Or the Interstate may still need a vent line and it was never connected. The OEM Varta had a vent on the very left side of the battery, and some genius made those vent tubes just barely long enough to reach the Varta vent. The Interstate probably has a vent in the middle, whereby the tube is too short. Make sure someone wasn't lazy and just didn't bother to hook up the tube because it was too short. You can get a prepackaged "kit" at an Interstate dealer (or call Interstate customer service) or a connector and additional tubing at any auto parts store.