Returnless fuel systems are becoming more common in new vehicles.

As we learned in a recent Jeff Smith Ask Away! column, the increased use is due in part to tightening federal environmental regulations.

Does that mean a returnless system is better for your engine than an earlier, return-style setup?

It’s an important question to answer, especially if you’re considering an electronic fuel injection (EFI) conversion.

But before we come to a conclusion, let’s understand how each system works.

mazda miata fuel tank & pump removed from car
(Image/Tom Tharpe – Tom’s Turbo Garage)

Return-Style Fuel Systems

Think of a traditional return-style fuel system as an endless cycle.

Your fuel pump (electrical or mechanical) constantly pumps gas from the tank, through a regulator, to either the injectors or carburetor.

The fuel pressure regulator is charged with sending the excess fuel back to the tank. It’s a simple mechanical device consisting of a diaphragm and a spring that is typically controlled by engine vacuum. As manifold vacuum changes with engine rpm, the diaphragm moves, opening/closing a secondary passage for the fuel to exit and return to the fuel tank.

For visual learners, here’s a cutaway image of a regulator, courtesy of Turbosmart:

Fuel Pressure Regulator
Here’s a cutaway of a fuel pressure regulator, the “brain” of a return-style system (Image/Turbosmart)

There are some key benefits to a return-style setup.

  • Vapor lock is mitigated because the constant cycle of fuel back into the tank cools the gasoline.
  • Engine tuning is easier because the fuel temperatures in a return-style system are typically more consistent.
  • Fuel pressure is more stable at the carburetor or injectors because you can place the regulator closer to the delivery point.

This system does have a few drawbacks however, beyond the potential negative environmental effects.

  • Return-style systems require fuel pumps to work all the time—it never shuts off as long as the engine’s running, which can shorten the life of the pump.
  • Your plumbing system can become more complicated because you have to run a separate return line from the regulator to the tank.

Returnless-Style Fuel Systems

A returnless fuel system uses an in-tank pump and regulator. A single fuel line exits the tank and travels to the engine.

Pressure is controlled by a computer, which monitors a series of engine sensors to determine how much fuel to deliver.

To regulate the fuel pressure, the ECU simply adjusts the pump speed or fuel injector rate.

From an auto manufacturer’s standpoint, it requires one less fuel line. That makes it easier (read: cheaper) to design and build a car around.

It also reduces the amount of harmful evaporative emissions released from the vehicle.

So…Which One?

The short answer is a return-style system, given its mechanical simplicity and consistency.

It’s an easy question to answer on vehicles that already have a fuel return line…or at least the routing room for one. Retrofitting a returnless system can get expensive, requiring a new pump, regulator, and plumbing.

But that’s not to say your vehicle’s returnless system isn’t pretty darn good. It’s perfectly fine for OE and many high performance applications—so don’t think you need to rip out the factory setup if you’re planning some power-adders on your late model car or truck.

Replacing your OE returnless system may bring potential regulatory concerns depending on where you live— Please check your local laws/regulations.

Want to see the differences between return and returnless fuel systems in more detail? Check out this video from our pals at FAST.

Author: Paul Sakalas

Paul is the editor of OnAllCylinders. When he's not writing, you'll probably find him fixing oil leaks in a Jeep CJ-5 or roof leaks in 1972 Corvette ragtop. Thanks to a penchant for vintage Honda motorcycles, he spends the rest of his time fiddling with carburetors and cleaning chain lube off his left pant leg.