You need to be more specific about idle vacuum. With a manual transmission it should be specified as "Hg @ RPM in neutral. My 2.6 manual is about 15" @ 700.

Idle vacuum for an automatic should be measured with the transmission in Drive, and both RPM and vacuum will usally be lower than the same engine with a manual idling in neutral.

I don't have any data for a 2.6 automatic idling in Drive, but if you post yours, perhaps someone with a 2.6 automatic can chime in with their reading.

Duke