2001 E450 SD Vacuum check valve??
Hello...I have a 2001 Ford V10 E450 running my class C and just this weekend we were driving and notice the fan for AC or Heat or vent will go from dash air to defrost when the motor is under stress l...
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I had the same problem with my 2005 E450. Their is a RED plastic vacuum tube under the hood on the passenger side near the oil fill & the windshield washer fill. I installed a checkvalve($5.00 at Auotzone)between the red plastic tube and the black rubber hose it goes in to. Use a small zip tie to secure the checkvalve to the rubber hose. Worked for me.
