Emissions Control
Systems
Below are general
descriptions of emissions control systems that are inspected
during an emissions inspection. Not all vehicles will have all
of these systems installed on them. The inspection process will
only check for systems that were installed on the vehicle by the
vehicle manufacturer. Different vehicle manufacturers may use
slightly different versions/configurations of these systems. For
more detailed specifications for your vehicle, visit a local
dealership or repair shop.
The air injection system
is designed to introduce outside air into the exhaust stream to
assist in burning the gases produced by the engine. Air is
injected into the exhaust system at one of several locations
including the cylinder head, exhaust manifold, or directly into
the catalytic converter. Outside air is drawn by an air pump
(sometimes referred to as a smog pump), a pulse air valve, or a
reed valve. The air pump is run by a fan/accessory belt on the
front of the engine or, in some cases, a small electric motor.
The pulse air and reed valves work by sensing the drop in
manifold vacuum produced by an exhaust valve closing. Each time
the drop in vacuum is noticed, a "puff" of fresh air is pulled
into the exhaust stream.
Some related parts of an
air injection system can include, but are not limited to: air
pump, check valve(s), gulp valve, diverter valve, air supply
tube, and/or an air manifold.
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An exhaust gas
recirculation system is designed to reduce nitric oxide
emissions from an engine. Nitric oxide is produced when the
temperature of the combustion chamber rises above 2,500 degrees
Fahrenheit. At these high temperatures, the nitrogen (78% of
outside air) and the oxygen (21% of outside air) chemically
combine to create nitric oxide, a major contributor to ground
level ozone. The name of the system describes it's operation.
During times of slight acceleration or constant load (the
dynamometer simulates this load), a small amount of exhaust gas
is recirculated back into the combustion chamber of the engine.
This helps cool the temperature of the combustion chamber which
in turn lowers the amount of nitric oxide produced. High
temperatures in the combustion chamber can also produce a
knocking or pinging sound in the engine, indication of a problem
which can, over time, damage the engine.
Some related parts of an
exhaust gas recirculation system can include, but are not
limited to: the EGR valve, EGR solenoid, vacuum line(s), and
intake manifold (EGR passages within).
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The positive crankcase
ventilation system keeps blowby gases from escaping from the
engine. During combustion, a portion of the burned air/fuel
mixture is forced past the piston rings and ends up in the
crankcase and oil pan. These gases are a raw and untreated
pollutant. The PCV system is designed to draw these untreated
gases from the crankcase and reintroduce them into the intake
stream. Once they enter the intake stream, they can be burned
off by the combustion process and other emissions control
components (such as an air injection system and/or a catalytic
converter). Blowby gases that are not evacuated from the
crankcase and oil pan can contribute to engine corrosion, oil
dilution, and engine deposits or sludge. Since the system must
be sealed to operate properly, oil fill caps and dipsticks need
to be in place and properly seated or tightened.
Some related parts of a
positive crankcase ventilation system can include, but are not
limited to: PCV valve, vacuum hose(s), sealed oil cap and
dipstick, and fresh air intake tube.
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The catalytic converter
(or catalyst) is probably the simplest and most effective
emissions control system on a vehicle. As exhaust gases pass
through the honeycomb or pellets contained inside the shell, a
chemical reaction takes place. The platinum and/or palladium
that coats the honeycomb or pellets speeds the change of
hydrocarbons (unburned fuel) and carbon monoxide (partially
burned fuel) into water and carbon dioxide. Some catalytic
converters also contain the metal rhodium. These catalysts are
referred to as "three-way catalyst." The rhodium's purpose is to
reduce amounts of nitric oxide. In many cases, an air injection
system may be routed directly to the catalytic converter. The
extra oxygen introduced by the air injection system helps the
catalyst perform better under certain conditions.
An air/fuel mixture that
is overly rich (too much fuel, not enough air), can destroy a
catalytic converter over time, as can any lead in the
fuel. This can create extremely high temperatures in the
catalyst and can ultimately damage and destroy the honeycomb or
pellets inside the catalyst. In extreme cases, the outside shell
of the catalyst actually glows orange or red from the extreme
heat inside, and the contents of the catalytic converter can
actually melt and clog the exhaust system creating an
undriveable vehicle.
