87 5ktq cabin vacuum leak

["Doyt W. Echelberger" <Doyt@nwonline.net>]

Frequently, when my 87 5kcstq is just coming up to maximum vacuum in 4th
gear, a soft but audible 'bzzzzzt' will come from somewhere in the dash,
sort of on the driver side. When this happens, engine power sags during the
sound, but doesn't chop off totally like when the fuel cut-off activates.
This event is happening when I am holding the accelerator pedal constant
and the engine is just starting to level off and float at that setting,
when the vacuum is at it's maximum.

I don't think this is classical fuel pump cut off, because I have
experienced that at maximum boost with the wastegate lower hose clamped.
This power sag isn't even at a time when I am using boost. Fuel pump
cut-off is far more total and alarming.

This event happens about the same time (or after) the little yellow upshift
light would normally announce 'time to be in another gear.'   But the light
doesn't work on my car.

I suspect that there is a vacuum leak in some component, which appears at
high vacuum, and which is associated with the acceleration sag. Or maybe
there is a valve in the system which senses maximum vacuum and which cuts
back or cuts off fuel, like a deceleration valve.  Another candidate is the
CC solenoid control assembly behind the glove box. I got this same noise
several times when I switched on the Climate Control panel, when
approaching peak vacuum. In my opinion, this points toward those controls
or the peripheral components.

Another candidate is the little yellow dash light system that tells me to
shift gears(whatever that is); cruise control vacuum lines; and multiple
'other' devices and components (which I am unaware of as yet.) I also
suspect a check-valve between the egg-crate vacuum reservoir in my driver
fender, and the rest of the system. And I don't know where this check-valve
is.

More confusing information: This seems more frequent on really hot days.
Cold weather seems to drive this gremlin into hiding temporarily.

My strongest candidate is the solenoid group for the vacuum servos, and the
lines to those servos, and I have a Bentley, but it doesn't show me how to
get to the individual servos. I think I take out the glove box and look
behind it for the main module.  But the sound seems to come from the driver
side dash, so I would guess there is a vacuum line to one or more of the
individual servos under there somewhere.

So, anybody able to focus me a little tighter? Tearing into the dash is
something I've never done, and I'd appreciate any BTDT's.


Doyt Echelberger
86 4kq
87 5kcstq  153k