Rough Idle / Cold Start Fix - IACV Diaphragm Installed Incorrectly

Car had a rough idle and cold start problems after the IACV (Idle Air Control Valve) had been removed and cleaned during winter maintenance.

The rubber diaphragm/spacer inside the IACV on the intake manifold was reinstalled in the wrong orientation. The rubber seal was placed on top of the actual valve instead of being positioned on top with the valve sitting at the bottom underneath it.

Moderate confidence — Original poster confirmed this resolved the issue