If it's still stuck open it will cause a lean code. Three inch pipe is a bit of overkill.

 

If it's stored but not active, it could have the senor locked out and substituting a fixed setting. A good reason for falling in & out of a lean condition during operation. You get this since you reset the KAM (kept addaptive memory)?
If it was an active code I'd agree w/ JCMS that the EGR may be failing to close. He's usually a good bet anyway. But that's more of an issue at idle & lower end of the cruise range & upon deceleration. A grey area, secondary air lowers cylinder temps during acceleration, but the engine is picky as to when it's allowed to be introduced or continue.

& 3 inches? An interesting experiment would be to find a way to temporarily reduce that a little (like a plumbing "No-hub reducing coupler"/ If not familiar w/ that, it's hard rubber & attaches w/ a large hose clamp), but I think the prob is ahead of that.

 

Pulled the codes again. This time in the memory was 332,EGR valve opening not detected. I checked my vacuum lines and it came back to the vacuum canister. I pushed down on it and saw a bunch of rust fall out the bottom. Been running my engine with no vacuum. I have silicone in it hardening right now, will be picking up a new one when the wreckers open on monday. Hopefully this is it.

This has gone a long way from asking about an 02 extension.

 

That's actually "Insufficient flow detected"

 

Either way that did it. Drove a total of 60 km without the light coming on. Idles like a dream as well. Pulled 16 mpg mixed highway/city. Did a baseline test before I upgrade the ignition. I'll post results when its done tomorrow. If I never see that light come on again I will die happy.