"Profile correction" software is used to "learn" and correct for mechanical inaccuracies in the crankshaft position
wheel tooth spacing. Since the sum of all the angles between crankshaft teeth must equal 360
o
, a correction factor
can be calculated for each misfire sample interval that makes all the angles between individual teeth equal. . The
LDR misfire system will learn one profile correction factor per cylinder (ex. 4 correction factors for a 4 cylinder
engine), while the HDR system will learn 36 or 40 correction factors depending on the number of crankshaft wheel
teeth (ex. 36 for V6/V8 engines, 40 for V10 engines).
The corrections are calculated from several engine cycles of misfire sample interval data. The "mature" correction
factors are the average of a selected number of samples. In order to assure the accuracy of these corrections, a
tolerance is placed on the incoming values such that an individual correction factor must be repeatable within the
tolerance during learning. This is to reduce the possibility of learning corrections on rough road conditions which
could limit misfire detection capability and to help isolate misfire diagnoses from other crankshaft velocity
disturbances.
To prevent any fueling or combustion differences from affecting the correction factors, learning is done during
decel-fuel cutout. This can be done during closed-throttle, non-braking, de-fueled decelerations in the 60 to 40 mph
range after exceeding 60 mph (likely to correspond to a freeway exit condition). In order to minimize the learning
time for the correction factors, a more aggressive decel-fuel cutout strategy may be employed when the conditions
for learning are present and are typically learned in a single 60 to 40 MPH deceleration, but can be learned during
up to 3 such decelerations, or over a higher number of shorter duration decelerations..
For Hybrid Electric Vehicles profile is learned by using the electric drive to spin the crankshaft on the first engine
shutdown during which time profile is calculated.
Since inaccuracies in the wheel tooth spacing can produce a false indication of misfire, the misfire monitor is not
active until the corrections are learned. In the event of battery disconnection or loss of Keep Alive Memory the
correction factors are lost and must be relearned. If the software is unable to learn a profile after three 60 to 40
mph decels, a P0315 DTC is set.