Disabled the Decel valve and now AFRs are higher...why?
 

I'm testing out how the car does without the decel valve, so I have temporarily disabled it by pulling off the vac line and plugging it. Car runs and idles great...and my initial impressions are I prefer it without the valve. I did notice that my AFRs seemed to go leaner with the valve disabled, and I'm trying to reason out why this would happen. Before, my idle afr was around 13.5 (going for 2.5% CO). On the test drive after disabling the valve my idle afr is over 14.

It shouldn't. All air ingested into the engine goes trough CIS barn door, so removing throttle bypass or not doesn't do any difference as long as system is airtight.

Sure you don't have leaks somewhere? It's a maze of hoses, one might have sprung a leak or you didn't tightened something?

You may have a ruptured diaphram in the decel valve itself

Did you plug the nipple on the valve as well or just the vacuum line?

Certainly I need to check all the obvious stuff. I've removed and reinstalled the airbox, I/C and BOV assembly off so many times now I am well familiar with every hose. I'll double check everything...the only spots that I think would be suspect are the three hoses off the I/C (wastegate, AAV and AAR) and I/C itself. Thing is, it runs and idles just fine.

I did not plug the nipple on the decel valve, and that crossed my mind as well. Just a few weeks ago I checked the operation of the decel valve with a mity vac while I had the BOV assembly off for new seals. It held vac just fine and seemed to operate as it should.

Hmm, maybe it sucks unfiltered air trough it and raises the AFR's?

The decel valve is looking for a vacuum signal to open it. If you don't plug the nipple it shouldn't matter. The valve never opens. As Mike and Goren have pointed out, it might be leaking, however.
I'd remove it and plug up the intake and i/c openings as a way of troubleshooting the problem.