Emissions System Diagnosis

The link between vehicle emis­sions and air pollution was first identified by studies that began in California during the early 1940s. By the end of that decade, research­ers determined that the smog above California’s Los Angeles Basin was the result of hydrocarbon and oxides of nitrogen emissions from motor vehicles . As automobile manufactur­ers looked into isolating the source of these gases, they discovered that vehicle exhaust also contained car­bon monoxide (CO). While not a component of smog, CO is a by­ product of the combustion process and a highly toxic gas. In this sec­tion, we will study the three primary pollutants; including how they are produced and the systems designed to control them.


The air we breathe consists of 78% nitrogen (N2), 21% oxygen (02), and 1% other gases, such as carbon dioxide (C02). Gasoline is a liquid hydrocarbon (HC) that consists of a complex arrangement of hydro­gen and carbon atoms. The air/fuel mixture that enters the combustion chamber is ultimately transformed through a chemical reaction brought about by heat and pressure. Every engine emits varying levels of the following gases:

Hydrocarbons (HC) fuel molecules that pass through the engine without burning. Accept­ able levels of HC are character­ istic of the normal combustion process, while excessive HC lev­ els result when a cylinder misfires.
Carbon Monoxide (CO) forms when there is insufficient oxygen available to support proper combustion. CO is al­ways the result of a rich mixture and can only be produced when combustion takes place. When a cylinder misfires, due to an open plug wire for example, CO cannot be produced since the hydrogen and carbon in the fuel have not been separated.
Carbon Dioxide (C02) not considered a pollutant. However, it is classified as a greenhouse gas (contributes to global warming) because of its ability to absorb heat in the atmosphere. C02 is an indicator of combustion efficiency and typically measures between 13% and 17% under ideal conditions. Carbon Dioxide is also used as a verification gas during a state emissions test.
Oxygen (02) levels can be compared to other gases to de­ termine the relative richness or leanness of the mixture, as well as evaluate the condition of the catalytic converter. Oxygen levels will vary depending on the air/fuel ratio and combustion effi­ciency.
Oxides of Nitrogen (NOx) result when combustion cham­ ber temperatures exceed approx­ imately 2500°F (1371OC). In this high-temperature environment, nitrogen and oxygen combine to form various NOx compounds such as Nitric Oxide (NO) and Nitrogen Dioxide (N02). Under the right atmospheric conditions, these compounds combine with hydrocarbons to form photochemical smog.

When combustion occurs in a controlled environment, it is a pollutant-free process that yields oxygen, nitrogen, carbon diox­ide, water and heat. A controlled environment allows all of the hydrocarbons to separate into their component parts of hydrogen and carbon. Both of these elements then combine with the oxygen required to form water and carbon dioxide. Except for absorbing heat, the nitrogen in the air, along with any remaining oxygen, exits the tailpipe unchanged. In the real world however, the air/fuel mixture never burns completely. This is due to variables such as charge density, cylinder temperature, and engine load, among oth­ers. Consequently, vehicles emit varying levels of hydrocarbons, carbon monoxide and oxides of nitrogen.

Stoichiometry: ‘Stoichiometry’ is the branch of chemistry that deals with the relationships between compounds in­ volved in a chemical reaction. While the word ‘stoichiometry’ may be unfamiliar to many technicians, its application to the internal combustion engine is universal. An air/fuel ratio of 14.7:1 represents the chemically correct proportions of air and fuel necessary to become a ‘stoichiomet­ric mixture.’ This ideal combination of air and fuel allows an engine to produce the most power, best econ­omy and least exhaust emissions per pound of gasoline . This is why mod­ern day fuel systems are designed to deliver a stoichiometric mixture under most operating conditions. When the amount of air in the mixture is less than 14.7 lbs., the mixture is said to be ‘rich.’ Con­versely, air in proportions greater than 14.7 lbs. will cause the mix­ture to be ‘lean.’ While maintain­ing the air/fuel ratio at 14.7:1 helps keep emissions at their lowest levels, a stoichiometric mixture is too lean for conditions such as cold starting and acceleration. In these situations, engines require a richer mixture to provide acceptable performance. To understand how emission levels react to changes in the air/fuel ratio, study the accompanying stoichiometric chart.


