[FIXED] 08 TrailBlazer p0411

2008 Chevy trailblazer 4.2l 6cyl
235000 miles (long post, be warned)

P0411-secondary air flow incorrect.

System design:
Service info pressure specifications are in kpa. The secondary air system consists of the air pump, plumbing and a diverter valve mounted to the cylinder head above the exhaust manifold. This is a 5 wire solenoid which has a built in pressure sensor to verify AIR system operation.
On a cold start, the pump relay and solenoid are both activated, allowing fresh air into the exhaust via a channel in the cylinder head above the exhaust ports.
At this point the PCM is watching for a pressure increase of 8 to 10kpa above baro. That is phase one of the test in which a p0411 will set. This TrailBlazer reaches 13-16kpa above baro setting the code every time the rest runs. The next phase closes the valve to essentially "dead head"the pump and the PCM watches for a further increase in pressure.
Testing:
While bidirectional control of the system is used (koer because that's how the system runs the self check), the pump runs and the valve opens and closes. With the plumbing off the valve and the valve activated, exhaust can be heard from the valve. When closed, it cannot. When the pump is activated and the valve is not, the scan tool pressure reading matches what my vacuum/pressure gauge displays during a dead head test, and is also within service info specs.
With the valve removed from the head (open to atmosphere) with the air pump & valve active, pressure rises 7kpa per service info.
Resting pressure is 100kpa and my baro PID reflects that at 101kpa. So I believe the sensor portion of the valve to be accurate and that the cause of the P0411 is unexpectedly high pressure.

So my suspicions for the pressure being too high during the first phase of the AIR system check are partly blocked catalytic converter or carboned cylinder head AIR passages.

Exhaust back pressure test shows 0psi. Likely not a blocked exhaust. Also there is no drivability concern, just the engine light.

There are 6 casting plugs in the head to gain access to the AIR port. Knocking one out shows an abundance of carbon build up. (You may need to zoom on pic)
After popping out the plugs and picking around, I found the most effective method for cleaning between the ports to be a length of park brake cable in my drill, brake clean and a propane torch, cause I'm just that kind of guy.
So the ports are clean to the best of my knowledge and ability. I know that the system will run the test since it took a long damn time playing around with the head, so I have the scanner ready with the necessary data.
The good news is that it passed the IM test this time around.
However, the pressure did not appear to significantly decrease, now staying 13-14 kpa.
If it passes the IM test in the next couple cold starts, I'm going to send it, but the evidence doesn't support a fix.

I realise there is a TSB concerning intermittent p0411 setting, but this seems to be a hard failure.
Has anyone been down this path before with one of these?

Thanks for sticking it out and reading the whole post!
Noah

Another one fixed. With zero parts fired.

I don't suppose you looked at Mode $06 data during this process? No worries if you didn't - I wouldn't have, either. :silly: Mostly wondering if the test results would have confirmed the active test results.

I hope you're right, I was hoping for some more clear indication in the data...

Update: passed the second time in a row. Calling this one fixed until further notice.

WHAT DID I TELL YOU ABOUT FIXING CARS THE FIRST TIME? NOW THEY KNOW YOU CAN FIX ANYTHING

It's baaaaack....
With a VVT code :woohoo:

I've seen this truck several times since this thread was started for routine repairs, and still no light.

Forgot about this one until the most recent SD premium video.
Great stuff as always, check it out.
www.scannerdanner.com/scannerdanner-prem...xhaust-passages.html