FORD MOTOR COMPANY REVISION DATE: SEPTEMBER 17, 2001 PAGE 6 OF 50
Misfire Monitor
There are two different misfire monitoring technologies used in the 1997 MY. They are Low Data Rate (LDR) and
High Data Rate (HDR). The LDR system is capable of meeting the FTP monitoring requirements on most
engines and is capable of meeting “full-range” misfire monitoring requirements on 4-cylinder engines. The HDR
system is capable of meeting “full-range” misfire monitoring requirements on 6 and 8 cylinder engines. HDR is
being phased in on these engines to meet the” full-range” misfire phase-in requirements specified in the OBD-II
regulations.
Low Data Rate System
The LDR Misfire Monitor uses a low-data-rate crankshaft position signal, (i.e. one position reference signal at 10
deg BTDC for each cylinder event). The PCM calculates crankshaft rotational velocity for each cylinder from this
crankshaft position signal. The acceleration for each cylinder can then be calculated using successive velocity
values. The changes in overall engine rpm are removed by subtracting the median engine acceleration over a
complete engine cycle. The resulting deviant cylinder acceleration values are used in evaluating misfire in the
“General Misfire Algorithm Processing” section below.
“Profile correction” software is used to “learn” and correct for mechanical inaccuracies in crankshaft tooth spacing
under de-fueled engine conditions (requires three 60 to 40 mph no-braking decels after Keep Alive Memory has
been reset). These learned corrections improve the high-rpm capability of the monitor for most engines. The
misfire monitor is not active until a profile has been learned.
High Data Rate System
The HDR Misfire Monitor uses a high data rate crankshaft position signal, (i.e. 18 position references per
crankshaft revolution [20 on a V-10]). This high-resolution signal is processed using two different algorithms. The
first algorithm, called pattern cancellation, is optimized to detect low rates of misfire. The algorithm learns the
normal pattern of cylinder accelerations from the mostly good firing events and is then able to accurately detect
deviations from that pattern. The second algorithm is optimized to detect “hard” misfires, i.e. one or more
continuously misfiring cylinders. This algorithm filters the high-resolution crankshaft velocity signal to remove some
of the crankshaft torsional vibrations that degrade signal to noise. This significantly improves detection capability for
continuous misfires. Both algorithms produce a deviant cylinder acceleration value, which is used in evaluating
misfire in the “General Misfire Algorithm Processing” section below.
Due to the high data processing requirements, the HDR algorithms could not be implemented in the PCM
microprocessor. They are implemented in a separate chip in the PCM called an “AICE” chip. The PCM
microprocessor communicates with the AICE chip using a dedicated serial communication link. The output of the
AICE chip (the cylinder acceleration values) is sent to the PCM microprocessor for additional processing as
described below. Lack of serial communication between the AICE chip and the PCM microprocessor, or an
inability to synchronize the crank or cam sensors inputs sets a P1309 DTC.
“Profile correction” software is used to “learn” and correct for mechanical inaccuracies in crankshaft tooth spacing
under de-fueled engine conditions (requires three 60 to 40 mph no-braking decels after Keep Alive Memory has
been reset). If KAM has been reset, the PCM microprocessor initiates a special routine which computes correction
factors for each of the 18 (or 20) position references and sends these correction factors back to the AICE chip to
be used for subsequent misfire signal processing. These learned corrections improve the high rpm capability of the
monitor. The misfire monitor is not active until a profile has been learned.
FORD MOTOR COMPANY REVISION DATE: SEPTEMBER 17, 2001 PAGE 7 OF 50
Generic Misfire Algorithm Processing
The acceleration that a piston undergoes during a normal firing event is directly related to the amount of torque that
cylinder produces. The calculated piston/cylinder acceleration value(s) are compared to a misfire threshold that is
continuously adjusted based on inferred engine torque. Deviant accelerations exceeding the threshold are
conditionally labeled as misfires.
The calculated deviant acceleration value(s) are also evaluated for noise. Normally, misfire results in a nonsymmetrical
loss of cylinder acceleration. Mechanical noise, such as rough roads or high rpm/light load conditions,
will produce symmetrical acceleration variations. Cylinder events that indicate excessive deviant accelerations of
this type are considered noise. Noise-free deviant acceleration exceeding a given threshold is labeled a misfire.
