It's a thermo vacuum switch. It switches the operation of the egr depending on engine temperature. Various solenoid valves and other gizmos switch the vac around, too. THis is why the 2.6 has so many vacuum lines. I once spent some time tracing out the 2.6 vac diagram, figuring out what did what, and whatever path you traced, it ended up at the egr or venting to atmo except for one isolated bunch that went to the sas and distributor.

Not responsible for advice not taken...