Recently purchased a truck with a Impco cng system. System was not working right and I was smelling gas in my cab. Had the regulator changed, and the smell of gas went away. However when I switch over to cng my gas gauge is still decreasing with the mileage even though my cng gauge light is green. When I switch back over to gas my has gauge does not go back up. Is this normal? Or is there something I'm doing wrong