A filler neck adapter to be inserted through the open fuel mouth of a capless filler neck of a fuel tank of a motor vehicle. The adapter has a hollow flow body to create a continuous flow path between a source of pressure or vacuum and the fuel tank to enable the fuel tank to be tested for leaks. A resilient seal surrounds the flow body of the adapter so as to lie between movable and stationary seal stops. A force-generating member is moved along the flow body to push the movable seal stop and the seal towards the stationary seal stop. Accordingly, the seal is compressed between the movable and stationary seal stops so as to bulge outwardly towards and thereby create an air-tight seal against the fuel mouth of the filler neck.