Second header CARZZ Audi Q5 1550x236 BB

Secondary Air Injection System

Secondary Air PumpGear case (3)Secondary air injection failure is a common problem worth a page to explain it. Basically, it is a system to push extra air into the exhaust system after start up with the engine cold or warm. This makes the exhaust more diluted for lower emissions; and the oxygen sensors and catalytic converters start working sooner. Without going into all the parameters on when and why the system goes on, let us give a brief rundown on the components, their functions, and the problems.

The secondary air injection system is composed of a blower motor which is shaped rather like a hairdryer, that pushes air through a rather fragile plastic pipe to a combi valve which is mounted on the exhaust manifold or cylinder head. There are two of these valves on Vee motors. The Combi valve is what allows the air to pass into the engine exhaust when it opens. The combi valves on all early systems are controlled by a vacuum supply, which is controlled by a solenoid. Which is controlled by the engine control unit. Which also controls a relay which controls the blower motor. Clear enough? All these components and functions combine to form the Secondary Air Injection System, or SAI. Which has to function correctly to pass a smog test.

The most common source of secondary air injection failure is the vacuum hoses, all across the gamete of motors. Occasionally we find a blown fuse for the blower motor. And some blower motors are assembled with rivets that can vibrate loose and make the motor leak. Karmakanix has a bolt kit to replace the rivets. We have even seen some blower motors that blew apart when the car hit a puddle and gulped water into the pump. This problem was pretty much always accompanied by a missing belly pan that would have protected the pump.

Understand that the diagnostic time to sort out a secondary air injection failure depends entirely on what is wrong. And there can be more than one problem. The easy stuff can take under an hour to diagnose and repair. If we have to make all the way to the bottom of the troubleshooting tree, then it can take as much as two hours just to figure out what went wrong. Most systems take about an hour. First, we check for the diagnostic trouble code, or DTC, with a computer. We clear the code and make sure it comes back, meaning the problem is active. A quick check that the hoses and the fuse are in order. Then we run output tests that make the blower motor turn on, and the solenoid click. Check the combi valve(s) to make sure it holds vacuum, and with the engine running, the sound should change when vacuum is applied. Then we check if the blower motor puts out enough pressure. Still did not find it?

Use a vacuum pump to check that the solenoid passes vacuum when actuated. Use a vacuum gauge to check the vacuum at the combi valve when actuated. Check for rust in the blower motor. Check for breaks or clogs in the plastic air pipes. Still did not find it? Check for a mouse in the air filter housing where the air supply comes from the air filter housing (really!). Then hook up with a computer, some volt meters and/or an oscilloscope to read the rear oxygen sensor voltages. Turn on the system with the engine running. If those voltages don’t drop, then the exhaust manifold and/or the cylinder head is clogged with carbon. On a Vee motor, this can refer to one side or both. And that’s the bottom of the tree. See why it can take two hours. And that is if you know exactly what to do.

One can see that the time to repair a secondary air injection system is quite variable depending on what is wrong. Obviously, removing the exhaust manifold to clean it and the cylinder head takes the most time. Please feel free to ask your Service Advisor for any further explanation.

Gear case (3)