Re: what Rich Wagner said

["Rich Wagner" <drsavage@domain.elided>]

Eric wrote:

>I'm slightly confused about open loop mode and how the system enters
>it. Is the shorting of the full-throttle switch leads also a way to
>arrive at open loop? And if it is not, what is happening when this
>switch is closed or the leads shorted together?

The L-Jetronic system has two basic modes of operation:  open- and
closed-loop.

In open-loop mode, fuel is delivered based solely upon measurements made by
the Air Flow Meter, and calculations from the internal fuel delivery map. 
The injection system operates in this mode when it can't trust the readings
from the O2 sensor, when a strong power transient occurs (strong
acceleration), when the wide-open-throttle switch is closed, or (on
Milano's and those GTV6's which have the dual-position switch) when the
idle-throttle switch is closed.

In closed-loop mode, the system still makes its fuel delivery calculations
from the map and measurements of the AFM, but it augments those
calculations with an adjustment based upon the measurements made by the O2
sensor.  Based on the O2 sensor's output, it leans or enriches the mixture
(from what the map says it should be) enough to keep the mixture just on
the lean side of peak.

In closed-loop mode, the system is self-correcting, and will keep the
mixture set to give a balance of low emissions and decent performance
regardless of the outside conditions.  In open-loop mode, the system is not
self-correcting.  It doesn't test to see how good a job it's doing.

The open-loop mixture of the L-Jetronic system can (and should be) set by
adjusting the bypass screw on the AFM.  Turning the screw in reduces the
amount of air that gets passed by the AFM, and enriches the mixture. 
Backing the screw out leans out the mixture.  According to the Alfa shop
manual, the correct way to set the mixture is to unplug the O2 sensor
(forcing the system into open-loop mode), sample the exhaust upstream from
the cat (there is a plug just for this purpose), and set the mixture to
yield a CO output of 0.5-0.7% for the 2.5L engine, 0.5-0.9% for the 3.0L. 
This places the open-loop mixture just on the rich side of peak.  (Peak
mixture, depending upon the blend of fuel, yields about 0.25% CO output.)

So you see that, properly set up, the system will go from slightly lean to
slightly rich when it switches from closed- to open-loop mode.  (When the
O2 sensor is disconnected, or when either of the throttle switches are
closed.)


>The reason I inquire is I'm encountering not reduction of power but a
>great increase of power in the 1500 to 4000 RPM range. With leads
>properly attached, the car has limited power under 4000. With leads
>shorted, the car is all the way up the range. I am certain it's
>running rich with shorted leads.
>
>The fact that the engine has Sperry Stage 3 heads and what may be hot
>cams has something to do with this phenomenon. The car has the stock
>Bosch brain. Could it be that this setup has exceeded the range of
>normal Bosch L-Jetronic delivery maps?

In my experience, once the AFM is properly set, it doesn't drift much with
age.  If yours hasn't been adjusted, or better yet, "adjusted" by the PO,
it should work well with a factory spec engine.  Your engine, on the other
hand may be modified enough to make a difference.  With enlarged intake
ports, increased lift, and increased duration, the engine will rebreathe
below a certain RPM.  That is, some of the intake charge will be forced
back out of the cylinder and into the intake runners.  On the next intake
stroke, this already fuel-charged air will combine with the incoming
charge, get doubly fueled, and make the mixture go rich.  If the valves and
cams are highly modified, the mixture can go REALLY rich--so much that the
idle is unstable, and the plugs build carbon.  It's possible that the PO
adjusted (leaned out) the AFM to provide a more stable idle.  In that case,
I would expect the mixture to go lean above a certain RPM.

Note that, because of the dynamics of the AFM, and the design of the
L-Jetronic system, the mixture on these cars does go pretty lean under
acceleration.  However, I don't think the difference between open- and
closed-loop mode should be very dramatic.  You may have to play a bit
before you figure out what's going on with your car.

One thing to try, is to connect a digital VOM to the disconnected O2
sensor.  Connect the positive lead to the O2 sensor's output wire, and the
negative lead to ground.  A voltage reading above 0.5 volts is rich, below
0.5 is lean.  If you run the leads back into the cockpit, you can watch
what the mixture is doing as you drive.  Just be careful not to become too
distracted!


Rich Wagner
Montrose, CO
'82 GTV6 Balocco

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