Spark-ignition engines produce the largest amount of pollution during a cold start. Until the engine and the catalytic converter reach operating temperature, more fuel is needed, producing larger quantities of carbon monoxide and unburnt hydrocarbons.

Air injection reaction systems reduce pollution and fuel consumption. (Drawing courtesy of General Motors)

Automakers introduced air injection reaction in 1968 to achieve the correct air/fuel mixture adjustment in a shorter period of time and for the next decade or so, they equipped most car and truck engines with an engine-driven air pump and air injection fittings in the exhaust manifolds or cylinder heads.

The secondary air injection system quickly brings the catalyst to operating temperature. (Drawing courtesy of Ford)

Today the air pump is back, but it serves a different purpose. Instead of being engine driven, the pump is electric, and it’s turned on just a few seconds after cold start. It pumps air into the exhaust, upstream of the oxygen sensor, either to a single point in the manifold or directly to each exhaust port. At the same time, the powertrain control module may lengthen the injector pulse. The extra fuel and air react in the exhaust manifold, causing a rapid rise in exhaust gas temperature that brings the catalyst up to operating temperature quickly.

Quick tips for diagnosing air pump system issues:

Air check valve. (Drawing courtesy of Ford)

Rarely does a secondary air pump fail by itself. The actual cause is frequently found in a sticking non-return valve between the secondary air pump and the exhaust manifold.

Secondary air system. Drawing courtesy of Volkswagen.

The combination valve is the device which keeps exhaust gas and water out of the pump, and is most commonly overlooked. Water from the exhaust stream contains acids that can damage the valve and carbon that can clog the valve or prevent it from sealing when closed.

Sometimes due to exhaust gas leaking backwards through the secondary air system, the pump discharge hose becomes brittle, cracked, or otherwise deteriorated enough that it obviously should be replaced. The vacuum hose to the combination valve (on vacuum operated systems), is especially prone to deterioration as the hose ages.

Solenoid valves and relays rarely present problems but verify them for proper operation as part of a whole-system diagnosis.

  1. A scan tool is useful for testing the system as a whole because it can be used to command the pump and solenoid valve. Without a scan tool, you can still hear the pump run for at least 15 seconds after a cold start, and you can use a hand-held vacuum pump to test the combination valve.
  2. Tee a vacuum gauge between the solenoid valve and combination valve, and then start the engine and/or command the system to operate and/or open the valve with the vacuum pump. If the combination valve does not open, the pump will make enough noise to hear it from inside the car. Some people describe it as a squealing sound or sounding like a shop vacuum.
  3. With the pump’s air inlet hose disconnected from the intake duct, command the pump to run and operate the combination valve manually, listening for a change in the pump sound.
  4. Stop the pump and listen to the noise coming from its inlet hose with the engine running. If you hear noise and feel or smell exhaust when the pump is not running, the combination valve is not closing properly. With the pump running, you should be able to feel it drawing air though the hose. You’ll hear some pump noise through the open hose, but it should sound smooth, even with your thumb momentarily blocking the hose.
  5. To inspect the pump, start by removing the inlet and outlet hoses to see if there is water inside. It’s common on models with the pump mounted low in the engine compartment. Often but not always, when the pump locks up from corrosion, it will blow a fuse. The only other inspection is to run the pump with both hoses disconnected and listen for unusual noises, as described earlier.

To summarize, on most systems, the air pump is rarely the first part to fail. Verify that the whole system, component by component, is operating properly. After making any repairs, be sure to reset the readiness monitors.

Whether it's related to emissions or another system, check ChiltonDIY for comprehensive online repair procedures, specifications, diagnostics, and technical bulletins to help you service your car or truck.

A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 25 years experience in the automotive industry.