To better understand how the various pieces of the emissions/engine control puzzle fit together, let's look at some of the individual components starting with some of the "traditional" emission control systems first.
POSITIVE CRANKCASE VENTILATION (PCV) SYSTEM. The positive crankcase ventilation (PCV) system recycles crankcase vapors back into the engine so they can be burned up and eliminated. Intake vacuum pulls the blowby vapors from the crankcase through a hose connected to the PCV valve which is usually mounted on a valve cover.
Crankcase emissions come from combustion byproducts and moisture that blow past the piston rings. The crankcase is sealed so these vapors can't escape into the atmosphere, instead they are vented back to the intake manifold via the PCV system. This also helps to rid the crankcase of moisture and fuel vapor that thins the oil and causes sludge.
The PCV system functions as a calibrated vacuum leak. The PCV valve has an orifice with a spring-loaded valve that controls airflow according to engine load and vacuum. If the PCV valve or hose leaks, too much air can be sucked into the intake manifold. This will have a leaning effect on the air/fuel ratio, which the oxygen sensor in the exhaust manifold will detect. The computer will then attempt to compensate for the air leak by enriching the mixture, which may cause fuel consumption as well as carbon monoxide and hydrocarbon emissions to rise.
If the PCV valve or hose becomes clogged, it won't flow any air. The air/fuel mixture will go rich causing the computer to lean the fuel mixture. This, in turn, may create a lean misfire condition and higher hydrocarbon emissions. Moisture and fuel vapors will also build up rapidly in the crankcase, diluting the oil and allowing the formation of engine-damaging sludge.
For preventative maintenance, the PCV valve should be replaced every 50,000 miles along with the crankcase breather element (if used) which is typically located inside the air cleaner housing.
EXHAUST GAS RECIRCULATION (EGR) SYSTEM. The EGR system reduces oxides of nitrogen (NOX) emissions in the exhaust by reducing combustion temperatures. This is done by allowing intake vacuum to pull a small amount of exhaust gas back into the intake manifold. The exhaust dilutes the air/fuel mixture just enough to keep combustion temperatures below the 2,500 degree F. NOX formation threshold.
The EGR valve opens a fixed orifice passageway that connects the exhaust and intake manifolds.
Most EGR valves have a vacuum actuated diaphragm that opens a poppet valve when ported vacuum is applied to the diaphragm. On many late model engines, a "positive backpressure" or "negative backpressure" EGR valve is used to fine-tune valve operation. This type of valve requires a level of backpressure in the exhaust before it will open when vacuum is applied. On some newer GM engines, an electrically-operated "linear" EGR valve is used. The engine computer regulates the opening of the EGR valve by driving a small electric motor on the valve.
The EGR system is more complicated that the PCV system because it often includes extra control hardware. The vacuum control plumbing to the EGR valve may include a temperature vacuum switch (TVS) or solenoid to block or bleed vacuum until the engine warms up. Or the computer may operate a vacuum solenoid to modify the opening of the valve. Many newer vehicles have an EGR valve position sensor to provide feedback to the computer about the valve's operating position.
Like the PCV system, the EGR system can effect the air/fuel mixture as an air leak when it opens. Because of this, EGR is not used at idle because doing so would cause the engine to idle roughly, misfire and possibly stall (all of which may be symptoms of a leaky EGR valve). NOX emissions are naturally low at idle anyway because the engine is not under load. So the EGR system doesn't do its thing until the engine is at normal operating temperature and the engine is under load.
If the EGR valve fails or becomes plugged with carbon, it can affect engine performance two ways. One is that the engine may experience detonation (spark knock) when accelerating. This occurs because combustion temperatures are higher than normal. The other is that the engine will produce more NOX in the exhaust.
Unlike PCV valves, EGR valves have no recommended replacement interval for preventative maintenance. They're usually only replaced on an "as needed" basis. Just make sure your customer gets the correct EGR valve for this engine because the calibration of the valve must match the application.
CATALYTIC CONVERTER & AIR PUMP. The catalytic converter acts like an afterburner to destroy hydrocarbons and carbon monoxide in the exhaust. The converter contains "catalysts" (platinum and palladium) that trigger the burning of these two pollutants. Used primarily on older vehicles (1975 to 1980), these are usually referred to as "two-way" converters. Newer vehicles (1981 & up) mostly use "three-way" or "three-way plus oxygen" converters that contain an extra catalyst (rhodium) that reduces NOX in the exhaust.
For a three-way converter to function efficiently, the engine's air/fuel ratio must be maintained within a certain range and flip-flop back and forth from slightly rich to slightly lean. This job is handled by the engine computer and oxygen sensor.
Some converters also need additional oxygen, which is provided by an "air pump" or "aspirator valve." On engines that have an air pump, the pump may be used to route air to the exhaust manifolds and/or converter to reduce HC, CO and/or NOX emissions depending on the operating conditions at the time.
The air pump is belt driven, and feeds air to the exhaust manifold through a "diverter valve" that may be computer-controlled. Problems here can route air to the wrong place at the wrong time, causing emissions to rise.
Converters operate at high temperatures once they light off, but can be damaged if they get too hot. This may occur if the fuel mixture is excessively rich or if unburned fuel is passing through the engine because of ignition misfiring or leaky exhaust valves that don't hold compression.
The extra fuel in the exhaust can make the converter's temperature soar causing a partial or complete meltdown of the substrate inside that supports the catalyst. The result is a "plugged converter" that creates excessive backpressure or may even block the flow of exhaust causing the engine to stall.
Replacing the converter may temporarily solve the blockage problem, but unless the underlying cause of the failure is diagnosed and corrected, the replacement converter may suffer the same fate.
