Misfire Diagnostic - Volvo B6324S Functional Description

If the fuel/air mixture does not burn correctly, then
the generated torque will be less than intended and the
engine rpm will drop suddenly, (decelerate) the engine
is said to be misfiring. The control module can detect
misfiring by measuring the time between successive
segments on the flywheel /carrier plate.
If there is a misfire then there will be a stepchange
in the size of these successive time measurements, if
there is a misfire the lost torque will be noticed as a
slowing down of the flywheel rotation. The prerequi-
site for reliable misfire detection is accurate segment
period measurement. However, the period between
two top dead centers (TDC), at constant speed, is also
subject to variations due to manufacturing tolerances
and off center installation. These inaccuracies are sys-
tematic, so they can be "learned" during fuel cut off
periods and used for compensation. By this way, the
systematic error introduced by the tolerances of the
target flywheel is largely eliminated. The segment time
can vary due to the following reasons:
- Misfiring
- Flywheel mechanical tolerances
- Driveline oscillations
- Normal variations caused by uneven
combustion
- Poor roads.
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Misfire diagnostic

Since mechanical tolerances and driveline oscillation
interfere with the signal, it is difficult to ascertain
whether or not this interference is due to misfiring. To
eliminate mechanical faults in the flywheel the flywheel
signal is adapted. Two crankshaft revolutions are divi-
ded into six periods, (on a 6 – cylinder engine), if the
engine has no external load all six periods should be
equal. This is to even out the signal, so that a mecha-
nical fault in the flywheel is not registeredas misfiring.
After adaptation there is someinterference in the signal
due to oscillations in the drive train and normal engine
irregularities.The flywheel signal is adapted when:
- Engine speed is between two targets
- The fuel shut-off system is operating and has been
active for 100 revolutions.
A DTC is stored when misfiring leads to increased
emissions and a DTC is stored when misfiring could
cause damage to the TWC. The engine control module
registers and stores the engine speed, load and warm-
up status in which the misfiring occurred. See part
"Diagnostic functions, Overview".
If misfire exceeds catalyst damage threshold, the sys-
tem will cut the fuel on those cylinders that experience
misfire if one of the two following conditions are
fulfilled:
1. Misfire on single cylinder
2. One or two cylinders misfiring all the time.