For more than 40 years, the auto industry has devoted a major portion of its engineering resources to reducing exhaust gas emissions. In the U.S., it all began with positive crankcase ventilation, introduced in 1967. Air Injection Reaction (AIR) was introduced the following year. In those days, engines ran so rich that at least some of the unburned hydrocarbon (HC) in the exhaust would react chemically with oxygen just by injecting fresh air into the exhaust stream at its hottest point. This is commonly known as afterburning, and it really does work. For the next decade or so, most car engines were equipped with an engine-driven air pump and air injection fittings in the exhaust manifolds or cylinder heads.
By the late 1980s, combustion chamber shapes and fuel injection technology had improved enough to reduce engine-out HC emissions to about the same level provided by AIR, so fresh air was injected further downstream to provide oxygen for the catalytic converter. Eventually, fuel injection systems and catalytic converters were developed to the point that most engines could provide enough oxygen for the catalyst to do its job without additional air.
Today the air pump is back, but it serves a different purpose. Now almost all HC and CO emissions leaving the tailpipe are generated immediately after cold-start, before the catalytic converter reaches operating temperature. As emissions regulations have continued to tighten, engineers have been working on ways to get the catalyst up to operating temperature quickly, and the secondary air pump is part of the answer. The pump is electric, and it’s turned on just a few seconds after cold-start. Air is pumped into the exhaust upstream of the oxygen sensor, either to a single point in the manifold or directly to each exhaust port as it was in the 1970s.
At the same time, the PCM might 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. By comparing the front and rear oxygen sensor signals, the PCM can determine when the catalyst has begun working. This may take as little as 15 seconds or as long as two minutes, and the air pump might continue running for several seconds even after the system goes into closed-loop.
Though every manufacturer is using a secondary air pump on at least some models, it’s more commonly found on European cars. On those cars, some of the most frequently set fault codes are for the secondary air system. A quick Internet search shows there’s a brisk business for replacement pumps, but a failed pump rarely is the original problem. Unfortunately, many dealers and aftermarket shops have replaced multiple pumps on the same vehicle because they haven’t checked the rest of the secondary air system.
A failed pump is almost always badly corroded and usually has water inside. Often customers are told it’s a result of driving through high water, but because the pump draws air through the engine’s air intake duct, this is highly unlikely. The water that causes corrosion in the secondary air pump comes from the exhaust gas, the same water that produces steam at the tailpipe in cold weather. The device that keeps that exhaust gas and water out of the pump causes the problem that’s most commonly overlooked.
PAGE 2Combination valve
In addition to the pump, there is also a Secondary Air System (SAS) valve. On some European models, it might be called the combination or “combi” valve. It’s called that because on the old AIR systems, it also operated as a vent valve during deceleration to prevent backfiring through the exhaust, and the name just stuck. (In the States we called it an anti-backfire valve). The combi valve is a normally closed flow control valve. It might be operated by vacuum that is controlled with a solenoid valve, or it might have its own built-in direct acting solenoid. The solenoid might be either on/off or pulse-width-modulated, depending on the vehicle.
Because the PCM only operates this system for a minute or so after cold-start, the combi valve is closed most of the time. One side of the valve constantly is exposed to exhaust gas from the hottest part of the exhaust. It’s far enough away that extreme heat is not an issue, but the temperature drop through the connecting tube causes condensation. Because there’s no flow most of the time, water collects in parts of the tube or in the combi valve itself. If that valve doesn’t seal properly, some of that water can leak through the valve and collect in the pump.
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. This will result in MIL illumination with two fault codes: P0410, secondary air injection system; and P0411, secondary air injection incorrect flow detected. On a Vee engine with two combi valves, codes P0491or P0492 might also be present, indicating incorrect flow to cylinder bank 1 or bank 2. The same codes will set if the pump’s inlet tube is clogged, if the outlet air tube leaks or if the pump is not moving enough air.
