Secondary Air System
Pierburg was one of the first OEM suppliers to develop a secondary air pump and has since evolved a secondary air system that includes the pump and associated valve.
Why Secondary Air?
The reliable cold start of a petrol engine requires a “rich mixture” – a mixture with excess fuel. But since the monolith of the catalytic converter has not yet reached its working temperature (in the range of 300 to 350°C) a large quantity of carbon monoxide (CO) and unburnt hydrocarbons (HC) is produced.
Blowing oxygen-rich secondary air into the exhaust gas produces a post-oxidation (catalytic post-combustion) of pollutants. Even though the secondary air system is active for only 90 seconds after a cold start, it will considerably reduce HC and CO emissions during the cold start phase. Furthermore, the heat produced by post-oxidation reduces the warm-up time of the catalytic convertor. Because diesel cars always run with excess air – even during a “cold start” – there has never been any need to blow in secondary air.
Driven by a DC electric motor, the secondary air pump provides the fresh or ‘secondary’ air that enables the post-oxidation process. The pump is sealed off from the engine compartment in order to protect the motor from the corrosive effects of exhaust gas condensate.
The other key component in the system is the secondary air valve. The valve combines the function of a non-return valve and a shut-off valve, preventing backflow of exhaust gases into the pump. It also prevents air from entering the exhaust manifold after the cold start phase.
Faulty pump? Check the valve
With European on-board diagnostics (EOBD), the secondary air system is only checked with regard to the electrical connection of the pump, but not for its effect. A faulty secondary air system is often only detected by an unusual whistling noise from the secondary air pump or by a varying idle speed in the cold running phase.
In most cases, damage to the secondary air pump is caused by exhaust-gas condensate in the pump. As the secondary air valve is designed to prevent dirt and water entering the pump, the valve must, therefore, be faulty. Simply replacing the pump will not resolve the problem as the valve remains faulty and the new pump will inevitably fail again in a short time.
So, when garages call to order a new pump, ask them if they have checked the valve. A simple ‘finger test’ will tell you if the valve is faulty. Simply loosen the connecting hose on the valve leading to the secondary air pump, and wipe through the opening of the valve with one finger. If there are deposits on this side of the valve, the non-return valve is faulty and must be replaced along with the secondary air pump.
The message from Pierburg is simple: the OBD system often captures only the symptoms, the causes need to be detected by the expert. The malfunctioning of just one component in the secondary air system can result in damage to other components. It is therefore essential to check all the components when problems arise.