Stumbles and surges a bit at idle. Problem started about a week ago and rapidly progressed from once in a while to constant. Starts fine and runs smooth and strong except at idle. No codes.

Changed plugs and coils about 4 months ago. Changed IAC last night. MAF is clean.

What's next on the list to check?

 

Did you confirm your IAC and maf are operating correctly? New and clean don't mean they are working properly

 

Vacuum leak ?

 

Yup. Hose from intake to EVAP valve had a big honking split and was hissing like an angry snake.

 

At idle it's almost always unmetered air/vac. Also, ends up being within the PCV system, usually in back by the firewall at the intake.

 

Yup, just out of sight on intial inspection.