KJ Jones Senior Technical Editor
November 5, 2009
Photos By: Courtesy of Anderson Ford Motorsport

Horse Sense:
The electronic returnless fuel system found on Mustang GTs has been around since 1999. It's comprised of a high-pressure pump that delivers fuel directly to the fuel rail, which effectively eliminates two of the most important components of a power-adder Mustang's fuel system: the bypass fuel-pressure regulator and return line. While the move to this type of fuel system came about because of mandates made by the Environmental Protection Agency and the California Air Resources Board that required new vehicle manufacturers to meet ever more stringent levels of evaporative emissions control, its result dealt yet another blow to modern high-performance and left 'Stangbangers searching for a remedy. See More At 50mustangandsuperfords.Com

In a nutshell, returnless fuel systems were designed and implemented to offer improved fuel economy, lower fuel temperatures, and decrease vapor in the fuel tanks of today's fuel-injected vehicles. They're the OEM's best solution for meeting the new evaporative-emissions-control requirements. By avoiding flow through the fuel rail, heat transfer to the fuel is minimized, resulting in fewer fuel vapors in the tank that could enter and pollute the atmosphere.

In the static, returnless fuel system on '99-'04 'Stangs, there is considerably less fuel flow than that of the return-style (bypass) fuel system in earlier fuel-injected Mustangs. Without a return line, the only fuel flowing in the system is that which the engine can use at any given moment. A specialized, speed-controlled (by the ECU) turbine fuel pump is utilized to create and maintain a preset pressure level, somewhere between 30 and 70 psi. The command fuel pressure is determined by many factors, including engine load, manifold vacuum or boost, and even underhood temperature. A sensor mounted on the fuel rail monitors both fuel and manifold pressures, and updates the ECU on both.

By all accounts, Ford's returnless fuel system represents the leading edge of EEC technology. But this technology makes it more difficult to modify or enhance the fuel system to deliver the critical flow necessary to support significant horsepower gains. Time and time again, 'Stangbangers with New Edge cars are finding out the hard way that pushing the limits of the factory returnless fuel system (with 93-octane fuel and the big three power adders) is like playing with matches at a gas station.

From a performance standpoint, return-style fuel systems have their good points-voltage is constant to the fuel pump, adjustable fuel-pressure regulators can be used, additional inline pumps can be added easily-and their bad. We'll focus on the negative aspects. Heat can become a factor in the efficiency of a return-style system. When recycled fuel is heated by the engine, exhaust and road-surface temperatures, or in-tank fuel temps, and if its temperature gets too high, another hot-fuel-handling problem usually arises-fuel-pump cavitation and vapor builds in the fuel lines and fuel rails. As we know, bad things happen when engines don't have fuel. "Lean is very mean," according to Rick Anderson of Anderson Ford Motorsport. "Without a good supplement to any EFI Mustang's stock fuel supply, the road to meltdown is fairly short if you're trying to make horsepower with a blower, turbo, or nitrous."

With a returnless system, since cool fuel isn't fed to the fuel rail at all times, the standing fuel in the rail also can become superheated. So, additional pressure is required to prevent fuel from boiling and vapor locking the rail.