mech259

 Posts: 1760
Joined: 1/22/2006 From: Lumberton, Texas Status: offline
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You probably have a vacuum leak. The supply vacuum from the engine should be what the engine vacuum is. Usually, the defrost position is the "at rest" position. If the supply is good, you might have a leak at the selector vacuum switch, which is obsolete from Delco and from Chevy. I know, because my 86 has similar problems. I found one in a wrecking yard.
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L.B. ASE Master Certified L1 Emissions Certified 1986 C4 1969 RS Z28 Camaro 1970 Chevelle SS 396 (under construction)
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