[urq] intermittent ignition cutout, tach drops to zero

[Buchholz, Steven]

... sorry that I haven't been able to respond earlier ... 

Your indication that the tach drops to zero ... and I'm assuming that
this means that the engine is still turning because the transmission is
in gear and the car is coasting ... effectively you have lost power
while the car is in motion.  I'm pretty skeptical that there is any
connection between this and the vacuum work that you did.  The only
complete WAG that might explain it would be if the ECU itself were
locking up somehow ... but still I can't necessarily correlate this to
the vacuum system work.  

I just went back and read the post that started the thread ... one thing
that I'm wondering is whether or not there is any apparent correlation
to temperature ... if so, I'd wonder if the issue isn't with the
reference or speed sensors.  I would bet that you've already checked
this, but is the Hall Effect Sensor timing set up correctly?  I suppose
it could be marginal to the point where it might cause this ... but even
that is a bit of a stretch.  

I wish I had copies of the wiring diagram for the car ... I know that
there's an oddity in the ground reference between the ECU and the
ignition driver module.  I think some cars have a direct connection from
the ECU to a ground on the coil driver while others have the connection
left open.  Does anyone else remember this?  I think you can see it by
looking at the similar circuits on the urq and the '85 4kSQ in the
Bentley.  

No matter what, unless you happen to find one of these wrong you may
need to instrument things up a bit to try to get more information about
what is working and what isn't.  

One more thing ... I wanted to explore one of your other comments
relating to the OXS Freq Valve ... you said that the OXS Freq Valve
*started* working ... it should be buzzing any time the engine is
running.  It sounds like  perhaps you triggered the diagnostic mode by
closing the WOT switch at idle ... this forces the duty cycle to 50% ...
IME this causes the idle speed to increase.  If you are sure that you
did not trip the WOT switch, then perhaps there is a grounding issue
between the ECU and the engine block.  There should be a single ground
wire that attaches to the back of the intake manifold ... this
establishes the reference for the signals the ECU uses to determine the
engine parameters.  

... wish I could be more definitive!  Good luck Ed!
Steve B
San Jose, CA (USA)
> 
> Actually, my tach needle does not bounce around.  It drops to zero and
> stays there until the power to the ignition comes back.  I do not
> believe this is fuel related in any way.  My recent work had to do
> with vacuum leaks and I merely wondered if there could possibly be any
> connection, an intersection point of some sort in a single device that
> has both vacuum and power input/outputs... the frequency valve maybe?