leak. A buddy of mine has a work truck with the same vacuum set up as mine, and when we pull the vac lines of the little unit which I believe to be the check valve (?) it still has vacuum to it even after a day of sitting. Mine never has any even if only sitting for 10 min. So we took that check valve off his and put it on my truck, went for a drive, and it seem to work, also had vacuum when he wouldnt let me keep it and took it off. Is that what you would call the check valve, mounted on the right side fender? Has two vac lines and three wires going to it. On my buddys 09 it is mounted on top of the rad, but had the same part # on it. And any idea what they are priced at in the states? Here in canada they are about 185.00 dollars which if it fixed it for sure would be o k, but I am still not convinced for some reason.
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Old 10-22-2009, 20:29   #10 (permalink)
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Re: mind of its own or vacuum leak?

I've never seen a check valve with wires to it. They do usually have 2 vacuum lines though. They are usually on the firewall and a reservoir
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