Daewoo CIELO EURO III Manual page 591

misfire DTC stored. The misfire history counters (Misfire
Hist #1-4) indicate the total number of cylinder firing
events which were misfires. The misfire history counters
will display 0 until the misfire diagnostic has failed and a
DTC P0300 is set. Once the misfire DTC P0300 is set,
the misfire history counters will be updated every
200 cylinder firing events. A misfire counter is main-
tained for each cylinder.
If the misfire diagnostic reports a failure, the diagnostic
executive reviews all of the misfire counters before re-
porting a DTC. This way, the diagnostic executive re-
ports 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.
Use diagnostic equipment to monitor misfire counter
data on On-Board Diagnostic (EOBD) compliant ve-
hicles. Knowing which specific cylinder(s) misfired can
lead to the root cause, even when dealing with a multiple
cylinder misfire. Using the information in the misfire
counters, identify which cylinders are misfiring. If the
counters indicate cylinders numbers 1 and 4 misfired,
look for a circuit or component common to both cylinders
number 1 and 4.
The misfire diagnostic may indicate a fault due to a tem-
porary fault not necessarily caused by a vehicle emis-
sion system malfunction. Examples include the following
items:
D Contaminated fuel.
D Low fuel.
D Fuel–fouled spark plugs.
D Basic engine fault.
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
DAEWOO T-154 BL2,3
ENGINE CONTROLS 1F – 421
at their limits for a calibrated period of time, a malfunc-
tion 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 val-
ue 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 intru-
sive test. This test determines if a rich condition is being
caused by excessive fuel vapor from the Evaporative
(EVAP) Emission canister. In order to meet EOBD re-
quirements, 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 2 DTC may set).
Use a scan tool 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 oth-
er times. No fuel trim DTC would set (although an en-
gine idle speed DTC or HO2S 2 DTC may set). Use a
scan tool to observe fuel trim counts while the problem is
occurring.