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86 740 heat on all of the time?

posted by  Chris Mullet on  Thu May 15 01:20 CST 2008
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last visit: Fri Oct 24 09:08 CST 2014
[RELATED]

I believe the vacuum diaphragm works opposite of what you would expect - it takes a good vacuum source to keep the valve CLOSED. Verify that the vacuum hose is in good shape, is connected well, and has a good vacuum pull.

You could verify the valve itself by connecting a vacuum hose to it and running the hose into the passenger compartment. With the (warm) engine running and heater blower on, suck on the hose and maintain the suction for a couple minutes and see if the warm air coming out of the vents turns cool.

I "think" I'm right about this, but I could be wrong, in which case maybe someone else will chime in. Look at the old valve and suck on the vacuum nipple and see if I'm right or wrong.






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