RE: [T3] FI Wiring Harness and Lowered Compression

["Fuhriman, Mark" <mfuhriman@fugro.com>]

> I had no pressure swtch installed in my car so I have no holes
in my
> runners. What
> is this made of ? A rubber diaphram ?

Leon, Your 69 would have come originally with a pressure switch,
whose function is to enrich the mixture under full throttle.  The
switch is a vacuum-actuated diaphragm.  At full throttle, the
pressure in the manifold approaches atmospheric.  The pressure
switch senses this pressure change and tells the brain to enrich
the mixture.  It's mounted under the RHS intake runners.  There is
a small hose that goes to a vacuum pick-off on the manifold.  This
pick-off is well hidden under one of the RHS intake runner
connections.

This is a 68-69 feature only.  See Fig 7.4 of the fuel system
chapter in Bentley.  This thing is wired directly to the ECU.  So
you must have modified your FI wiring harness, or maybe you or a
PO changed your FI system (including the wiring) to the '70
version that takes the "C" brain and a throttle valve switch that
does the enriching at full throttle.

If you no longer have a pressure switch but still have the 69
manifold, you may have a vacuum leak at this small vacuum hose
connection if it's not already plugged with something.  Check it
out.  On my '69 I once had fuel economy problems (around 20 mpg)
until I discovered the missing hose for my pressure switch.  When
I connected it up right, the switch finally worked right and my
mileage shot up to around 25mpg for around-town driving.

As for your pressure sensor experimentation, ... well, I don't
think I'd have the mettle to play around with it so much.  I
wonder what would happen if you found a "normal", unadjusted
pressure sensor that is matched to the rest of your FI system.  I
wonder if you'd see an improvement in performance/MPG.  But if you
can really get a pressure sensor made from parts scavenged from
two different cars calibrated right through trail and error,
you're a better and more patient man than I.

-Mark Fuhriman

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