vacuum line for (8V) knock box (long)

[drew@dyermaker.cs.uoguelph.ca (drew)]

I replaced the knock box on my winter beater (87 Jetta) last night to see
if I could track down an intermittent stumbling/hesitation/lack of power
problem. The engine is JH with RD pistons, G grind cam, RD knock sensor
ignition, and straight CIS (no cat, no K-lambda)

In addition to swapping the box, I was questioning the vacuum source for
the control unit.  I had it connected to the vacuum port on the throttle
body where the vacuum advance was connected (intake and throttle body were
from a GX code A2 CIS motor.)  It seemed like the place to plumb the knock
box when I installed the system, and then I looked at a certain yellow 16V
MKI for a reference point and noticed on that engine swap the knock box
recieved its vacuum from the ports on the line to the brake booster (same
as the MFA.)

I T'ed into one of the lines coming off the brake booster line (one is MFA
and the other an in-dash vacuum guage on my car.) and tried it out.

The car ran fine on the commute this morning, which is not unusual, as it
often behaves fine when its colder, only acting up when its warmer (say
above freezing) and wet.  I should know definitively if the swap fixed the
problem on the ride home tonight, as it's quite a bit warmer now, and that
damned ice that was all over the roads this morning is gone so I should be
able to get into it a bit better.

My vacuum advance theory is a little fuzzy, as it stems more from by
air-cooled carburetted days (as short as they were.)  As I understand it,
the characteristics of ported vacuum vs. manifold (or full) vacuum are
pretty much opposite - manifold vacuum is high at idle and near zero at
WOT, and ported vacuum is near zero at idle and increases with the amount
of air flowing through the venturi - all of which is fine theory for a
carburetted engine.  The million dolalr question is does the CIS
throttlebody have a ported vacuum connection?   I seem to recall this on
older EGR equipped cars (that had the restrictor in them,) but I never
looked that closely at the newer ones.

Where does the vacuum connect for stock 16V CIS-E (8v CIS-E should be the
same location?)  What function does the vacuum feed for the knock box
perform?  My understanding was it allowed the knock box to sense load
based on throttle position, and that off-idle advance was controlled by
the closing of the idle-throttle switch.

Anybody have any input?

Drew


Drew MacPherson - '84 Wolfsburg Edition Scirocco TurboDiesel