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Old 07-20-2012, 03:07 PM   #1
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Location: Erath, Louisiana
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loosing vacuum duing 4-wheeling

I have a F350 4x4 Dually Crew Cab with the 7.3 lit. engine. When I turn my 4 wheel drive on to 4 low I loose my A/C dash vents and it gose to defrost vents. Does this mean that I have a leak from my vacuum line to my front hubs are my vacuum pump is wearing out? This is my first 4x4 truck I ever owned, any help would be appreciated.

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Old 07-21-2012, 11:44 AM   #2
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Join Date: Jan 2000
Location: Bakersfield, Ca. U.S.A.
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It sounds like you have a vacuum leak. Does this also happen when you are in 4 high? Or only in 4 low? If it only happens in 4 low it may be the switch. To help you out, with a helper and the truck running, place the truck in 4 low ,have the hood open and see if the pump is : 1. Is the pump running? 2. If you have a vacuum gauge see how much vacuum the pump has and maintains. 3. If running and has good vacuum then try a spray bottle and spray the hoses very from the pump to each hub ( use a lot of spray)if the vents change you will have found the leak. If not it may be in the hubs . They have an " Oring" in them.

Hope this helps.


99 F-350 4X4 PSD pulling a 24' Nash travel trailer
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Old 07-22-2012, 04:06 PM   #3
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Thanks, JRNOLL. I will test it after work tomorrow. I was hoping that it would be a hose leak. After this repare I will start on replacing upper and lower ball joints on both sides. Thanks again.
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