The number of misfires are counted over a continuous 200 revolution and 1000 revolution period. (The revolution
counters are not reset if the misfire monitor is temporarily disabled such as for negative torque mode, etc.) At the
end of the evaluation period, the total misfire rate and the misfire rate for each individual cylinder is computed. The
misfire rate evaluated every 200 revolution period (Type A) and compared to a threshold value obtained from an
engine speed/load table. This misfire threshold is designed to prevent damage to the catalyst due to sustained
excessive temperature (1600°F for Pt/Pd/Rh conventional washcoat, 1650°F for Pt/Pd/Rh advanced washcoat and
1800°F for Pd-only high tech washcoat). If the misfire threshold is exceeded and the catalyst temperature model
calculates a catalyst mid-bed temperature that exceeds the catalyst damage threshold, the MIL blinks at a 1 Hz
rate while the misfire is present. If the threshold is again exceeded on a subsequent driving cycle, the MIL is
illuminated. If a single cylinder is indicated to be consistently misfiring in excess of the catalyst damage criteria, the
fuel injector to that cylinder may be shut off for a period of time to prevent catalyst damage. Up to two cylinders
may be disabled at the same time. This fuel shut-off feature is used on many 8-cylinder engine and some 6-
cylinder engines. It is never used on a 4-cylinder engine. Next, the misfire rate is evaluated every 1000 rev period
and compared to a single (Type B) threshold value to indicate an emission-threshold malfunction, which can be
either a single 1000 rev exceedence from startup or a subsequent 1000 rev exceedence on a drive cycle after
start-up.
Profile Correction
"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 360o, a correction factor
can be calculated for each misfire sample interval that makes all the angles between individual teeth equal. To
prevent any fueling or combustion differences from affecting the correction factors, learning is done during decelfuel
cutout.
The correction factors are learned 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. The corrections are typically learned in a single deceleration, but can be learned during up
to 3 such decelerations. The "mature" correction factors are the average of a selected number of samples. A low
data rate misfire system will typically learn 4 such corrections in this interval, while a high data rate system will learn
36 or 40 in the same interval (data is actually processed in the AICE chip). 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.
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.
FORD MOTOR COMPANY REVISION DATE: SEPTEMBER 17, 2001 PAGE 8 OF 50
Misfire Monitor Operation:
DTCs P0300 to P0310 (general and specific cylinder misfire)
P1309 (no cam/crank synchronization, AICE chip malfunction)
Monitor execution Continuous, misfire rate calculated every 200 or 1000 revs
Monitor Sequence None
Sensors OK CKP, CMP
Monitoring Duration Entire driving cycle (see disablement conditions below)
Typical misfire monitor entry conditions:
Entry condition Minimum Maximum
Time since engine start-up (5 sec or 240 sec on
1996/97 vehicles)
5 seconds 5 seconds
Engine Coolant Temperature 20 oF 250 oF
RPM Range (FTP Misfire certified) Idle rpm ~ 2500 rpm
RPM Range (Full-Range Misfire certified) Idle rpm redline on tach or fuel
cutoff
Profile correction factors learned in KAM Yes
Typical misfire temporary disablement conditions:
Temporary disablement conditions:
Closed throttle decel (negative torque, engine being driven)
Fuel shut-off due to vehicle-speed limiting or engine-rpm limiting mode
Accessory load-state change (A/C, power steering)
High rate of change of torque (heavy throttle tip-in or tip out)
Typical misfire monitor malfunction thresholds:
Type A (catalyst damaging misfire rate): misfire rate is an rpm/load table ranging from 40% at idle to 4% at
high rpm and loads
Type B (emission threshold rate): 1% to 3%
FORD MOTOR COMPANY REVISION DATE: SEPTEMBER 17, 2001 PAGE 9 OF 50
J1979 Mode $06 Data
Test ID Comp ID Description Units
$50 $00 Total engine misfire rate and emission threshold misfire rate (updated
every 1,000 revolutions)
percent
$53 $00 - $0A Cylinder-specific misfire rate and malfunction threshold misfire rate
(either cat damage or emission threshold) (updated when DTC set or
clears)
percent
$54 $00 Highest catalyst-damage misfire and catalyst damage threshold misfire
rate (updated when DTC set or clears)
percent
$55 $00 Highest emission-threshold misfire and emission threshold misfire rate
(updated when DTC set or clears)
percent
$56 $00 Cylinder events tested and number of events required for a 1000 rev
test
events
Conversion for Test IDs $50 through $55: multiply by 0.000015 to get percent
Conversion for Test ID $56: multiply by 1 to get ignition events
Profile Correction Operation
DTCs P1309 – AICE chip communication failure
Monitor Execution once per KAM reset.
Monitor Sequence: Profile must be learned before misfire monitor is active.
Sensors OK: CKP, CMP, no AICE communication errors, CKP/CMP in synch
Monitoring Duration; 10 cumulative seconds in conditions
(a maximum of three 60-40 mph defueled decels)
Typical profile learning entry conditions:
Entry condition Minimum Maximum
Engine in decel-fuel cutout mode for 4 engine cycles
Brakes applied No No
Engine RPM 1300 rpm 3700 rpm
Change in RPM 600 rpm/background
loop
Vehicle Speed 30 mph 75 mph
Learning tolerance 1%
very thankful for that, he got autolite XP104 plugs