Many people think that
they can simply replace the catalytic converter and pass the
emissions inspection. If the problem is the catalytic converter,
of course this would be the right solution. But, in many cases,
a new catalytic converter will simply "mask" an underlying
problem with the air/fuel ratio, ignition system, or some other
problem with the engine. A catalytic converter should only be
replaced if it is operating inefficiently or not at all. There
are tests that can be performed on a catalytic converter to
determine whether or not it needs to be replaced.
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About 20% of hydrocarbon
emissions emitted by a vehicle can be caused by the evaporation
of fuel. The fuel evaporative system is designed to hold the
hydrocarbon vapors and introduce them into the engine to be
burned. A vapor line running from the fuel tank and carburetor
(if equipped) transports fuel vapors to an evaporative canister
(or charcoal canister). The hydrocarbon molecules attach
themselves to the charcoal contained in the canister. When the
engine is started (or shortly thereafter), the vapors are drawn
from the canister and pulled into the engine to be burned along
with the air/fuel mixture. Gas caps are also part of the fuel
evaporative system. They prevent pressurized vapors from exiting
the fuel tank through the filler neck. The gas
cap pressure test is used to verify the condition of the gas
cap.
Some related parts of a
fuel evaporative system can include, but are not limited to: a
charcoal canister, vapor hose, purge hose, purge solenoid, gas
cap, and vapor-liquid separator.
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This system regulates
air temperature flow into the engine. The thermostatic air
cleaner system is designed to draw heated air from around the
exhaust manifold of the engine during a cold engine "start-up"
and as the engine warms up to normal operating temperature, a
valve changes position to let cool air enter the engine.
Some related parts of a
thermostatic air cleaner system can include, but not limited to:
exhaust manifold heat shroud, hot air pipe (tube), air door
(valve) assembly, vacuum diaphragm and hoses, and a temperature
sensor.
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One function of the gas
cap is to prevent harmful fuel vapors from escaping into the
atmosphere. A gas cap pressure test is performed to test the gas
cap's ability to hold pressurized vapors inside the fuel tank.
This test is automated by the analyzer. A small amount of air
pressure is applied to the gas cap. If the gas cap releases too
much of this air pressure, it will fail the test. If the
original cap fails, a new gas cap may be purchased at the
station if the customer wishes, tested, and installed on the
vehicle during the inspection in order to save time and avoid
having to return to the station for a retest.
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HC is the number when the engine is running rich. CO is your
oxygen sensors. NO is your Catalytic converter and/or EGR
system.
The easiest fix is to change the CAT and O2 sensors, check the
base timing, and do a good tune up, wires, plugs, cap, rotor,
filters, and clean the throttle body.
Other things that affects high NO readings:
- engine too hot, high combustion chamber temperatures
- aftermarket intake modifications
- aftermarket exhaust headers
- aftermarket CAT
- brittle or cracked vaccuum lines, especially line to the EVAP
& EGR
- Missing EGR system
Check your cooling system. If the cooling system can not
keep the engine temperature cool enough the NO reading will go
up.
Fixing the CO and NO first will bring the HC number down
automatically.
High HC = burning rich > controlled by O2
sensors, fuel / air ratio = high CO reading. Fixing CO first
will usually bring HC number down.
Changing the CAT will further bring down the
numbers.
An original CAT from the dealer will cost you approximately ten
times as much. Aftermarket CATs will work, but they don't last
long.
After CAT. replacement` drive the car for a day or
two to get it burned in. It needs to be very HOT in order to
operate correctly.
Just before getting it retested, drive it good and hard for
about 20 minutes.
COPING WITH REJECTION
When a vehicle fails an emissions test, the motorist usually
receives a printout that show the test results of the vehicle’s
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’s 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.
PERFORMANCE CHECKS
There are four 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 sensor’s output with a scan tool, or directly with a
digital storage oscilloscope or voltmeter. If everything’s 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’s 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’s 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.
READING EMISSIONS
Though a good technician can often diagnose and repair
emission problems without having to actually check tailpipe
emissions, it’s 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’s 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 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 DON’T
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. 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 won’t 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 an engine’s emissions are marginal.
- Dirty air filter.
A restricted air filter will choke
off the engine’s 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’s 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.
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