The PCM can detect and compensate for variances in the engine and its components. To learn these variations, the
PCM uses the input of the actual crankshaft rotation pattern and ideal crankshaft rotation pattern that has been
calibrated into the PCM. The PCM then compares the two patterns. The variation between the two values is the
Adaptive Numerator. If the Adaptive Numerator is not learned by the PCM, the misfire monitor will not run and the
Multi-Cylinder Displacement System (MDS) will not operate. Without MDS operation, the customer will experience
decreased fuel economy. If the customer experiences decrease fuel economy, use the scan tool to ensure that the
Adaptive Numerator is learned.
FUEL SYSTEM MONITOR
To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the
emission of hydrocarbons, oxides of nitrogen and carbon monoxide. The catalyst works best when the air fuel (A/F)
ratio is at or near the optimum of 14.7 to 1.
The PCM is programmed to maintain the optimum air/fuel ratio. This is done by making short term corrections in the
fuel injector pulse width based on the O2S output. The programmed memory acts as a self calibration tool that the
engine controller uses to compensate for variations in engine specifications, sensor tolerances and engine fatigue
over the life span of the engine. By monitoring the actual air-fuel ratio with the O2S (short term) and multiplying that
with the program long-term (adaptive) memory and comparing that to the limit, it can be determined whether it will
pass an emissions test. If a malfunction occurs such that the PCM cannot maintain the optimum A/F ratio, then the
MIL will be illuminated.
CATALYST MONITOR
To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the
emission of hydrocarbons, oxides of nitrogen and carbon monoxide.
Normal vehicle miles or engine misfire can cause a catalyst to decay. A meltdown of the ceramic core can cause a
reduction of the exhaust passage. This can increase vehicle emissions and deteriorate engine performance, drive-
ability and fuel economy.
The catalyst monitor uses dual oxygen sensors (O2S’s) to monitor the efficiency of the converter. The dual O2S’s
strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both
reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The
upstream O2S is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic con-
verter. The PCM calculates the A/F mixture from the output of the O2S. A low voltage indicates high oxygen content
(lean mixture). A high voltage indicates a low content of oxygen (rich mixture).
When the upstream O2S detects a lean condition, there is an abundance of oxygen in the exhaust gas. A function-
ing converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter absorbs the
oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream O2S will indicate
limited activity in this condition.
As the converter loses the ability to store oxygen, the condition can be detected from the behavior of the down-
stream O2S. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen
will be the same downstream as upstream. The output voltage of the downstream O2S copies the voltage of the
upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2S’s.
To monitor the system, the number of lean-to-rich switches of upstream and downstream O2S’s is counted. The
ratio of downstream switches to upstream switches is used to determine whether the catalyst is operating properly.
An effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero.
For a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in the device.
The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over
the legal limit, the MIL (Check Engine lamp) will be illuminated.
NATURAL VACUUM LEAK DETECTION (NVLD)
The Natural Vacuum Leak Detection (NVLD) system is the next generation evaporative leak detection system that
will first be used on vehicles equipped with the Next Generation Controller (NGC). This new system replaces the
leak detection pump as the method of evaporative system leak detection. This is to detect a leak equivalent to a
0.020
9
(0.5 mm) hole. This system has the capability to detect holes of this size very dependably.
The basic leak detection theory employed with NVLD is the
9
Gas Law
9
. This is to say that the pressure in a sealed
vessel will change if the temperature of the gas in the vessel changes. The vessel will only see this effect if it is
indeed sealed. Even small leaks will allow the pressure in the vessel to come to equilibrium with the ambient pres-
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EMISSIONS CONTROL
LX