Lay the tire flat and slowly pour some water around the bead area. The water will collect and fill the low spot at the tire/wheel interface and enable you to spot any bubbles.

Can also be the valve stem or the core.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles