Heat stuck on
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New User
Joined: Aug 2004
Posts: 16
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From: so cal
Heat stuck on
I have a 1987 F-150 4X4 4.9L and the heat is stuck on and it will only blow hot air even when all the switches are off. I was told possibly the heatercore. If this is correct is it a difficult fi
Im 16 and cant afford to take it in). Any feedback is appreciated.
Thanks
Im 16 and cant afford to take it in). Any feedback is appreciated. Thanks
Senior User
Joined: Mar 2003
Posts: 288
Likes: 0
From: Northeast Wisconsin
My son has an '87 F150 I6 2-wheel drive, so I'm assuming his is much like yours. Open the hood and find the two hoses in the cooling system that run to the heater core. They'll pass through the passengers-side firewall. One of them will have a vacuum-operated valve that stops the circulation of the hot water to the core. That's stuck open. Pull off & plug (otherwise you'll have a vacuum leak to the engine) the small vac line going to the actuator diaphragm. Push or pull the actuator rod on the diaphragm in the opposite direction from where you find it (wire or hold it there), run the engine, and see if you have heat. If you still do, the damper in the valve is broken or rusted off.
It's NOT your heater core. The valve is either broken or (most likely) just stuck open. With no vacuum to the diaphragm motor, the valve should be shut (this is the default position, ie, no heat).
Please post back as to what you find.
It's NOT your heater core. The valve is either broken or (most likely) just stuck open. With no vacuum to the diaphragm motor, the valve should be shut (this is the default position, ie, no heat).
Please post back as to what you find.
Senior User
Joined: Apr 2003
Posts: 280
Likes: 0
From: Iowa
Yep, it aint the heater core.. sounds like the heater core is workin great! 
my '94 uses a blend door instead of an actual cutoff valve (which I like.. I always had problems with the valves in other cars)
You can always clamp shut one of the hoses! --> preferrably the inlet hose
my '94 uses a blend door instead of an actual cutoff valve (which I like.. I always had problems with the valves in other cars)
You can always clamp shut one of the hoses! --> preferrably the inlet hose
