EVAP DTC Triggers Rich Condition?

[project-pegasus]

I recently installed a supercharger on my 2003 5.3L Yukon. I must have messed up something with the EVAP hoses going to the supercharger, because I soon got trouble codes corresponding to a large EVAP leak and solenoid voltage.

After the MIL came on for the EVAP codes the vehicle started running poorly (stalling at stops, running very rich with long term fuel trims going to +20%, AFR of 12ish)

I disconnected the EVAP solenoid in the engine compartment and pulled the EVAP vacuum line from the supercharger and capped that port. And I cleared the DTC codes.

It ran fine on the first start up: the stalling went away and the LT fuel trims were closer to +3%, AFR back to 14.6.

On the next start up, when the DTCs for the EVAP leak were triggered again, it started running poorly again. Stalling, running rich. But the vacuum port was still capped and I could not find any other vacuum leaks.

So I cleared the DTCs again, and it ran fine again, at least until the second start up.

I'm still working on the finding the EVAP leak and solenoid issue, but I'm wondering if anyone knows why the system goes rich when it sees these EVAP DTCs? I didn't realize this was part of the OS.

[TheMechanic]

ECM/PCM is expecting to see a certain amount of enrichment when it pulses the EVAP purge solenoid. If it has failed open it introduces more than it calculated and will run rich. Very rich. If you cap it, it will expect to enrich but gets none. It will run lean giving similar drive ability symptoms and just a different code. A circuit code is not a leak. A circuit code (you didn't give the DTC) will only result from a failed solenoid, wiring/connector issue, ECM/PCM/software or a combo of the three.
Oh and the "System" is always attempting three things in this order. 1. Safe. 2. Clean. 3. driveability.

[project-pegasus]

Excellent info. Thanks for that.

[SaskCutlass]

ECM/PCM is expecting to see a certain amount of enrichment when it pulses the EVAP purge solenoid. If it has failed open it introduces more than it calculated and will run rich. Very rich. If you cap it, it will expect to enrich but gets none. It will run lean giving similar drive ability symptoms and just a different code. A circuit code is not a leak. A circuit code (you didn't give the DTC) will only result from a failed solenoid, wiring/connector issue, ECM/PCM/software or a combo of the three.
Oh and the "System" is always attempting three things in this order. 1. Safe. 2. Clean. 3. driveability.

What does a guy do when swapping that same 5.3 into 84 G-body with no evap system?
im getting 3-4 evap codes that recur as soon as i drive it.
My car runs quite well but i suspect its running very rich from black smoke at WOT and soot in tailpipe
Datalogger shows it running in open loop but it says because of low engine temps not evap problems

So how do i disable thsi evap system entirely in the PCm