Fuel Trim System Monitor Diagnostic Operation; Fuel Trim Cell Diagnostic Weights - Isuzu 1999 RODEO Workshop Manual

If the misfire diagnostic reports a failure, the diagnostic
executive reviews all of the misfire counters before
reporting DTC. This way, the diagnostic executive
reports the most current information.
When crankshaft rotation is erratic, a misfire condition will
be detected. Because of this erratic condition, the data
that is collected by the diagnostic can sometimes
incorrectly identify which cylinder is misfiring. Misfires are
counted from more than one cylinder. Cylinder #1 has the
majority of counted misfires. In this case, the Misfire
Counters would identify cylinder #1 as the misfiring
cylinder. The misfires in the other counters were just
background noise caused by the erratic misfire rotation of
the crankshaft. If the number of accumulated misfires is
sufficient for the diagnostic to identify a true misfire, the
diagnostic will set DTC P0300 – Misfire Detected.
Use diagnostic equipment to monitor misfire counter data
on OBD II-compliant vehicles. Knowing which specific
cylinder(s) misfired can lead to the root cause, even when
dealing with a multiple cylinder misfire.
information in the misfire counters, identify which
cylinders are misfiring. If the counter indicate cylinders
numbers 1 and 4 misfired, look for a circuit or component
common to both cylinders number 1 and 4.
Misfire counter information is located in the "Specific
Eng." menu, "Misfire Data" sub-menu of the data list.
The misfire diagnostic may indicate a fault due to a
temporary fault not necessarily caused by a vehicle
emission system malfunction.
following items:
f Contaminated fuel
f Low fuel
f Fuel-fouled spark plugs
f Basic engine fault
Fuel Trim System Monitor Diagnostic
Operation
Fuel Trim System Monitor Diagnostic
Operation
This system monitors the averages of short-term and
long-term fuel trim values. If these fuel trim values stay at
their limits for a calibrated period of time, a malfunction is
indicated. The fuel trim diagnostic compares the
averages of short-term fuel trim values and long-term fuel
trim values to rich and lean thresholds. If either value is
within the thresholds, a pass is recorded. If both values
are outside their thresholds, a rich or lean DTC will be
recorded.
The fuel trim system diagnostic also conducts an intrusive
test. This test determines if a rich condition is being
caused by excessive fuel vapor from the EVAP canister.
In order to meet OBD II requirements, the control module
uses weighted fuel trim cells to determine the need to set
a fuel trim DTC. A fuel trim DTC can only be set if fuel trim
counts in the weighted fuel trim cells exceed
specifications. This means that the vehicle could have a
fuel trim problem which is causing a problem under
certain conditions (i.e., engine idle high due to a small
vacuum leak or rough idle due to a large vacuum leak)
while it operates fine at other times. No fuel trim DTC
RODEO 6VD1 3.2L ENGINE DRIVEABILITY AND EMISSIONS
Using the
Examples include the
would set (although an engine idle speed DTC or HO2S
DTC may set). Use the Tech 2 to observe fuel trim counts
while the problem is occurring.
A fuel trim DTC may be triggered by a number of vehicle
faults. Make use of all information available (other DTCs
stored, rich or lean condition, etc.) when diagnosing a fuel
trim fault.

Fuel Trim Cell Diagnostic Weights

No fuel trim DTC will set regardless of the fuel trim counts
in cell 0 unless the fuel trim counts in the weighted cells
are also outside specifications. This means that the
vehicle could have a fuel trim problem which is causing a
problem under certain conditions (i.e. engine idle high due
to a small vacuum leak or rough due to a large vacuum
leak) while it operates fine at other times. No fuel trim
DTC would set (although an engine idle speed DTC or
HO2S DTC may set). Use the Tech 2 to observe fuel trim
counts while the problem is occurring.
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