[urq] intermittent ignition cutout

[Ed Kellock]

What does the "decel valve" do?

It has 3 connections, one goes to the turbo to throttle body hose, another goes back to
the bottom of the intake manifold, and the 3rd is a smaller vacuum line which goes to the
vacuum tree on the top of the intake manifold.

I ask about its operation because this 3rd smaller hose is the one which had the worst
vacuum leak that I fixed a couple of weeks ago when the ignition cutout seemed to really
make itself known.

The relevance that I'm wondering about is based on the assumption that the smaller vacuum
hose to the decel valve is likely the trigger for its operation and because the hose was
split at least half way around at the intake manifold fitting, I figure the decel valve
wasn't previously doing much of what it was supposed to do.

It seems fairly obvious that its purpose is to manipulate manifold pressure before and
after the throttle plate.  Yes, no, sorta kinda?  Does the decel valve operate at all like
a bypass valve?

Earlier today I began to wonder about the closed throttle, open throttle nature relative
to my ignition cutout.  In addition to acutating the idle switch, this activity also
causes manifold pressure to fluctuate.  I sat down and started rereading the IST docs and
began to wonder about the pressure sensor in the ecu.  I was pretty sure it has the
ability to affect ignition and I went back thru some old email and found this from Dennis
below which states that the pressure sensor can cut ignition when it thinks there's
overboost.

It seems I may have discovered a possible tangible link between opening and closing the
throttle and the ignition cutout.  I'm thinking it's a big maybe.  Anybody care to weigh
in on this?

Ed


----- Original Message ----- 
From: DGraber460 at aol.com
To: ekellock at adelphia.net ; urq at audifans.com
Sent: Wednesday, March 23, 2005 8:28 PM
Subject: Re: [urq] intermittent ignition cutout

I'm purely shooting from the hip here, but my impression is that the only true ignition
cut off these cars have is the over-boost pressure sensor in the ECU, and oil pressure
loss? The pressure transducers do go bad once in a while from what I hear. The load
reduction relay will not cut ignition- only lights and accessories, and the fuel pump cut
off will not make the tach drop to 0.
I would try to pinch off the tube going to the ECU, eliminating any input to the pressure
transducer, and see what happens.
The flywheel sensors are the only other "usual suspect" in my thoughts. If the ECU can't
"see" TDC, it will cut spark. Why that would be heat related is strange but could happen I
suppose. The sensors on the head will only make it run rich or lean, but won't shut it
down.
Your car has taken on the early recalcitrant behavior mine showed early on. Defying logic
and being somewhat of a bitch.
Have you swapped distributors with a known good unit yet?

Dennis
Denver


In a message dated 3/22/2005 10:28:09 P.M. Mountain Standard Time, ekellock at adelphia.net
writes:

Had hoped that my IC and burnt wire adventures would yield some change to the cutout.
Still chasing the wire, so won't know for sure until its resolved.

But, I've gotten a pretty clear fix on the behavior of the cutout, purely subjectively of
course.  The cutout does appear to be temperature sensitive to some degree, though not
summer heat temp type sensitivity.

The car starts up beautifully and runs nicely as it warms, but as it does, the cutout
manifests.  It's almost always momentary and as long as it's not a idle (even sometimes if
it is), the cutout is short enough that the engine will catch and continue running.

I bought some resistors to put in place of a couple of sensors so as to fake out the ecu
and either eliminate or isolate the issue.  In looking at the tdc and rpm sensors, there
is a 3rd wire for ground.  I haven't quite sorted out how to rig that to fake out the ecu.

I have swapped in a spare ignition module (thanks Dennis!), however the behavior was
unaffected.

The cutout is definitely electrical in nature.  The tach drops to zero and I think even
the fuel pump quits, though I'm not positive about that.  I replaced the ignition switch
and the fuel pump relay about a year or 2 ago while diagnosing a no-start situation.

I've been wondering about the load reduction relay.  Mine appears to be original.  I do
hear a click when the ignition cuts out.  Haven't been able to isolate it yet though.

Just a recap and thinking out loud.  I'm open to your thoughts.

Ed