Technically, fuel pumps can run without relays, but running a fuel pump without a relay is quite dangerous for the electrical system and the overall reliability of the pump. Relays are just high current switches, so this allows the pump to pull all the current it needs, right from the battery, without pulling too much through the ignition switch or small wires. While upward of 12-14 volts is fine for fuel pumps, but you really want to be sure, and relays can minimize resistance and distance in the electrical path. The ignition switch carries all the electrical load without a relay which can cause overheating and failure in the switch and the ignition switch is a common point of failure in high-current systems which require 15-20 amps.
Excessive Current → Protection for Wiring Relays Bypassing a relay can cause low-current wires to overheat in a high-load application, which can lead to electric-arc fires. In fact, more than 15% of the vehicle fires related to electrical systems were caused by bypassed or malfunctioning relays, according to the National Highway Traffic Safety Administration (NHTSA). The ignition circuit can be used, but any high-performance vehicle or engine that has been modified to require more fuel flow than stock will quickly burn out the circuit since it simply is not designed to handle high current over long periods of time.
Non-relay fuel pumps can also be getting a less reliable power supply as direct connections can be a source of voltage drop. This inconsistency results in surging pressure in the pump, which for most fuel injected engines must remain stable in the range of 40 — 70 PSI. Fluctuating voltage can cause lean conditions, misfires, and eventually damage to fuel injectors. Relays are the backbone of a reliable fuel system, as they supply safe and stable power to fuel pumps and other high load components, says automotive specialist Ed China.
Choosing a quality Fuel Pump, with a Complete Relay Setup within assists in fuel pump performance, and electrical component protection, long-term fuel system reliability, limited risk of expensive damage to vehicle wiring, and ignition system.