During the power stroke, a portion of the combustion gases leak past the piston rings into the crankcase. Known as ‘blowby,’ these gases consist primarily of unburned fuel and water vapor. The purpose of the PCV system is to remove these gases before they condense and combine with the oil to form sludge. If left continue to rise the engine oil is forced past seals and gaskets. The PCV valve contains a spring loaded plunger that regulates the flow of vapors from the crankcase to the intake manifold. Since manifold vacuum drops during acceleration, the spring inside the PCV valve unseats the plunger to provide the required space. When the volume of blowby exceeds the flow capacity of the PCV valve, as is the case under extreme loads, the increase in crankcase pres­sure forces the gases through the breather tube into the air cleaner. At this point , the blowby is drawn back into the engine and reburned to provide backfire protection. If the mixture should ignite inside the intake manifold, the resulting pressure will force the PCV plunger toward the crankcase end of the valve.This prevents the flame from enter­ing the crankcase and igniting any fuel vapor. While all vehicles are equipped with a crankcase ventilation system, not all of them use a PCV valve. On some vehicles, crankcase vapors are drawn through a fixed metering orifice.


There are two methods used to control idle speed on a fuel injected engine; air bypass and direct throt­tle plate control. As the name im­plies, the air bypass technique al­lows a controlled amount of air to go around the closed throttle plate. An ECM-controlled air valve regulates the amount of bypass. With direct control, the ECM adjusts idle speed using a reversible DC motor that op­erates the throttle plate directly. Idle speeds that are too low can cause the engine to idle rough or even stall, especially on deceleration. In addition, a low idle speed in­creases hydrocarbon emissions since it causes a reduction in combustion efficiency. When idle speed is too low, it is typically the result of throt­tle body contamination. Higher than normal idle speeds will cause harsh engagement when the transmission is placed into gear. This condition can result from a malfunctioning idle speed device, a binding throttle plate, or a vacuum leak (speed density systems only).


Exhaust Gas Recirculation is a process designed to reduce com­bustion temperatures, which is the key to controlling oxides of nitrogen (NOx) emissions. The process involves the dilution of the air/fuel mixture with metered amounts of exhaust gas. There are two reasons that exhaust gas dilution is effective at reducing NOx. First, exhaust gas is ‘inert’ (non-reactive), since it does not contain the oxygen necessary to support combustion. Second, dilut­ing the charge allows the inert gas to take up the space normally oc­cupied by additional air and fuel. Consequently, a diluted mixture burns more slowly, reducing com­bustion temperatures by as much as 300°F (149°C). Because NOx out­put is relatively low at idle or when the engine is cold, it is not necessary to dilute the air/fuel mixture under these conditions. Rather, exhaust gas recirculation is used during accelera­tion and part throttle cruise, when combustion temperatures have the greatest tendency to increase. The heart of the Exhaust Gas Re­circulation system is the EGR valve. This device regulates the volume of exhaust gas entering the intake man­ifold. Depending on the system, the EGR valve may be vacuum operated or electronic. In a basic vacuum operated de­sign, a ‘pintle’ is attached to a spring­ loaded diaphragm. The spring keeps the pintle closed until vacuum is applied to the area on top of the diaphragm. At this point, the dia­phragm overcomes spring pressure and raises the pintle off its seat. Pin­tle travel, and consequently EGR flow, is directly proportional to the strength of the vacuum signal. As a result, there is no EGR at wide-open throttle, since vacuum is zero under this condition.

