A friend of mine just had their kitchen remodeled with a new design including an island sink. Everything is finished including flooring, cabinetry, etc. The island sink is about 8' (up stream) from the wall separating the kitchen from the garage. This wall has the drain for the wash machine and a laundry tray facing into the garage. The vent for these two fixtures is 1 1/2". The new island sink has its own separate vent, but no c/o for the drain and no c/o for the vent. I'm almost positive the vent/drain for the island was improperly installed. Without cutting the concrete slab and tearing the drywall open, it's hard to say what or how screwed up it is. Nevertheless, the kitchen sink drains well.
The problem is that when the washing machine, as described above, drains, the kitchen sink girgles and groans loudly. Looks like the draining washing machine creates too much strain on its own vent that it pulls from the kitchen sink trap. Would up sizing the laundry group vent to 2" help? Or is this a tear open and redo all situation?