Converters can also die as a result of old age or contamination. Leaded gasoline is now history but many an older converter was ruined by using leaded gasoline. On newer converters, silicon contamination from coolant leaks in the combustion chamber, or phosphorous contamination from excessive oil burning can have the same adverse effect on the catalyst.
To monitor the operating efficiency of the converter, newer vehicles equipped with the OBD II self-diagnostic system (1996 & up, but also some 1994 & 1995 models) have an additional oxygen sensor aft of the converter. The readings of the two O2 sensors are compared to see if there's a significant reduction in oxygen levels (indicating the converter is working). No change triggers the "Check Engine" light and sets a diagnostic trouble code.
WARRANTY RULE CHANGES. In 1995, the replacement rules governing catalytic converters changed. Up to 1995, converters and all other emission control parts (including the computer and sensors) on new cars were covered by a 5-year, 50,000-mile federal emissions warranty. Since 1995, the converter and computer are covered under the federal emissions warranty for 8 years or 80,000 miles - but all the other emission control parts and sensors are only covered for two years or 24,000 miles - which is good news for the aftermarket.
THE PCM & FEEDBACK FUEL CONTROL LOOP. The engine computer, oxygen sensor and coolant sensor are the main components in the feedback fuel control system which is often referred to as the "loop" because of the way that input from the O2 sensor affects the computer and vice versa.
The brains of the system is the powertrain control module (PCM), which used to be called the "engine control module" (ECM) but was renamed in 1995 by the Society of Automotive Engineers (SAE) to better describe the computer's role in today's vehicles. The computer receives inputs from its various sensors and then performs the various control functions based on internal programming. The newer the vehicle, the smarter and more sophisticated the PCM.
The coolant sensor is a variable resistor that monitors the temperature of the engine's coolant. The sensor changes resistance as the temperature of the coolant goes up and down. The coolant sensor is a sort of master switch that can affect the operation of many of the PCM's control functions.
The oxygen sensor monitors the level of unburned oxygen in the exhaust. The O2 sensor is unique in that it generates its own voltage signal. The level of that signal, which varies from about 0.1 (lean) to about 0.9 volts (rich) is used by the PCM to determine the relative richness of the fuel mixture so corrections can be made. The computer does this by varying the duration (on time) of the fuel injectors. On older vehicles with feedback carburetors, it varies the dwell of the mixture control solenoid.
When a cold engine is first started, the air/fuel mixture must be richer than normal for the engine to idle smoothly.
As the engine warms up, the fuel mixture is gradually leaned until it reaches a balanced mixture. The computer regulates the fuel mixture and uses a fixed setting when the engine is cold. This is called "open loop" operation because the computer is not yet using the input from the oxygen sensor to alter the fuel mixture.
As things start to warm up, the computer begins to receive an input signal from the oxygen sensor (O2 sensors don't produce a signal until they reach 450 to 600 degrees F, so most late model O2 sensors have a third wire for an internal heater element that reduces the sensor's warm-up time). If the engine is warm enough, the computer will begin to use the oxygen sensor's input to modify the fuel mixture. This is called "closed loop" operation because the output of the O2 sensor affects the computer's control over the air/fuel mixture.
If the coolant sensor is defective, the computer may be fooled into thinking the engine is always cold (which it really isn't). This will prevent the computer from going into closed loop, resulting in a richer than normal fuel mixture, increased fuel consumption and elevated carbon monoxide and hydrocarbon emissions.
If the oxygen sensor is defective, the same thing can happen. But it can also happen if the oxygen sensor has become sluggish with age. O2 sensors slow down with age. This decreases the rate at which the computer can make corrections to the air/fuel mixture, resulting in higher emissions and fuel consumption. Deterioration of the O2 sensor can be caused by a variety of substances that find their way into the exhaust.
Some car makers have recommended replacement intervals for O2 sensors while others do not. Generally speaking, unheated 1 or 2-wire wire O2 sensors on 1976 through early 1990s vehicles should probably be replaced for preventative maintenance at 50,000 miles.
Heated 3-and 4-wire O2 sensors on mid-1980s through mid-1990s applications can be changed every 60,000 miles. On 1996 and newer OBD II-equipped vehicles, the sensor should go 100,000 miles.
OTHER SENSORS. Other sensors that can effect emissions include:
Manifold Absolute Pressure (MAP) Sensor. This sensor is mounted on or connected to the intake manifold to monitor intake vacuum. It changes resistance or frequency as the pressure changes. The computer uses this information to measure engine load so ignition timing can be advanced and retarded as needed. On engines with a "speed density" type of fuel injection, it also helps the computer estimate airflow. Computerized engine control systems that do not use a MAP sensor reply on throttle position and air sensor input to determine engine load. Problems with this sensor can affect the fuel mixture as well as ignition timing to increase emissions.
Throttle Position Sensor (TPS). Mounted on the throttle shaft of the carburetor or throttle body, it changes resistance as the throttle opens and closes.
The computer uses this information to monitor engine load, acceleration/deceleration and when the engine is at idle for fuel, timing and other control functions.
Many TPS sensors require an initial voltage adjustment when installed. On some engines, a separate idle switch and/or wide open throttle (WOT) switch may also be used. Problems here will primarily affect the fuel mixture.
Mass Airflow Sensor (MAF). Mounted ahead of the throttle body on multiport fuel injected engines to monitor how much air is entering the engine. The sensor uses either a hot wire or heated filament to measure both airflow and air density. A bad airflow sensor will upset the air/fuel mixture causing both emissions and driveability problems.
Vane Airflow Sensor (VAF). Performs the same function as above but uses a mechanical flap to measure airflow.