I don't have access to a scanner for the Volvo

I don't have access to a scanner for the volvo. There is a problem in the fuel system. THe car was in storage in a humid climate for 8 months. The gas tank appears to have been broken into and 1/2 of the tank of gas removed. A local mechanic cleaned the injectors, said they were very dirty but still the car starts very roughly and dies. Once its running it runs ok and doesn't dies while driving. Should I add fresh gas to the tank or should I try to use this tank up. I can't get another mechanic to look at it for a week and need the car.