5k freq valve

[Arryn Milne <arryn@home.com>]

Anyone know what this does or how to troubleshoot it?

On an '86 5kcsTQ, having what seems like a vacuum leak, but isn't.
Eliminated that by plugging every vacuum line I can find.  Using a
stethescope I used to hear humming from the Freq valve (few months ago),
but it is silent now.  Putting the steth on top of the black plastic boot
(from the fuel dist), I hear a humming sound (like I used to hear from the
Freq valve), but can't hear it on anything metal.

Thanks for any help,
Arryn.

PS: also sprayed carb cleaner along and around vacuum lines and there were
no changes in rpm.