Your Ford E350 may not be starting due to a faulty fuel pump driver module, which can cause symptoms like rough idle, loss of power, hesitant acceleration, and intermittent engine performance. You can check for a fuel pump failure by listening for an audible noise while the engine is cranking, or by hitting the fuel tank with a rubber mallet while an assistant cranks the engine. The fuel pump relay is located in the engine compartment fuse box on the left side of the engine compartment.