Ac
Fleet Owner
Joined: Oct 2005
Posts: 20,783
Likes: 1,751
From: Kentucky
More than likely you have the infamous white vacuum supply line for the HVAC system that has aged and cracked. Here is a thread about the subject and resolution from another forum: A/C and/or heater stuck on defrost - FSB Forums
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Junior User
Joined: Oct 2011
Posts: 91
Likes: 0
From: Lake Butler Fl
ok i put a new rubber line over that white cracked line it blows nice and hard up to 65 and once i hit 70 75 if i let off the accelrater a lil bit and starts blowing in my face. could i not be getting a good enough vaccumn?
Logistics Pro
Joined: Dec 2003
Posts: 3,774
Likes: 860
From: Foothills of NC
Look at the check valve coming from the engine vacuum. It should be a big line coming in, then the check valve is a T. These will go bad, causing vaccum to leak back into the engine during acceleration. You should be able to suck through the engine side connector, but not blow through it.
Fleet Owner
Joined: Oct 2005
Posts: 20,783
Likes: 1,751
From: Kentucky
Don't forget the little plastic vacuum reservoir mounted to the side of the blower motor case. Mine was cracked. These trucks are way under powered, low to almost no vacuum at higher speeds is not unusual. A tired engine or poor state of tune brings these vacuum operated systems to their knees. You need to check the HVAC circuits with a vacuum pump/gauge. The thread URL I posted has some good info of where those vacuum lines go.
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