BMW P0492 Secondary Air Flow Too Low M54 M56 6-cylinder & M62 V8
This answer is applicable for many BMWs in addition to the one listed below.
I am using the excellent article in the 2008 newsletter (Diagnosing Secondary Air Systems – DIY) to troubleshoot my P0492 code (and check engine light) on my 2002 X5 3.0 (M54 engine).
Last May I had this light come on and the dealer replaced the secondary air pump, the pump relay, and the check valve (vacuum operated valve controlling air from the pump) and the valve gasket. The light came on again a week after their work and then went off by itself. It has recently come back on again (same P0492 code) and I am using your article to try to address it myself.
The pump seems to be working fine…blows air hard when I disconnect the line from the valve. The vacuum pressure to the valve seems OK (I used a gauge and it shows ~ 13-14″).
When I open the valve myself (using a mityvac) to check for exhaust gas escaping the nipple (connection to the pump) I hear a change in noise (I hear engine valve ticking), but I do not feel exhaust gas. With the engine off, when I operate the valve I hear what I believe to be the valve opening when I pump up the vacuum pressure, and hear a clunk when I release it (seems like something in the valve is being operated by the vacuum pressure). I also get the same result with the vacuum line correctly connected on a cold start…I believe the valve opens (I get a noise change) but I feel no exhaust gas.
So my question is: Is it possible for the valve to be working correctly and NOT feel any exhaust gas coming from the valve’s inlet port? Could there be a blockage somewhere in the air path between the valve and the manifold(s).
Excellent diagnostic work, Peter! Ya done well.
The P0492 code is: Secondary Air flow insufficient, Bank 2.
If this is the only code that appears, I would think about the faults that could cause this on only the rear cylinders (Bank-2 = cylinders 4 through 6). This could be a carbon clog in the air injection passages going to one or more of the rear cylinders. This would cause these cylinders (or even just one) to be getting less air than the front cylinders, and less than the system requires. If this is the case, it’s likely that the front cylinders are carboning up as well.
This carbon build-up is a not-uncommon problem with the V8 engines, which have similar internal air passages to each exhaust port. We have not yet seen this issue as a common fault on the later 6-cylinder engines that have the internal passages (M54 and M56 6-cylinder engines, from about 2000 through the late 2000s). The earlier M52 6-cylinders have an external piping manifold that connects to the exhaust manifolds.
On the V8 engines, the official repair is to remove the cylinder heads in order to attempt a cleaning of the air ports. I would assume that BMW is specifying a similar repair for the 6-cylinder models. We have not tried any DIY cleaning of the ports while the head is installed. You may be able to remove the check valve and run a somewhat flexible, yet stiff, rod through the passages. Perhaps removing the manifolds would allow you to go up into the air ports, through the open exhaust ports.