Misfire Monitor Diagnostic Operation; Misfire Monitor Diagnostic Operation 6E1-51; Misfire Counters - Isuzu 1999 RODEO Workshop Manual

f Exhaust leaks
f HO2S contamination
f Alternate fuels
Exhaust system leaks may cause the following:
f Preventing a degraded catalyst from failing the
diagnostic.
f Causing a false failure for a normally functioning
catalyst.
f Preventing the diagnostic from running.
Some of the contaminants that may be encountered are
phosphorus, lead, silica, and sulfur. The presence of
these contaminants will prevent the TWC diagnostic from
functioning properly.
Three–Way Catalyst Oxygen Storage Capacity
The Three–Way catalyst (TWC) must be monitored for
efficiency. To accomplish this, the control module
monitors the pre–catalyst HO2S and post–catalyst HO2S
oxygen sensors. When the TWC is operating properly,
the post–catalyst oxygen sensor will have significantly
less activity than the pre–catalyst oxygen sensor. The
TWC stores and releases oxygen as needed during its
normal reduction and oxidation process. The control
module will calculate the oxygen storage capacity using
the difference between the pre–catalyst and post catalyst
oxygen sensor's voltage levels. If the activity of the
post–catalyst oxygen sensor approaches that of the
pre–catalyst oxygen sensor, the catalyst's efficiency is
degraded.
Stepped or staged testing level allow the control module
to statistically filter test information. This prevents falsely
passing or falsely failing the oxygen storage capacity test.
The calculations performed by the on–board diagnostic
system are very complex. For this reason, post catalyst
oxygen sensor activity should not be used to determine
oxygen storage capacity unless directed by the service
manual.
Two stages are used to monitor catalyst efficiency.
Failure of the first stage will indicate that the catalyst
requires further testing to determine catalyst efficiency.
The second stage then looks at the inputs from the pre
and post catalyst HO2S sensors more closely before
determining if the catalyst is indeed degraded. This
further statistical processing is done to increase the
accuracy of oxygen storage capacity type monitoring.
Failing the first (stage 1) test DOES NOT indicate a failed
catalyst. The catalyst may be marginal or the fuel sulfur
content could be very high.
Aftermarket HO2S characteristics may be different from
the original equipment manufacturer sensor. This may
lead to a false pass or a false fail of the catalyst monitor
diagnostic. Similarly, if an aftermarket catalyst does not
contain the same amount of cerium as the original part,
the correlation between oxygen storage and conversion
efficiency may be altered enough to set a false DTC.
RODEO X22SE 2.2L ENGINE DRIVEABILITY AND EMISSION
MISFIRE MONITOR DIAGNOSTIC
OPERATION

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft
rotational velocity (reference period) variations. The PCM
determines crankshaft rotational velocity using the
crankshaft position sensor and camshaft position sensor.
When a cylinder misfires, the crankshaft slows down
momentarily. By monitoring the crankshaft and camrhaft
position sensor signals, the PCM can calculate when a
misfire occurs.
For a non–catalyst damaging misfire, the diagnostic will
be required to monitor a misfire present for between
1000–3200 engine revolutions.
For catalyst–damaging misfire, the diagnostic will
respond to misfire within 200 engine revolutions.
Rough roads may cause false misfire detection. A rough
road will cause torque to be applied to the drive wheels
and drive train. This torque can intermittently decrease
the crankshaft rotational velocity. This may be falsely
detected as a misfire.

Misfire Counters

Whenever a cylinder misfires, the misfire diagnostic
counts the misfire and notes the crankshaft position at the
time the misfire occurred. These "misfire counters" are
basically a file on each engine cylinder. A current and a
history misfire counter are maintained for each cylinder.
The misfire current counters (Misfire Cur #1–4) indicate
the number of firing events out of the last 200 cylinder
firing events which were misfires. The misfire current
counter will display real time data without a 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 maintained for each cylinder.
If the misfire diagnostic reports a failure, the diagnostic
executive reviews all of the misfire counters before
reporting a 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.
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. 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.
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
6E1–51