Fuel Trim System Monitor Diagnostic Operation; Fuel Trim System Monitor Diagnostic Operation 6E1-52; Fuel Trim Cell Diagnostic Weights - Isuzu 1999 RODEO Workshop Manual

6E1–52
RODEO X22SE 2.2L ENGINE DRIVEABILITY AND EMISSION
emission system malfunction. Examples include the
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
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.