heater and a/c problems 1998 ford expedition. - Ford Forums

11-16-2005, 19:01

when heat is on and i speed up it switches to a/c and vis versa. you can
here the heater box shut when it happens.

11-16-2005, 22:01

"delvil" wrote:

> when heat is on and i speed up it switches to a/c and vis versa. you can
> here the heater box shut when it happens.
>
>

you have a vacuum leak, or a bad check valve (vcv) in that line.

check all the way to the intake manifold tap.

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