Gremlins have made it down under -

[Scott Larson]

Dunno, I would hold something other than a vacuum leak suspect. Start with the EGR since the CEL leads you there. You start tearing down too much too soon, you might actually create a vac leak related to 171/174. Baby steps.

Definitely a possibility. I missed that his CEL had lit, a code would indicate it had. Good catch!