EMISSION CONTROLS AND PROBLEMS

Emissions Testing: Will Your Car Pass?

Emissions testing or Smog testing is used in many areas of the country to improve air quality. But even if your community is not affected, you still need to know about emissions testing and smog checks because problems that often cause emission failures can also cause a variety of drive ability and performance complaints.
Mandatory emissions testing has become a fact of life in many areas as a means of identifying vehicles that are "gross polluters" so they can be fixed. 

Everybody is for clean air as long as somebody else pays for it. That is why all new cars and light trucks built from 1981 through 1995 have had a 5 year, 50,000 mile federal emissions warranty. This warranty covered all emission control components as well as the fuel delivery system (except the pump and filter), ignition system (except spark plugs), and engine management system including all sensors.
Most inspection programs include "waiver" provisions that limit the amount of money motorists have to spend on emission repairs. If an emissions problem cannot be resolved within the specified waiver limit (which may be anywhere from $50 up to $450 depending on the local regulations and applicable model year), the vehicle gets a "pass" even though it may still be a polluter.
The objective, therefore, is to get the most bang for your repair buck (the most pollution reduction for the least out-of-pocket repair expense).
When a vehicle has multiple problems (one or more fouled or worn spark plugs, one or more bad plug wires, plus worn rings and valve guides), zero in on the repairs that will make the most noticeable improvement. A single misfiring plug, for example, can increase hydrocarbon emissions enormously (10 times normal!). Replacing the spark plugs (and plug wires if necessary) will eliminate the misfire problem (at least temporarily) and make a dramatic difference in reducing the engines overall emissions. The vehicle may still not be in compliance because the engine is burning oil, but it will run cleaner than it did before and have cost far less than an engine overhaul.

EMISSIONS TESTING: WHAT THEY CHECK

Each state or municipality determines its own cut points for emissions testing as well as the specific tests that must be performed. Most test programs look at only two pollutants: unburned hydrocarbons (HC) and carbon monoxide (CO). Most also measure carbon dioxide (CO2) but only for diagnostic purposes since CO2 is not a pollutant (though it is a "greenhouse gas" that may contribute to global warming).
Although most emission test programs have now gone to a simple OBD II plug-in test (which does NOT measure tailpipe emissions but checks your vehicle computer for any faults that might increase emissions), some inspection programs may require visual checks of various emissions-related equipment for evidence of tampering. These include:
  • Checking the restrictor in the fuel tank filler neck to make sure it has not been knocked out or enlarged to accept regular leaded gasoline.
  • Inspecting the gas cap to make sure it is the correct type for the application and seals tight.
  • Looking under the car to see that the catalytic converter is in place.
  • Checking the instrument panel to see if the "Check Engine" or malfunction indicator lamp is illuminated.
  • Checking under the hood to make sure the engine has all the required emission control components (California primarily).
  • Checking any non-stock aftermarket parts on the engine to make sure they are emissions certified (California primarily).
  • If an engine has been swapped or replaced, making sure it has all the required emissions equipment for the original model year and application (California primarily).
  • In areas that do an I/M 240 test, they check oxides of nitrogen (NOX) emissions as well as total emissions in grams per mile as the vehicle is run at various speeds and loads on a dyno. An I/M 240 test may also require checking the integrity of the fuel system for air leaks (evaporative emissions) and the flow capacity of charcoal canister purge valve.
  • In areas that do an OBD II plug-in emissions check, they want to make sure the Check Engine light is functioning and is OFF, and that there are no trouble codes present.

EMISSIONS TEST STANDARDS

The cut points for acceptable HC and CO levels are generally based on the emission standards that a vehicle was required to meet when it was new, so older vehicles have more lenient emission standards than new ones (see chart).

EMISSIONS CUT POINT CHART

Model year...Typical Cut Points...Well-tuned engine
..............CO%.....HC ppm..........CO%.....HC ppm
pre-1968..... 7.5-12.5... 750-2000..... 2.0-3.0... 250-500
1969-70..... 7.0-11.0... 650-1250..... 1.5-2.5... 200-300
1971-74..... 5.0-9.0... 425-1200..... 1.0-1.5... 100-200
1975-79..... 3.0-6.5... 300-650..... 0.5-1.0... 50-100
1980........ 1.5-3.5... 275-600..... 0.3-1.0... 50-100
1981-93..... 1.0-2.5... 200-300..... 0.0-0.5... 10-50
1994 & up..... 1.0-1.5... 50-100..... 0.0-0.2... 02-20
Notice in the emissions cut point chart that the actual emissions produced by the average well-tuned engine are substantially less than the cut points required to pass an emissions test. The actual cut points are more lenient because the goal of emissions testing is to identify the gross polluters so they can be fixed and brought back into compliance.

