Fuel shut off solenoid. Does it look like this, or similar to it:
If so, they are easy to replace, and test. There are 3 wires to run it. A ground, another that is the holding coil (always has power while key on), and the third is a pull in winding (connected to the starter). When the starter is engaged, power is also sent to the solenoid to pull the linkage, and allow fuel to the injection pump.