Secondary air injection system driving me nuts

[ThreeGermans]

Sorry that my first post is a cry for help...


This is a fairly new-to-me 2006 X5 M5 (e53 with m54 3.0L straight 6 gas engine). I bought it a year ago and put 5000 miles on it.

As of 2 months ago, I keep getting Check Engine Light with codes P1419 (split air #2 circuit malfunction) and/or p0491 (secondary air injection system bank #1 malfunction- insufficient flow).

I sprayed the secondary MAF (yes I know it's not the same as the main intake MAF) with MAF cleaner. I looked at the little air filter assembly that the MAF attaches to, but it seemed pretty clean. This didn't help. With the engine off, I applied 12V to the secondary air pump and felt a lot of air coming out of the exhaust (which I believe proves that the air pump and valve are functional, and that the head is not clogged/blocked with carbon).

I removed the valve. I understand from searching this forum and others that some of the valves are vacuum-actuated and some are not. This valve has NO vacuum line, so there is no vacuum circuit diagnosis that needs to be done. I can blow through the valve both ways if I blow hard, which I believe is how these are supposed to work (the pressure from the secondary air pump opens the valve) and again, when I apply 12V to the fan, I get lots of air at the tailpipes. I'm a little confused on this point because the valve doesn't seem to be working like a check valve (ie if you blow hard from the engine side, you can still get air to go through).

After resetting the code, it comes back in a day or so.
I checked the hoses (from the little filter to the secondary air pump and from the pump to the valve); they are supple and have no cracks (and I would have heard whistling from the hoses when I got air out of the tailpipes, so I conclude that there are no leaks).
I replaced the secondary MAF with a new one. Still getting the codes.
I replaced the air pump (even though the one in the car appeared fine). Still getting the codes.

I pulled the relay that activates the air pump. I believe it is the salmon colored one in the e-box that's under the right side of the windshield. It is a normally open relay that tested infinite resistance normally (which is good) and 0.4 ohms when I apply 12V to the coil (which is good). I applied and removed 12v a few times to make sure the relay didn't stick. Relay seems perfect.

I replaced the valve mounted on the engine head. When I had it off, I sprayed a little brake cleaner in the hole; a small trickle of black gunk came out but I think that was just carbon residue coming off the walls of the channel, it was not fully blocked. The new valve 'tested' the same as the one that came off the car (I can get air through it if I blow hard).

Still getting the code.
What could possibly be causing these codes to keep coming back?

Is the valve operating correctly? Could the new one be bad as well? How does one test this? The info I find online seems to cover the vacuum-actuated ones, which is not what I have.

Thank you for reading all this!