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ASE P2 Test Preparation Guide: Emission System

• Identify major emission system components
• Identify emission system component functions
• Identify related items
• Provide basic use, installation and warranty information


* Positive crankcase ventilation (PCV) — Recirculates blowby vapors from the crankcase back into the intake manifold so the vapors can be reburned inside the engine. PCV prevents engine blowby vapors from escaping into the atmosphere. It also helps remove moisture from the crankcase to prolong oil life and prevent the formation of engine-damaging sludge. The PCV valve is usually located in a valve cover, and is attached to the intake manifold by a large vacuum hose. Recommended replacement interval is typically 50,000 miles. A clogged PCV valve can allow pressure and moisture to build up inside the engine.


* Evaporative emission controls (EVAP) — Prevents fuel vapors from the fuel tank and fuel system from escaping into the atmosphere. The fuel tank is sealed, and fumes are vented to a charcoal-filled storage canister. When the engine is running, a “purge valve” opens, allowing the fumes to be siphoned into the engine and burned. On 1996 and newer vehicles, the OBD II system does a pressure or vacuum test to check the EVAP system for leaks. A loose or missing gas cap, or a leaky vapor hose will set a fault code and turn on the Check Engine light. EVAP faults can be difficult to diagnose because small leaks can be hard to find.


* Exhaust gas recirculation (EGR) — Reduces oxides of nitrogen (NOx) emissions from the engine. When the EGR valve opens, it allows intake vacuum to suck exhaust gas back into the intake manifold. This dilutes the air/fuel mixture slightly and reduces combustion temperatures to lower NOx emissions. EGR is not used when the engine is cold or at idle. It is mostly used when the engine is working hard under a heavy load. EGR also prevents detonation (spark knock).

 Various types of EGR valves are used. EGR valves may be vacuum-operated, or they may have solenoids or a small stepper motor inside. The powertrain control module (PCM) decides when EGR is needed. With vacuum-operated EGR valves, the PCM commands a vacuum solenoid to open. With electronic EGR valves, it energizes the valve as needed to vary EGR flow. On many Ford applications, a “differential pressure flow sensor“ (DPFE) is used to monitor the operation of the EGR valve. Some EGR valves may also have a position sensor to monitor its operation. EGR problems can occur if the valve itself fails, if carbon builds up under the valve or clogs the EGR ports in the intake manifold, if vacuum hose connections are leaking or plugged, or if the DPFE or position sensor is defective.


EGR valves are calibrated for specific engine applications. Some aftermarket replacement EGR valves have various adapters to modify the flow rate so one part number can fit a broader range of vehicles.

* Catalytic converter — Located in the exhaust system, the catalytic converter is an “afterburner” that reduces pollutants in the exhaust. It contains “catalysts” that trigger chemical reactions to reduce unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). The catalyst has to be hot (above 600 degrees F) to light off, and runs very hot once it begins to work.

The catalyst can be contaminated by lead (leaded gasoline), silicon (coolant leaks) or phosphorus (oil burning). On 1996 and newer vehicles, the OBD II system uses a “downstream” oxygen sensor behind the converter to monitor its operating efficiency. If efficiency drops below a certain level, it will set a fault code and turn on the Check Engine light. Converters can also be damaged by overheating (often due to ignition misfire or an engine exhaust valve that is leaking compression). The catalyst can melt and create an obstruction. A plugged converter will create excessive backpressure, which hurts engine performance and fuel economy, and may even cause the engine to stall. Replacement converters must be the same as the original.


*Air injection reaction (AIR) — An air pump used on some older vehicles to pump extra oxygen into the exhaust system or to the catalytic converter to reduce pollution. A diverter valve assembly on the air pump controls the flow of air into the exhaust manifold.

* Oxygen sensors — Part of the fuel system feedback control loop, it monitors oxygen levels in the exhaust so the PCM can regulate the air/fuel mixture to minimize emissions. “Upstream” oxygen sensors are located in the engine’s exhaust manifold(s). There are various types of oxygen sensors. Most have a zirconia thimble or strip sensing element inside the vented steel cap that produces a rich (high) voltage signal when the level of oxygen in the exhaust drops below a certain threshold. Others are a “wide band” design that measures the exact air/fuel ratio based on oxygen levels in the exhaust. Oxygen sensors must be hot (above 600 degrees) to generate a signal, so an internal heater element is used to bring the sensor up to operating temperature quickly. 

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