Vacuum operated EGR valves are typically used in conjunction with one or more computer controlled solenoids. These valves contain one or more solenoids that open and close specifically sized metering orifices. The solenoids re­spond directly to command signals from the ECM. The solenoids are used to apply and vent the vacuum signal to the EGR valve. The computer oper­ates the solenoids based on inputs it receives concerning manifold pressure, throttle position and coolant temperature. Control solenoids are typically identified as being normally open (N.O.) or normally closed (N.C.). A normally open solenoid permits vacuum flow when de-ener­gized. In contrast, a normally closed solenoid blocks vacuum until the computer turns it on. Most solenoids are pulse width modulated, which means the computer turns them ON and OFF many times per second. This allows the vacuum signal to be modulated for better valve control. Some vacuum operated EGR valves contain an integral pintle position sensor. This device informs the com­puter about the actual mechanical operation of the EGR valve. Electronic EGR valves are hard wired to the computer and there force require no vacuum signal. The ECM controls these valves based on inputs such as mass airflow, throttle position, intake air temperature and coolant temperature.

Depending on the failure condition, a problem in the EGR system can result in excessive NOx or increased hydrocarbons. For example, if the valve fails to open or the pas­sages become clogged, the lack of exhaust gas recirculation will cause increased combustion temperatures and high NOx emissions. However, if the valve does open but won’t seal properly when closed, the engine will misfire at closed throttle. As a result, hydrocarbon emissions will become excessive. Because of the wide range of con­ trols used by manufacturers to oper­ate the EGR valve, consulting the appropriate service manual is a must for diagnosing an EGR-related prob­lem. Regardless of the system how­ ever, the EGR valve should always remain closed at idle, wide open throttle and whenever the engine is cold.


The catalytic converter is an in­tegral part of the vehicle’s exhaust system. Located between the manifold(s) and the muffler, the con­verter is intentionally placed closer to the engine. This allows it to capture the exhaust gases while they’re still very hot. This is important, since the ‘light-off temperature’ of the typical converter is at least 400°F (204°C). There are two basic types of converters including ‘pellet’ and ‘monolithic.’ Monolithic converters are smaller than pellet-type catalysts, produce less exhaust backpressure, and reach light-off temperature more quickly. A catalytic converter contains small amounts of noble metals that act as ‘catalysts.’ These metals in­clude platinum, palladium and rhodium. To assist in the oxidation and reduction processes, many converters produced since the early 1990s contain another element from the periodic table known as ‘cerium.’ Cerium is used to stabilize catalyst operation and enhance the effec­tiveness of the noble metals. It ac­complishes this by storing and re­leasing oxygen in the exhaust sys­tem. The performance of a catalytic converter is directly related to the air/fuel ratio. When the mixture is lean, the converter is more efficient at reducing HC and CO. However, lean mixtures cause an increase in combustion chamber temperature, resulting in higher NOx emissions. In contrast, the lack of oxygen in a rich mixture makes it more difficult for the converter to oxidize HC and CO, while the additional fuel inhib­its the production of NOx. That’s why a stoichiometric mixture is es­sential for the converter to operate at peak efficiency.

The most common symptom of a defective catalytic converter is the inability to pass a state emissions test. This is especially true regard­ing NOx failures. Although there are several different tests that can help determine converter efficiency, the best way to be sure the converter is the source of an emissions failure is to eliminate all other possibilities first. On OBD II vehicles, the cata­lyst efficiency monitor is the best barometer for evaluating converter performance. A defective converter can create other concerns beyond excessive emissions including:

  1. No-Start/Hard Start (clogged converter)
  2. Lack of Power (restricted converter)
  3. Knocking or Rattling (broken monolith)


Secondary Air Injection is a post­ combustion emission control sys­tem. The heart of the system is the air pump. Under certain conditions, pump air is delivered to the exhaust manifold(s), and on some vehicles, the catalytic converter. Check valves are used to prevent hot exhaust gases from backing up into the pump. When air is being supplied to the exhaust manifold for example, the check valve opens under pump pressure. When pump air is directed away from that location, exhaust sys­tem backpressure forces the check valve closed. The addition of air to the post-combustion gases provides secondary oxidation. This process helps convert residual hydrocarbons and carbon monoxide into water vapor and carbon dioxide. There are several types of pumps used by ve­hicle manufacturers.