FAILING AN EMISSIONS TEST

When a vehicle fails an emissions test, the motorist usually receives a printout that show the test results of the vehicles emissions as well as the applicable cut points. From this, you can determine if too much HC and/or CO caused the vehicle to fail.
Hydrocarbon failures mean unburned gasoline is passing through the engine and entering the exhaust. The three most common causes include ignition misfire, lean misfire and low compression (typically a burned exhaust valve). Ignition misfire can be caused by worn or fouled spark plugs, bad plug wires or a weak coil. Lean misfire results where there is too much air and not enough fuel, so check for vacuum leaks, dirty injectors or a fuel delivery problem. In addition to these, hydrocarbon failures can also be caused by oil burning due to worn valve guides, valve guide seals and/or rings.
Carbon monoxide failures indicate an overly rich fuel mixture. On older carbureted engines without electronic feedback controls, look for things like a stuck choke, misadjusted or fuel saturated float or a rich idle mixture adjustment. On newer vehicles with electronic carburetors or fuel injection, the system may not be going into closed loop because of a bad coolant or oxygen sensor.
If both HC and CO are high, the vehicle may have a bad catalytic converter or an air pump problem. 
NOX failures are usually EGR-related, since the EGR system is primarily responsible for reducing oxides of nitrogen. But NOX emissions can also be caused by a bad three-way converter or a computer control system that remains in open loop.

EMISSIONS PERFORMANCE CHECKS

There are five things you should always check on every vehicle that has a computerized engine control system:
1. Scan for fault codes Any codes that are found need to be dealt with before anything else.
2. Make sure the system is going into closed loop. No change in loop status often indicates a coolant sensor problem.
3. Confirm that the system is alternating the fuel mixture between rich and lean. This is absolutely essential for the converter to function efficiently. You can do this by observing the O2 sensors output with a scan tool, or directly with a digital storage oscilloscope or voltmeter. If everything is okay, the sensor should be producing an oscillating voltage that flip-flops from near minimum (0.1 to 0.2v) to near maximum (0.8 to 0.9v). O2 sensors in feedback carburetor applications have the slowest flip-flop rate (about once per second at 2500 rpm), those in throttle body injection systems are somewhat faster (2 to 3 times per second at 2500 rpm), while multiport injected applications are the fastest (5 to 7 times per second at 2500 rpm).
4. Confirm that the system responds normally to changes in the air/fuel mixture. To test the system response, pull off a vacuum hose to create an air leak (not too large or the engine will die). You should see an immediate voltage drop in the O2 sensor output, and a corresponding increase in injector dwell or mixture control dwell from the computer. Making the fuel mixture artificially rich by injecting some propane into the intake manifold should cause the O2 sensor output to rise and the computer to lean out the fuel mixture.
5. Use your scan tool to confirm all monitors have completed and that the vehicle is ready for emissions testing BEFORE it goes to a test station. If all monitors are ready, and no faults have been found, the vehicle should pass an OBD II plug-in emissions test.

READING EMISSIONS: EXHAUST GAS ANALYSIS

Though a good technician can often diagnose and repair emission problems without having to actually check tailpipe emissions, it is becoming increasingly necessary today to have an infrared exhaust analyzer with at least three gas and preferably four gas (or even five gas) capability. Why? To baseline vehicle emissions for diagnostic purposes, and to verify that the repairs made eliminated or reduced the emissions problem.
Reading HC and CO at the tailpipe to diagnose emission problems may not give you the complete picture because the catalytic converter "masks" many problems by significantly lowering HC and CO in the exhaust. That is where a three- or four-gas analyzer comes in handy. The relative proportions of carbon dioxide and oxygen in the exhaust can reveal whether the air/fuel ratio is correct or not as well as other problems that affect engine performance and emissions.
As combustion efficiency decreases, the oxygen content in the exhaust rises and carbon dioxide falls. An engine that is running at a nearly ideal air/fuel ratio of 14.5:1 will show about 14.5 percent carbon dioxide and 2.5 percent oxygen in the exhaust. Carbon dioxide readings of less than about 13 percent and oxygen readings greater than about 4 or 5 percent indicate poor combustion efficiency. This translates to an over-rich or over-lean air/fuel ratio, poor compression, or an ignition problem.

