Beginning in 1978 and retrofitted on many 1974-1977 models, Volvos are fitted with a fuel pump inside the fuel tank. This pre-pump, also called a primary or feed pump, moves the fuel out of the fuel tank to the main fuel pump. This prevents fuel starvation problems that may occur when the tank is less than half full. If the main pump has failed, the car will not run. If the pre-pump is inoperative, you'll have fuel starvation symptoms. These may include hard starting, louder than normal main fuel pump noise and a loss of power under load.

To check the operation of the pre-pump without removing it from the tank, locate the pre-pump fuse. Remove this fuse with the car idling. You should be able to detect increased noise from the main pump and decreased vibration of the fuel tank itself.