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'Strange' PCM behavior

thorney_5

Elite Explorer
Joined
October 14, 2012
Messages
146
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City, State
New Jersey
Year, Model & Trim Level
2000 XLT SOHC
In an attempt to satiate my incessant curiosity about the explorer computer and it's operation, i intentionally unplugged the cylinder 3 injector while replacing my intake gaskets. Typically, one would expect a misfire code be stored, in this case P0303. To my initial surprise, it did not. According to the Ford 2000 Model year OBD system theory and operation, the misfire monitor is continuous, with a misfire rate calculated every 200 or 1000 revolutions. The P0303 code should have been stored (at least pending) almost immediately. But a deeper look into how the system detects a misfire revals why it was not.

There are two different misfire monitoring technologies used in the 2000 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.

In short, the computer uses the velocity of the piston to determine if there are any present misfires. With an injector disabled, and no actual 'misfiring' occurring, the computer will instead store a lean bank code (either p0171 or p0174, depending on injector) due to unexpectedly low amount of fuel in the exhaust (or rather, increased oxygen percentage). Observation suggests the computer illuminates the check engine light on the third driving cycle, following 2 cycles with the given condition.
 






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