Tech Tips

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Emission System

General Emission Systems

With the increase in vehicle population and vehicle miles driven, look for more rigorous emissions testing to increase emissions parts sales.

In addition, sales will likely increase due to the Environmental Protection Agency's (EPA) recently changing warranty coverage for emissions control parts. In many cases, catalytic converters and engine computers have had their warranties extended to 80,000 miles.

Because it's illegal to sell components covered by mandated warranties, many emissions parts suppliers furnish wall charts that explain the duration and coverage of warranties for both new and old vehicle models.

Regardless, it's always a good idea to check warranty coverage before making an emissions parts sale for vehicles less than five years old.

EXHAUST PARTS
Because the catalytic converter is critical for reducing exhaust emissions, it's important to make sure the converter meets OEM specifications. Two-way catalytic or single-bed catalytic converters, for example, help oxidize hydrocarbons (HCs) and carbon monoxide (CO) into carbon dioxide and water. Three-way or dual-bed designs are specifically made to reduce nitrogen oxides (NOX).  Also in this configuration, air is pumped into a chamber located between the reducing and oxidizing catalyst beds to further reduce exhaust pollutants.

In addition to configuration, the converter must also be matched to engine displacement. Insufficient flow capacity through the converter may cause a vehicle to fail the rigorous IM/240 chassis dynamometer or treadmill test.

When selling a catalytic converter, remember worn ignition and fuel system parts can overheat the catalytic converter by allowing excess unburned fuel in the form of hydrocarbons into the exhaust stream.

AIR INJECTION SYSTEMS

Air pumps, diverter valves, air injection manifolds, and manifold check valves make up the air injection management system. Most air pump problems are due to excessive noise or bearing failure. When selling air injection manifolds, remember these manifolds corrode from the inside out. Because component salvage is difficult and uneconomical, manifolds should be sold with new check valves and hardware.

When a vehicle develops a severe backfire through the muffler, it's usually due to a malfunction in the diverter valve, vacuum source, or computer control system. Air diverter valves range from simple versions controlled by intake manifold vacuum to more elaborate computer-controlled types. In addition, air flow from the diverter valve speeds oxygen sensor warm-up time and aids oxidation in the catalytic converter immediately after cold engine start-up. Assuming its support systems are functioning correctly, the diverter valve should be replaced if the engine backfires on deceleration or the converter fails to oxidize correctly.

OXYGEN SENSORS
Oxygen sensors maintain a chemically correct or stoichiometric 14.7:1 air/fuel ratio by sensing the oxygen content of the exhaust stream. A zirconia-type oxygen sensor will produce a 0.5-volt signal to the engine computer when a 14.7:1 air/fuel ratio is present. Numerically higher ratios will decrease the voltage signal to 0.2 volts while numerically lower ratios will increase the voltage signal up to one volt. Oxygen sensors will cause a vehicle to fail an emissions test primarily because exhaust-born chemicals like oil ash, antifreeze and silicates will coat the sensor and reduce its sensitivity to changes in exhaust gas oxygen content.

When selling oxygen sensors, remember some imports use a titania-type sensor which changes resistance values rather than voltage to indicate oxygen content. Also remember that many V-8 and V-6 engines may incorporate a sensor on each exhaust manifold. In addition to two manifold sensors, 1994 and later OBDII equipped vehicles may also utilize a pre-catalytic and post-catalytic converter oxygen sensor to indicate converter efficiency. Most 1990 and later oxygen sensors are heated with battery voltage to accelerate sensor activity and decrease cold-start emissions.

EGR VALVES
Since the advent of 1M/240, EGR valve sales have risen because failure is more apparent when engines are load-tested on a chassis dynamometer. When the EGR fails to cool combustion temperatures by introducing a small amount of exhaust gas into the cylinder charge, NOX emissions rise dramatically. In addition, the engine may suffer mechanical damage by pre-igniting or detonating the fuel mixture.

lt's always important to sell the correct EGR valve for the application. Early EGR valves were simple devices operated by ported carburetor vacuum and thermal vacuum switches. Later EGRs used exhaust gas pressure to modulate exhaust gas flow through the valve into the intake manifold. Current EGR valves are either partially or fully electronically controlled. Many EGRs incorporate a valve position sensor to help the engine computer modulate exhaust gas flow into the intake manifold. The most recent EGR designs are completely controlled by electronics.

RELATED PARTS
If a poorly maintained vehicle fails an emissions test, the problem can often be solved by replacing worn ignition and fuel system parts. Worn spark plugs, leaking ignition cables, cracked distributor caps and perforated distributor rotors can cause engine misfire and converter damage.

The fuel system can also contribute to emissions if the system over-fuels the engine. Over-fueling can be caused by leaking fuel injectors, leaking or incorrectly calibrated fuel pressure regulators, and flooding carburetors. In addition to clogged air filters, defective intake manifold vacuum, throttle position, coolant temperature, inlet air temperature, and mass air flow sensors can increase exhaust emissions.

For IM/240 inspection areas, fuel cap sales will likely increase since evaporating gasoline vastly increases HC emissions. Fuel caps incorporate a vacuum valve to allow air to enter the tank as gasoline is consumed.

When tank pressure builds due to atmospheric pressure or heat, hydrocarbons are vented into a charcoal canister which is purged into the engine's intake manifold for combustion.

If either the cap, connecting hoses, operating system or canister itself are defective or missing, recommend a replacement before testing.

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