A belt-driven pump is a vane-type pump driven by a belt attached to the crankshaft. The pump provides low-pressure air directly to the ex­haust manifold and catalytic converter (if applicable). Intake air en­ters. the pump through a centrifugal filter positioned behind the drive pulley. The filter consists of small fins that deflect airborne contami­nants away from the pump as it ro­tates. On most vehicles with this sys­tem, computer controlled solenoids are used to direct airflow to a specific location depending on engine oper­ating conditions. Typically, air is di­rected to the exhaust manifolds dur­ing open loop, and then switched to the catalytic converter during closed loop. Under certain conditions, such as heavy acceleration, the addition of oxygen to the exhaust could cause a backfire. To prevent this, pump air is diverted to the air cleaner or a remote silencer during this time. On some vehicles, the pump is equipped with an electromagnetic clutch, similar to an NC compressor. The clutch allows the air pump to oper­ate on demand, which reduces en­gine power losses and fuel consump­tion. Power to the clutch is provided through a computer controlled relay.

In the early 1990s, many manufacturers began using electric air pumps on engines that required air injection to meet federal emissions standards. The pump is controlled by the ECM and is typically activated during open loop operation. This is when hydrocarbon and carbon monoxide emissions are greatest. The pulse-air system eliminates the horsepower penalty associated with a belt-driven pump. The heart of the pulse-air system is the reed valve, which responds to pressure pulses in the exhaust system. When an exhaust valve opens, a low­ pressure area is created in the line extending from the reed valve to the exhaust system. This causes the valve to open. Under this condi­tion, air flows from the air cleaner through the open reed valve and into the exhaust where it oxidizes unburned fuel and carbon monox­ide. When the exhaust valve closes, exhaust back-pressure forces the reed valve closed.

A faulty secondary air system can cause several problems including backfiring, excessive HC and CO emissions, and improper fuel control. The latter occurs when pump air is delivered to the exhaust manifold(s) during closed loop. This is because the oxygen sensor interprets the additional air as a lean condition. In response, the computer commands a rich mixture. Eventually, this condition will lead to poor fuel economy, rotten egg odor, an overheated converter and/or an illuminated MIL.


The EFE system is designed to im­prove fuel vaporization at low ambi­ent temperatures. It is used on many vehicles equipped with Throttle Body Injection (TBI). In order for a liquid to change to a vapor, a specific quantity of heat is required. Unless a heat source is provided for the mix­ture when the engine is cold, the fuel will remain in liquid form and pool inside the intake manifold. In addi­tion, moisture can freeze on the cold throttle plate when relative humidity is high. The EFE system prevents these conditions by pre-heating the mixture, resulting in the following performance benefits:

  1. no throttle body icing
  2. leaner cold-start fuel curves
  3. lower open-loop HC and CO emissions
  4. better cylinder-to-cylinder mix­ture distribution
  5. improved cold driveability

The EFE system uses a ceramic heater grid mounted between the throttle body and intake manifold. When the engine is cold the heater is turned on. This allows the mixture to be preheated before it enters the manifold. On most vehicles, the grid is controlled by the ECM based on the signal from the coolant temperature sensor.


The thermostatic air cleaner is designed to improve fuel vaporiza­tion at low ambient temperatures. It is used on virtually all engines equipped with throttle body injec­tion, and provides the same benefits as the Early Fuel Evaporation sys­tem. There are three main compo­nents used in the TAC system in­cluding a heat stove, hot air supply hose (or pipe), and thermostatic air cleaner assembly. The thermostatic air cleaner incorporates a damper to establish the source of incoming air. The heat stove is a sheet metal enclosure surrounding the exhaust manifold, while the hot air supply hose serves as the link between the stove and air cleaner. There are two types of thermostatic air cleaner sys­tems in use.