How does the PCM know that secondary air flow is incorrect? On most later models, it’s measured by the engine’s Mass Airflow Sensor (MAF). On some models, such as BMW E46 with the M54 engine, the secondary air system even has it’s own dedicated MAF sensor. On many earlier models, flow is calculated using the oxygen sensor signal. Typically, the air pump is turned on and the PCM monitors the time required for the oxygen sensor signal to go lean.
On a vacuum operated combi valve, the diaphragm can be tested for leaks by testing the system live with a vacuum gauge or by testing just the valve with a hand-held vacuum pump. Even if it holds vacuum, the valve might still be clogged or stuck open. It should be impossible to blow through the valve in either direction when there is no vacuum applied, and it should be easy when vacuum is applied. Avoid using shop air to blow through the combi valve; it’s only expected to seal the low pressure generated by exhaust backpressure and the secondary air pump.
Hoses
Again, sometimes due to exhaust gas leaking backwards through the secondary air system, the pump discharge hose often becomes brittle, cracked or otherwise deteriorated enough that it obviously should be replaced. However, because the pump and/or hoses are often buried in the engine compartment, you might be doing the customer a favor by just replacing that hose once you actually get to it, especially if it contains water.
Possibly the most common part to fail first in the secondary air system is the vacuum line to the combi valve, especially as the line ages. On some models, particularly some Volvos, that vacuum hose is pinched or kinked right from the factory. It still works when new, but eventually this will generate incorrect flow fault codes and make the pump noisy as it tries to blow through a closed combi valve, which could eventually damage the pump.
It’s rare that anything goes wrong with the pump’s air inlet hose unless underhood temperatures are consistently high enough to make it brittle. The MAF sensor might measure air flowing through that hose. Leaks or obstructions can cause incorrect flow codes, so check for missing clamps or kinks in the hose.
Solenoid valves and relays
The pump is operated with a relay that’s controlled by the PCM. There’s also a fuse somewhere in the system. The PCM monitors current draw on the relay circuit and also checks the solenoid valve circuits for opens and shorts to ground. The generic fault codes for these circuits are P0412 through P0419. On most models the PCM also monitors its own relay driver and the secondary relay circuit (the pump circuit itself). Malfunctions here will set enhanced fault codes (P14XX).
Secondary air pump
Pierburg makes all the secondary air pumps we’ve seen on European vehicles. Howard Pitkow, a Volkswagen specialist and owner of Wagenwerx in Wyndmoor, Pa., says the only pattern failure he’s seen on these pumps is the rivets that hold the pump body together. The rivets tend to loosen or break off, but this usually causes only a minor leak, not a catastrophic failure.
The fix is easy: Remove the rivets one at a time and replace them with screws and nuts. Otherwise, the most common cause of pump failure is corrosion. Pitkow notes that when the pump is mounted low in the vehicle, several ounces of water may spill out after removing either the inlet or outlet hose. According to a service bulletin issued by Pierburg, this is a sign of combi valve failure:
“In the case of a failed secondary air pump, the actual cause is frequently not found in the pump itself but instead in a sticking non-return valve between the secondary air pump and the exhaust manifold. For this reason, in the case of a failure, all components must be checked together.”
In other words, it doesn’t matter which code is set or which part has obviously failed; the only real fix is to check/repair the entire secondary air system.
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 the proper scan tool, you still can 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 combi valve.
Tee a vacuum gauge between the solenoid valve and combi valve, then start the engine and/or command the system to operate and/or open the valve with the vacuum pump. If the combi 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 vac. This could be a stuck combi valve or lack of vacuum to the valve. If the pump makes excessive noise even when the combi valve opens properly, it’s time to inspect the pump.
With the pump’s air inlet hose disconnected from the intake duct, command the pump to run and operate the combi valve manually, listening for a change in the pump sound.
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 combi 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.
To check the pump, remove the inlet and outlet hoses and 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 bottom line
On European secondary air systems, the pump rarely is the first part to fail. No matter what code is set or which part has failed, it pays to check the whole system. Not much can be done to fix design flaws, except maybe reroute kinked hoses.
However moisture, a noisy pump or any brittle/cracked hoses are a sure sign of combi valve problems. After making any repairs, be sure to reset the readiness monitors to confirm the fix.