An exhaust sampling system includes a dilution tunnel in which exhaust gas from an engine is diluted with a diluent gas, a sample probe in fluid communication with the dilution tunnel, a sample collector, a first flow path in fluid communication with the sample probe and the sample collector, a source of fill gas, a second flow path in fluid communication with the source of fill gas and the sample collector, and a controller that selectively supplies the diluted exhaust gas to the sample collector through the first flow path during a test phase, and that selectively supplies the fill gas to the sample collector through the second flow path during the test phase. At least a portion of the second flow path is different than the first flow path.