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Secondary Air Injection system consists of a belt-driven Air Pump, single or dual AIR Diverter valve(s), an AIR Bypass valve, and AIR Bypass solenoids, air silencer/filter, Powertrain Control Module and connecting wires and vacuum hoses. The Belt-Driven Secondary Air Injection system provides either upstream/bypass air or upstream/downstream/bypass air. The number of these system configurations vary significantly with AIR Bypass and AIR Diverter valve combinations (Figure 179).



1. The PCM requires ECT, IAT and rpm inputs to initiate Secondary Air Injection operation.

2. The PCM provides one or more signals that activate one or more AIR solenoids.

3. The AIR solenoids control one or more AIR Bypass valves and/or AIR Bypass Diverter Valves in order to route secondary air depending upon the specific configuration.

4. The belt-driven Air Pump is pulling air through the filter any time the engine is running. The state of the valves determines if that air is bypassed to the muffler, or diverted into the exhaust stream.

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Before madly ripping out all the emissions systems on your vehicle, read this article. This is a list of Automotive Terms & Abbreviations.

Unburned fuel is probably the single biggest concern in vehicle emissions, not only because it's the most detrimental to the environment, but also because it's a waste of money. As engine management technology has progressed, a continually-increasing proportion of fuel is burned within the combustion chambers where it produces useable energy. Possibly the single biggest step in this direction is EFI, which results in MUCH more precise control of fuel flow, MUCH better atomization, and consequentially higher engine efficiency & reliability. Electronic engine management has also contributed significantly by instantly adjusting fuel delivery to the engine's exact state, and to the operator's needs. But overfuelling still occurs frequently (for several reasons), resulting in unacceptable HC emissions. The earliest attempt to reduce these emmissions was the addition of a device to "re-burn" the exhaust & consume this fuel (a "thermactor"). Engineers found that pure Platinum metal facilitated the reaction between fuel molecules & oxygen in the hot exhaust stream, without consuming the Platinum (meaning that it "catalyzes" the reaction). So powdered Platinum was mixed with ceramic clay & formed into honecomb-shaped tube extrusions to be incorporated in the exhaust system. Given its high surface area, the vast majority of the unburned fuel could be catalyzed before being emitted, but the Lead that was being added to gasoline as an anti-knock agent coated the Platinum, requiring UNleaded fuel to be produced. (The anti-knock agents in unleaded fuel are cheaper than Lead, but oil companies recognized the opportunity to gouge consumers & priced the new fuel accordingly.) But the high cost of Platinum & the expenses associated with developing the technology caused early designers to undersize catalytic converters, resulting in exhaust restrictions that noticeably reduced engine performance. Their initial solution was to add air to the exhaust (secondary air) using a belt-driven pump so that the fuel would burn more easily. But again; the system was too complicated & poorly designed for the typical mechanic to understand, so it was often neglected, modified, or sabotaged causing most people to think it was counterproductive or unnecessary. Over time, the cost of producing catalytic converters has come down, and the quality of their construction has gone up, making them very reliable & effective. So effective, in fact, that most now don't require the addition of downstream air. They have also been improved with additional catalyst chemicals that reduce CO & NOx emissions (3-way cats). Currently, the single biggest threat to a catalytic convertor/thermactor is probably mechanical damage. Collisions, road debris, improper service technique, & fording can shatter the delicate ceramic structure, causing exhaust restriction, noise, & increased emissions. But another significant threat is severe overfuelling (either because of fuel delivery or misfiring) which can overheat the ceramic substrate to the point that it powders & erodes. Modern engine management systems include dedicated downstream Oxygen sensors to monitor the catalysts' performance, but this performance generally has [b]no impact[/b] on engine performance (exhaust restriction being the main exception).



The 2ndry air system is known to fail in a wide variety of ways. The check valves that prevent hot exhaust from entering the rubber hoses age, rust, leak, & crack open melting the plastic TAB & TAD valves, creating exhaust leaks that can damage other components, raising exhaust oxygen levels (setting lean codes or rich adaptive limit codes), and making rattling noises. The hard steel tubing between the exhaust & the check valve can rust or crack (especially the infamous "crossover tube" on the backs of V8 heads). The vacuum controls leak (including the "coffee can" reservoir on the R wheelwell), get misrouted during other repairs, or the diaphragms rupture. The electronics that control the vacuum controls can fail electrically or mechanically, or the wires can be damaged. The 2ndry air pump can lock up mechanically. But all of these failures are either A) relatively cheap & easy to repair, or B) cheap & easy to prevent with normal inspection & maintenance.

The early steel coffee can reservoir commonly rusts &/or cracks, and the later plastic reservoir is an easy upgrade.