WHY SOME VEHICLES THAT SHOULD PASS AN EMISSIONS TEST DO NOT

Most vehicles that are in good running condition and properly maintained should pass an emissions test. In some cases, though, minor problems may cause the vehicle to fail an emissions test. These include:
  • Engine and/or converter not at operating temperature. If a vehicle is only driven a short distance to the test facility, it may not be warm enough for the engine to be at normal operating temperature and/or the converter at light-off temperature. This will affect the emissions of the engine and may cause it to fail. Excessive idling while waiting in a test lane may also cause the catalytic converter and/or oxygen sensor to cool down enough where they may not control emissions properly causing higher than normal readings.
  • Idle speed too high. A few hundred rpm can sometimes make the difference between passing and failing an emissions test if emissions are marginal.
  • Dirty air filter. A restricted air filter will choke off the engines air supply, causing higher than normal CO readings.
  • Worn or dirty spark plugs. Excessive plug gap and fouling deposits can create ignition misfire resulting in excessive HC emissions.
  • Dirty oil. The oil in the crankcase can become badly contaminated with gasoline if a vehicle has been subject to a lot of short trip driving, especially during cold weather. These vapors can siphon back through the PCV system and cause elevated CO readings.
  • Pattern failures. Some vehicles tend to be dirtier than others for a given model year because that is the way they were built. It may be the design of the engine, or the calibration of the fuel or engine control system. These kinds of problems may require special "fixes" that can only be found in factory technical service bulletins.
In areas that have plug-in OBD II emissions testing for 1996 and newer vehicles, the vehicle will be rejected for testing if all of the required OBD II readiness monitors have not run. This may require driving the vehicle for several days until all the monitors have run. The vehicle will also fail the test if (1) the test computer cannot establish communication with the vehicle PCM (defective or disabled diagnostic connector), (2) if the Malfunction Indicator Lamp (MIL) is on, or there are fault codes in the PCM. If the OBD II system is working properly, the MIL is not on and there are no codes, the vehicle should pass the test.

How Fuel Injection Affects Emissions

Fuel injection provides a continuously adjustable air/fuel mixture that changes with engine speed, load, throttle position, air temperature, coolant temperature and other operating conditions. This allows EFI to provide an Air/Fuel (A/F) ratio that minimizes emissions while maximizing fuel economy.
The optimum air/fuel ratio for lowest emissions is 14.7 pounds of air to 1 pound of fuel. This is called the "stoichiometric" ratio (or on European vehicles as Lambda equals one). An A/F ratio of 14.7 to 1 is chemically balanced and results in the most complete combustion with minimal carbon monoxide (CO) and unburned hydrocarbons (HC). Oxides of nitrogen (NOx) can still be relatively high depending on combustion temperatures, but at an A/F ratio of 14.7 to 1 a three-way catalytic converter will break down most of the NOx in the exhaust. The end result is almost zero HC and CO coming out of the tailpipe, and very little NOx.
The flexibility of a fuel injection system allows it to rebalance the A/F mixture as the engine's operating conditions change. The engine computer is preprogrammed with a basic fuel mixture map that includes various combinations of engine speed, load, temperature (air and coolant), throttle position and airflow. The sensor data that is most important for making A/F mixture adjustments depends on the type of EFI system.

BASIC EFI SYSTEMS

There are two basic types of EFI systems: mass airflow and speed/density. With mass airflow systems, some type of airflow sensor is used to measure the volume of air entering the engine. 
The airflow sensor is located in the tubing between the air cleaner housing and throttle. The MAF sensor's reading is combined with other sensor inputs such as engine RPM, throttle position, engine load (via a Manifold Absolute Pressure sensor) and other information to calculate what the optimum A/F ratio should be for that particular moment.

Speed/density EFI systems do not have an airflow sensor. Airflow is estimated using engine RPM, inlet air temperature, throttle position and engine load (via the MAP sensor). A speed/density system is not as sophisticated as a mass airflow system, but it is less expensive for the auto maker because it requires no MAF sensor. A speed/density EFI system is also less sensitive to small air leaks that may occur in the tubing between the air cleaner and throttle, or in the intake manifold or vacuum hoses that connect to the manifold. Air leaks can throw off a mass airflow EFI system and cause it to run leaner than normal because "unmetered" air that sneaks into the engine on the down wind side of the airflow sensor is not factored into the A/F calculations.

FEEDBACK FUEL CONTROL

Electronic EFI systems also use one or more Oxygen sensors or Air fuel sensors to constantly fine tune the A/F mixture. Both types of sensors monitor unburned oxygen in the exhaust. The computer uses this information to determine if the A/F mixture is running rich (too much fuel, not enough air) or lean (too much air, not enough fuel). This creates a feedback loop that allows the EFI system to fine tune the A/F mixture beyond what the other sensor inputs are telling the computer.

The upstream oxygen sensor tells the PCM if the engine
is running rich or lean so the PCM can readjust the A/F ratio.
The downstream O2 sensor is used primarily to monitor catalyst efficiency.
On some applications, input from the downstream O2 sensor is also
used for additional long term fuel trim adjustments.

