Date: Sat, 11 May 1996 09:59:17 -0700
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: mwest@cdsnet.net (Mike West)
Subject: The Real FI Tuning
I apologize for previous. This is the real thing.
I hope it helps you, if it is of value, I'll repost to group.
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Diagnosing problems in computer controlled carbureted engines
>From: bohdan@uscbu.ih.att.com (Bohdan L Bodnar)
This is a description of the procedure I've used to diagnose air/fuel
mixture problems in computer controlled carbureted engines; the
outlined procedure can also be used to set the idle air/fuel mixture
without resorting to infrared exhaust gas analysis. The procedure is
based on the General Motors "System Performance Test" which was
developed for diagnosing problems in GM's carbureted engines. This
procedure will not work with carburetors which use a stepping motor to
control the a/f mixture.
Theory
The a/f mixture is controlled by a MIXTURE CONTROL SOLENOID (MC
solenoid). This is a valve which operates at a fixed frequency
(typically, 10 Hz) and whose duty cycle (valve's ON time divided by
period) is varied. That is, the valve is pulse width modulated. When
the valve is turned on, the incoming a/f mixture is fully leaned; when
off, fully enrichened. The former is called a "lean command" whereas
the latter is called a "rich command." By varying the duty cycle of the
MC solenoid, the AVERAGE a/f mixture can be varied. In GM products,
this valve directly varies the incoming fuel and air flow. In Fords,
only the incoming air is directly varied. In Chryslers, only the
incoming fuel flow is directly varied.
The valve has a two wires electrical connector. On wire is connected to
switched battery voltage whereas the other is connected to a power
transistor in the computer and is a source of switched ground.
During closed-loop operation the following will occur (assume the oxygen
sensor is sensing a lean condition -- its voltage will be low):
The computer gradually decreases the MC solenoid's duty cycle.
The exhaust eventually becomes rich enough that the oxygen sensor's
output will swing high (about 1 volt).
>The computer gradually increases the MC solenoid's duty cycle.
The exhaust eventually becomes lean enough that the oxygen sensor's
output will swing low (about 0 volt).
The cycle now repeats. A device for monitoring the solenoid's duty
cycle (such as a dwell meter) will show a constantly varying duty cycle.
The frequency of the oscillations will depend on the how fast the
computer varies the duty cycle and the engine's RPM. An AVERAGE duty
cycle of 50% corresponds to, on the average, NO average a/f correction.
Stated differently, everything is operating correctly. An average duty
cycle of LESS THAN 50% corresponds to, on the average, a rich command
(the computer is compensating for a lean condition). An average duty
cycle GREATER THAN 50% corresponds to, on the average, a lean command.
DIAGNOSIS AND SETTING IDLE A/F MIXTURE
Monitoring the MC solenoid's average duty requires (for most people) the
use of high impedance dwell meter. A low impedance dwell meter may be
used unless it affects engine operation; my recommendation is to not
use a low impedance dwell meter (that is, stay away from self-powered
dwell meters). Following the GM procedure, set the dwell meter to the
six cylinders scale REGARDLESS of the number of cylinders in the engine.
At this setting, 30 degrees will correspond to a 50% duty cycle, 60 to a
100% duty cycle, and 0 to a 0% duty cycle. Run the engine until closed
loop operation is present; this will be indicated by a varying dwell
(see footnote 1 for deviations from this procedure). Once the engine is
hot, not the average dwell -- the reading should vary equally above 30
degrees and equally below 30 degrees. The following is a brief trouble
listing:
DWELL NOT VARYING: system is operating in open loop.
>DWELL STUCK AT 10 DEGREES OR LOWER: full rich command is present;
the computer is compensating for WHAT APPEARS TO BE a massive fuel flow
reduction (check for dirt in carburetor, air injection system stuck in
upstream position, vacuum leaks, improper a/f mixture setting...).
DWELL STUCK AT 50 DEGREES OR HIGHER: full lean command is present
(check for float stuck low, valve seat damage, oxygen sensor's sense
lead shorted to battery voltage, etc.)
>DWELL OSCILLATING, BUT AVERAGE READING IS BELOW 30 DEGREES: average
rich command is present (check for vacuum leaks, dirt in carburetor's
jets, improperly set a/f mixture...)
>DWELL OSCILLATING, BUT AVERAGE READING IS ABOVE 30 DEGREES: average
lean command is present. Check for incorrectly set a/f mixture, float
stuck low, valve seat damage, clogged air filter, etc...).
Based on the above descriptions, it should be fairly clear on how to set
the idle a/f mixture: merely set the mixture so that the average dwell
is 30 degrees. Now, suppose the system's dwell is not varying, but the
sensors are working properly, the upper radiator hose is hot...
Several cars with small engines have the oxygen sensor mounted fairly
far away from the engines. Indeed, during idle conditions, the sensor
may cool off to the point that it will not operate (I had this
experience in a 1986 Mustang with 2.3 liters engine and EEC-IV system).
My recommendation is that all electrical accessories be turned off (so
as to provide a minimal load on the engine) and use the idle stop screw
on the carburetor to gradually increase the idle rpm until the sensor
begins oscillating. Ensuring a negligible load on the engine guarantees
that the carburetor will be operating mostly on its idle circuit. Now,
set the a/f mixture so that the average dwell is 30 degrees. On the
Mustang, this was done at about 1500 rpm.
Note that the a/f mixture setting procedure assumes that NO fuel
delivery problems (vacuum leaks, clogged carburetor, etc.) are present.
FOOTNOTES
[1] In some engines (e.g., GM cars with the "min-T" system -- Chevette)
the a/f mixture is varied REGARDLESS of whether the engine is in closed
loop operation or not. Consider setting the a/f mixture or diagnosing
at a slightly increased rpm.