Vacuum Operated TAC: On this system, the damper is con­trolled by a spring loaded actuator mounted on top of the air cleaner snorkel. Vacuum to the actuator is regulated by a temperature switch inside the air cleaner. When the engine is cold, manifold vacuum is ap­plied to the actuator and the damper closes off the snorkel. This allows only hot air into the air cleaner. As the heated air acts upon the tem­perature switch, the vacuum signal is bled off. This causes the actuator to begin opening the damper. Dur­ing this time, a mixture of heated air from the manifold and cold out­side air is drawn into the engine. Once the vacuum signal drops below a calibrated threshold, the damper opens completely. Under this condi­tion, only outside air is drawn into the engine.
Self-Regulating TAC: This system uses a sealed wax actuator to operate a spring-loaded damper. The actuator relies on the expansion and contraction of wax to move an internal piston. When the engine is cold, the wax is solid. Under this condition the piston is relaxed, and the spring keeps the damper closed (no outside air). As hot air from the heat stove acts upon the actuator, the wax inside begins to melt. This change from a solid to a liquid causes the wax to expand and move the piston forward against spring tension. As a result, the pis­ ton begins opening the damper door. During this time, a mixture of heated air from the manifold and cold outside air is drawn into the engine. Once the wax is completely melted, the piston opens the damper door fully. As a result, only outside air is drawn into the engine.


The Evaporative Emissions (EVAP) system prevents fuel vapors (hydrocarbons) from being released into the atmosphere. The center­ piece of the system is the charcoal canister, which traps and stores fuel vapors from the tank when the en­gine is off. Once the engine is run­ning and the conditions are appro­priate, the vapors are purged from the canister and drawn into the engine. Here the vapors are burned in the normal combustion process. The following components are used in a typical evaporative emissions system:

Charcoal Canister: The canister contains activated charcoal and serves as the storage receptacle for fuel vapor. Charcoal has the ability to absorb fuel vapors and then release them when fresh air passes over it.
Fill Control Tube: This tube runs along the top of the filler neck. It is used to shut off the pump nozzle during refueling once the tank is approximately 90% filled. This provides an expansion space at the top of the tank.
Gas Cap: The gas cap contains a pressure/vacuum relief valve. This device al­lows the tank to breathe in the event of a system malfunction, such as a kinked vent line.
Purge Solenoid: The purge solenoid controls the vacuum signal to the purge valve based on computer commands. De­ pending on the application, the sole­ noid may be normally dosed (N.C.) or normally open (N.O.) . A nor­mally open solenoid permits vacuum flow when De-energized. In contrast, a normally closed solenoid blocks vacuum until it is activated by the computer. The ECM will typically command purge when the engine is warm, the vehicle is traveling above a pre-determined speed, and the throttle is opened a specified amount.
Purge Valve: This is a vacuum controlled valve that regulates the flow of fuel vapor from the canister into the engine. When the purge valve is opened, the difference between manifold pressure and atmospheric pressure allows out­side air to flow through the canister and carry the stored vapors into the engine.
Canister Vent Solenoid: The vent solenoid is only used on vehicles with enhanced EVAP sys­tems (leak detection). The solenoid is located in the fresh air supply hose to the sealed charcoal canister. On a non-enhanced EVAP system, the canister is open to the atmosphere. The canister vent solenoid is nor­mally open, allowing fresh air to be drawn into the canister. The ECM activates the solenoid during the EVAP leak test to block the entrance of outside air. There are several symptoms associated with a faulty EVAP system including poor fuel economy, rough idle, stalling, rotten egg odor and/or high HC and CO emissions.

This post was written by: Martin Hand


If you find this information helpful please consider a donation. These articles, questions and comments are very time consuming so even a small donation gives me motivation to keep educating automotive owners. Donations will allow us to continue open questioning/comments, automotive education and repair tutorials in the future as the business grows. All proceeds go to the expansion and maintenance mdhmotors.com. Thank You

Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

Leave a Reply

Your email address will not be published. Required fields are marked *