WHY THE A/F RATIO IS ALWAYS CHANGING

The A/F ratio is not constant but is constantly readjusting according to changing operating conditions. For example a significantly richer A/F ratio is required following a cold start so the engine will idle smoothly without stalling. A richer mixture is also needed when the engine is under load for more power, and when the throttle suddenly snaps open to prevent hesitation. Sometimes the A/F ratio can be made leaner than usual to improve fuel economy, such as when the vehicle is cruising under light load and throttle. The fuel mixture can also be leaned out or cut off entirely during deceleration to conserve additional fuel. It all depends on the operating strategies programmed into the computer. Reprogramming the computer is sometimes necessary to correct an emissions or driveability problem.

HOW THE AIR/FUEL RATIO IS CHANGED

The A/F ratio is adjusted by varying the pulse duration, duty-cycle or on-time of the fuel injectors. Increasing the on time of each injector pulse sprays more fuel into the engine and richens the A/F mixture. Decreasing the on time of each injector pulse leans the A/F mixture and reduces the volume of fuel sprayed into the engine. The speed of the computer determines how quickly such changes can be made. Many late model EFI systems are so fast they can readjust the A/F mixture between individual cylinder firings!
Basically, the engine computer does the opposite of what the oxygen sensors are telling it. When the O2 sensors read lean, the computer richens the A/F ratio and increases injector duration to richen the fuel mixture. When the O2 sensors read rich, the computer shorten the injector pulses to lean the fuel mixture.

Fuel trim values can be read on a scan tool.

The STFT readings shown here are abnormally high
because the engine is off. As soon as the engine starts,
the actual values will start to display.

MONITORING FUEL TRIM

The computer monitors the constantly changing A/F ratio by tracking "short term fuel trim" (STFT) and "long term fuel trim" (LTFT). You can view the fuel trim values on a scan tool to get a good idea of what kind of corrections are being made, and whether or not these corrections indicate a possible problem.
STFT is what's happening right now. STFT is constantly jumping around as the computer adjusts and readjusts the A/F ratio.
LTFT is how the fuel mixture is trending over time, and is the best indicator to tell you if the engine is running too rich or too lean and is possibly experiencing an emissions problem.
POSITIVE (+) fuel trim numbers mean the EFI system is adding fuel to compensate for a LEAN A/F mixture.
NEGATIVE (-) fuel trim numbers mean the EFI system is subtracting fuel to compensate for a RICH A/F mixture.
The changes in fuel trim are based on what the oxygen sensors are telling the engine computer. Consequently, fuel trim values and adjustments are only as accurate as the oxygen sensor inputs.
Any number of things can create inaccurate oxygen sensor readings. A misfiring spark plug, a leaky exhaust valve, or an air leak in the exhaust manifold gasket can allow false air into the exhaust. This will cause the O2 sensor to read leaner than it should, which in turn will cause the computer to richen the fuel mixture unnecessarily and increase the fuel trim numbers. This can hurt both fuel economy and emissions.
Conversely, if an old oxygen sensor is sluggish, contaminated, unresponsive or dead, it may underreport exhaust oxygen levels. This will mislead the computer into thinking the mixture is richer than it really is. The computer will lean out the mixture unnecessarily, causing the fuel trim numbers to go negative. An overly lean A/F mixture will increase NOx emissions, increase the risk of detonation (spark knock), and increase the risk of lean misfire -- which will cause a drop in performance and fuel economy, as well as a big jump in hydrocarbon emissions.
Ideally, STFT and LTFT should both be within a few percentage points of zero when the engine is idling or being held at a steady RPM and load. Good LTFT readings should be as close to zero as possible, though they often range from 5 to 8 percent depending on the condition of the engine. If LTFT numbers are more than 10, it usually indicates a problem that needs to be diagnosed. LTFT values that go as high as 20 or higher will usually set a P0171 or P0174 lean code and turn on the Check Engine light.
High positive LTFT fuel trim numbers due to a lean A/F mixture may be the result of dirty fuel injectors a dirty mass airflow sensor, vacuum leaks at the intake manifold or in vacuum hoses, a weak fuel pump, or a faulty fuel pressure regulator that cannot maintain adequate pressure to the injectors.
Electronics cleaner can be used to clean a dirty MAF sensor, while fuel cleaner or an off-car injector cleaning machine can be used to clean dirty injectors. Vacuum leaks can often be found by lightly pressurizing the intake manifold (no more than 3 or 4 PSI) and spraying soapy water on suspected leak points. Bubbles anywhere indicate a leak. Mineral oil vapor can also be fed into the intake manifold under light pressure using a smoke machine to search for small leaks.
Excessive negative LTFT fuel trim numbers caused by a rich A/F mixture may be the result of oxygen sensors that need to be replaced, false air getting into the exhaust, compression leaks, ignition misfire (check the spark plugs and coils), one or more leaky fuel injectors, or excessive fuel pressure due to a faulty fuel pressure regulator.
LABELS: EMISSION