What is the purpose of a secondary air pump

A brief history of secondary air injection

In America, during the 1960s, regulations were introduced to combat high hydrocarbon and carbon monoxide emissions. These emissions were a result of incomplete combustion, often experienced when the mixture was excessively rich - such as during a cold start.

To alleviate these issues, air injection systems were introduced; Chevrolet, for example, unveiled its 'Air Injection Reactor' set-up in 1966. These systems, soon dubbed 'smog pumps' would use engine-driven pumps to deliver fresh air into the exhaust port or manifold of the engine, where the exhaust gases were hottest.

Other systems would rely on the pressure pulses in the exhaust to draw in fresh air but, in any case, the effect was similar - oxygen would be introduced to the exhaust stream and allow any remaining fuel to burn. However, these systems were often unreliable - partly because they relied on precise carburettor calibration - and many owners simply disabled them.

The introduction of catalytic converters and more accurate fuel metering systems, in the form of mass-produced electronic injection systems, also greatly reduced the need for air injection systems. That said, it was not uncommon to find air injection set-ups on large or high-performance engines which would produce excessive emissions when cold.

The 'S38' straight-six engine used in the E34 generation of M5 made use of such a system, as did the C4 generation of Corvette - and, when engaged, the system's operation would cause a pronounced change in tone. This additional volume would frequently be exacerbated when the pump itself was failing, which remains an issue in modern set-ups.

As emissions regulations continued to tighten and the control systems advanced, manufacturers again turned to air injection. This time, however, the aim of injecting air into the exhaust system was primarily to help warm up the catalytic converter more quickly.

No matter what automotive marketing geniuses tell you, engines simply can't run with 100 percent efficiency. So, there is always some portion of unburned fuel (known as hydrocarbons) that does not get converted to engine power during the combustion process (which takes place in the induction chamber, which is essentially the "primary" air injection system, even though no one really calls it that). These hydrocarbons are then simply churned out with the rest of the exhaust gases. This is due to a messy, imprecise, fluctuating combination of inevitable mechanical imperfection in mass-produced engine components, variations in fuel quality, environmental factors, driving conditions, and the overall condition of the car. Since hydrocarbons are the component of fuel that makes power, and they are also one of the toxins measured to determine a car's emissions levels, it's important that they're burned as much as possible. So until someone figures out a way to grab those tiny and nearly insignificant particles of precious fossil fuel out of the exhaust, sift them loose like a miner panning for gold, and inject them back into the engine for a second shot at a useful life, it'll go to waste. And as it's being wasted, it's polluting more -- double blow. The truth is, burned fuel is less toxic to the environment than unburned fuel, even when burning that fuel results in no power whatsoever.

How does secondary air injection work?

According to component manufacturer Pierburg, over 80 per cent of a car's driving cycle emissions are typically produced when the car is cold. Two factors contribute to this; the first is rich cold start mixtures, which cause excessive hydrocarbon and carbon monoxide emissions - both of which are a result of incomplete combustion.

The second issue is that the car's emissions-reducing catalytic converter doesn't become effective until it reaches around 300 degrees Celsius. This, in conjunction with the rich mixtures, causes high emissions in the initial phase of the driving cycle.

To help counter this, manufacturers fit secondary air injection systems. These feature an electrically driven air pump, which can be cycled on and off when needed, that supplies fresh air into the car's exhaust system. The air is injected upstream of the catalytic converter, where it can mix with hot waste gas from the engine's cylinders.

The presence of the oxygen-containing air in the hot exhaust gas allows for any remaining fuel to continue burning, which helps cut HC and CO emissions; the reactions that take place cause hydrocarbons to be oxidised into water and less harmful carbon dioxide, while carbon monoxide is oxidised into less harmful carbon dioxide.

Furthermore, the process increases the temperature of the exhaust gas. As the gas is hotter, it speeds the rate at which the catalytic converter warms up; when it is running efficiently, the air injection system can then be shut down - although there are sometimes variations in the way the systems operate.

Performing troubleshooting in the secondary air system

Tip 1: Test the electrical actuation of the secondary air pump by the engine control unit and the switching relay.

The function test for the secondary air pump can be performed when the engine is cold, as the pump will then be audible for up to 90 seconds. However, the test can also be carried out with the engine at operating temperature using a diagnostic tester or an external 12 volt voltage source. If no malfunction is identified, the following problems should be ruled out: signs of melting on the housing or on the plug contacts of the secondary air pump and/or a strong smell of burning. In these cases, it is essential to replace the switching relay.

Tip 2: Frequent causes of failure are water or exhaust gas condensate in the secondary air pump. This firstly causes loud noises and then leads to failure. Clear water or rust signal a leakage between the air filter and secondary air pump.

Tip 3: Exhaust gas condensate can only enter the pump through the secondary air valve, caused by the secondary air valve becoming stuck and no longer fully closing. You can use your finger to check whether there are any deposits. If there are deposits, this means the secondary air valve is faulty and must be replaced.

Tip 4: When it comes to pneumatically actuated secondary air valves, the solenoid valve being actuated and the vacuum should be checked. If a vacuum of at least 390 mbar is not reached, it is highly likely that there are leakages.