Smog testing or Emission testing is used in many areas of the country so as to improve air quality. But even if your community is not impacted, you still need to know about emissions testing and smog checks since problems that often cause emission failures can equally cause a variety of driveability and performance complaints.
Mandatory emissions testing has become a fact of life in many areas as a means of identifying cars that are “gross polluters” so they can be fixed. See the Vehicle Emissions I/M Programs to see if your state needs emissions testing. Such programs force car owners to have emission problems repaired that might otherwise be ignored. So when a car fails an emissions test or a smog check, most motorists are not exactly overjoyed to learn that repairs are required.
Everybody is for clean air as long as somebody else pays for it. That is why all new vehicles 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 parts as well as the fuel delivery system (except the pump and filter), ignition system (except spark plugs), and engine management system including all sensors.
In 1995, the federal emissions warranty requirements changed. The warranty was extended to 8 years and 80,000 miles on the powertrain control module and catalytic converter, but taken back to only 2 years and 24,000 miles on everything else. Even so, all of these parts are still covered for 3 years or 36,000 miles (or longer) by the individual car manufacturers bumper-to-bumper warranties. In California, the coverage is much longer.
Once a car is out of warranty, the burden of paying for emission repairs becomes the sole responsibility of the car owner.
Most inspection programs are inclusive of “waiver” provisions that limit the amount of money motorists have to spend on emission repairs. If an emissions issue 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 car 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 car has multiple issues (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 lowering the engines overall emissions. The car may still not be in compliance since 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 IS CHECKED
Each state or municipality pegs its own cut points for emissions testing as well as the specific tests that must be carried out. 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 add 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 car computer for any faults that might increase emissions), some inspection programs may need 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 nonstock aftermarket parts on the engine to make sure they are emissions certified (California primarily).
- If an engine has been swapped or changed, 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 presented.
WHAT ARE EMISSIONS TEST STANDARDS
The cut points for acceptable HC and CO levels are generally dependent on the emission standards that a car was required to meet when it was new, so older cars 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
Check that 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 get to be fixed and brought back into compliance.
FAILING AN EMISSIONS TEST
When a car fails an emissions test, the motorist usually gets a printout that show the test results of the car emissions as well as the applicable cut points. From this, you can determine if too much HC and/or CO made the vehicle to fail.
Hydrocarbon failures mean unburned gasoline is going 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 shows 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 cars 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 car may have a bad catalytic converter or an air pump issue.
NOX failures are often EGR-related, since the EGR system is primarily responsible for lowering 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 watch out for on every car that has a computerized engine control system:
- Scan for fault codes. Any codes that are found has to be dealt with before anything else.
- Make sure the system is going into closed loop. No change in loop status often shows a coolant sensor problem.
- 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).
- Confirm that the system responds normally to changes in the air/fuel mixture. To test the system response, pull off a vacuum hose to produce 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.
- Use your scan tool to confirm that 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 car should pass an OBD II plug-in emissions test.
HOW TO READ EMISSIONS: EXHAUST GAS ANALYSIS
Though a good technician can usually diagnose and repair emission problems without having to actually check tailpipe emissions, it is getting 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 cars emissions for diagnostic purposes, and to verify that the repairs made eliminated or lowered 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 oxyten in the exhaust can reveal whether the air/fuel ratio is correct or not as well as other issues that affect engine performance and emissions.
As combustion efficiency goes down, 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 indicate 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 issues.
WHY SOME CARS THAT SHOULD PASS AN EMISSIONS TEST DO NOT
Most cars that are in good running condition and properly maintained should pass an emissions test. In some cases, though, minor issues may cause the car to fail an emissions test. These include:
- Engine and/or converter not at operating temperature. If a car 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 impact 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 creating 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 cut 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 leading to excessive HC emissions.
- Dirty oil. The oil in the crankcase can become badly contaminated with gasoline if a car has been subject to a lot of short trip driving, especially during cold weather. These vapors can siphon back through the PCV system and create elevated CO readings.
- Pattern failures. Some cars tend to be dirtier than others for a given model year since 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 need 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 cars, the cars will be rejected for testing if all of the needed OBD II readiness monitors have not run. This may need driving the vehicle for several days until all the monitors have run. The car 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 functioning properly, the MIL is not on and there are no codes, the car should pass the test.