Returnless Fuel Systems

Motor, Oct 2004 by Seyfert, Karl

To ensure adequate fuel pressure and volume under constantly changing operating conditions, conventional fuel systems always pump more fuel to the injectors than is ultimately needed. That means one molecule of fuel might make as many as 30 trips to the engine via the fuel rail before it finally passes through an injector and is converted to energy. Each trip adds heat to the fuel, which is returned to the fuel tank. In-tank fuel temperatures can exceed 160°F on a hot summer day-a prime breeding ground for fuel evaporation. Even if it's properly contained, vaporized fuel can contribute to a variety of driveability problems. Remember carburetor vapor lock?

To combat these problems, several vehicle manufacturers have introduced fuel systems that reduce the number of tank-to-engine trips to just one. Because these systems do not have a return line to return the unused fuel from the engine to the tank, they have been dubbed refumZeas fuel systems. This is something of a misnomer, as the unused fuel really i; returned to the tank. It just doesn't have quite such a long return trip to make. These systems were introduced in the mid-'90s, and their use is rapidly expanding.

On returnless systems, fuel is picked up via the fuel screen sock at the bottom of the tank, then routed to the fuel pump. The pump supplies the needed fuel pressure and volume to the engine and the excess is directed back into the tank after passing through a pressure regulator. The regulator lacks the vacuum connection to the engine that we're accustomed to seeing on conventional pressure regulators, so its job is to maintain a steady pressure independent of any changes in engine operating conditions. To make sure the engine always receives just the right amount of fuel for current conditions, the PCM makes rapid changes in injector pulse width instead. On some newer fuel systems, a pressure sensor keeps the PCM informed of system fuel pressure. The PCM responds by modifying the pulse width to the fuel pump power supply, adjusting system pressure and volume on the fly. These newer systems completely eliminate the need for a separate pressure regulator.

One advantage of the older returntype fuel systems was the constant filtering of the fuel they provided. Each time the fuel made a trip to the engine, it had to pass through the fuel filter. The filter trapped tiny particles of dirt or debris that may have found their way into the system. This cut down on component wear and service was relatively simple because the filter was usually in an easily accessible location outside the fuel tank.

Returnless fuel systems also have fuel filters, but where those filters are placed can have a big effect on the longevity of the system, as well as its ease of service and repair. The filter can be placed in any of three different locations. The first is the conventional location outside the tank. While this is the easiest to service, it also means that any unused fue] that's returned to the tank is never iiltered until it makes its first and only trip to the engine. So if it contains any contamination, the same junk will make repeated trips through the pump and back into the tank. A load of rusty fuel could pass repeatedly through the fuel pump, grinding the rust particles into increasingly smaller particles and making short work of the pump itself.

Placing the filter in front of the pump protects the pump from contamination, but creates other problems. The most obvious drawback is that the filter is now in the tank, so replacing it is a major deal. A filter that's become clogged with debris means the pump now has to suck extra hard just to get the fuel it needs. This causes low fuel pressure, fuel boiling and pump cavitation. So-called lifetime filters are supposed to alleviate all of these drawbacks.

The last option is to place the filter after the pressure regulator, before the unused fuel is returned to the tank. This allows the unused fuel to be filtered repeatedly, until it's finally used. A restricted fuel filter is less likely to have an adverse effect on the fuel pump, and the filter can be placed in a location on top of the tank that is easily accessible for replacement.

Fuel system cleanliness is important on any system, but never more so than on a retumless system. Any dirt left behind will quickly damage any new components you install.

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