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Thread: Return-less Fuel vs Return Fuel Setups 101

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    Return-less Fuel vs Return Fuel Setups 101

    The commonly misunderstood concepts of return-less fuel setups vs fuel return setups. I'll do my best to make the difference clearer to everyone, and address a few important points along the way.

    Engines with “return-less” electronic fuel injection (EFI) systems have the regulator inside the fuel tank. The regulator is part of the fuel pump assembly and is usually located 'downstream' of the in-tank fuel filter. Many manufacturers have chosen this type of setup to reduce EVAP emissions from the fuel system. This was done due to a requirement for 'lower EVAP emissions' alteration in federal emission regulations in 2004. This led to adjustments to return-less fuel systems.

    Here's some history for you: on a conventional port fuel injection system, fuel is routed to a fuel rail on the engine to supply the injectors. Fuel pressure is controlled by a vacuum-operated mechanical regulator mounted on the fuel rail. Intake vacuum is routed to the regulator through a vacuum hose and pulls against a spring-loaded diaphragm to maintain a preset pressure differential in the fuel system. When intake manifold vacuum is high, excess fuel pressure is vented through the regulator bypass valve and routed back to the fuel tank via a fuel return line. Unfortunately, this carries a lot of engine heat back to the fuel tank and increases EVAP fuel vapor emissions. Relocating the regulator to the fuel tank eliminates the circulation of fuel between the engine and tank so the fuel stays cooler. It also simplifies the fuel system plumbing by eliminating the return line. Another advantage is that putting the regulator in the fuel tank keeps it away from engine heat and extends its life.

    Now, there are two different types of return-less EFI systems in use: mechanical and electronic. In the mechanical systems, a mechanical regulator is mounted on the fuel pump module to provide a (more or less) constant fuel pressure to the engine. This type of setup works well with a speed-density EFI system that uses throttle position, intake manifold pressure and engine rpm to determine engine load rather than an airflow sensor. Electronic return-less EFI systems, aka “On Demand” return-less EFI systems, use a fuel tank pressure sensor to monitor fuel pressure. The PCM varies the speed of the fuel pump to increase or decrease fuel flow using pulse width modulation (PWM) of the pump’s supply voltage. The PCM determines how much fuel is required based on engine load and inputs from its other sensors. This type of system is typically used on an engine that has an airflow sensor to monitor engine load.

    Now that we have the basic knowledge and history down, let's get into some more specifics:

    Return-less systems typically operate at a higher pressure than return systems. This is necessary to reduce the risk of fuel boiling and vapor lock in the fuel rail during hot weather (since there is no recirculation of fuel from the engine back to the tank to keep the fuel supply rail cool). Because of this, a return-less system may not perform well if fuel pressure or flow is at less than optimal specs. Return-less systems also need to use higher 'cc' injectors than return setups because there is no return for the excess fuel to go back to the tank. This creates a 'dead end' at the fuel rail. The way most people increase/adjust their fuel pressure on a K-series return-less setup is to 'crush' the OEM FPR. Many people use the Comptech/CT-Engineering 'crush' tool to increase their fuel pressure.

    Fuel return systems operate at a lower pressure than return-less systems, as there is a full loop in the fuel system and allows excess fuel to 'return' back to the fuel tank. Typically, the fuel is pumped to the fuel rail that has a fuel pressure regulator at the end of the line. The FPR "regulates" or bleeds off pressure that is then carried back to the tank by the fuel return line. If you had your FPR at the tank, you would be regulating pressure there and not need a return line, but if you run a system with the regulator further down the line, the regulator works best in conjunction with a fuel return line (as the excess pressurized fuel needs a place to go - back to the tank). You can send more fuel (volume) to the rail since your FPR (no longer regulated at the pump) can send excess fuel back using the return line. So, in theory, you can increase the amount of overall fuel that will reach the injectors. This will allow you to use a lower 'cc' injector to reach the same goal as a higher 'cc' injector on a return-less setup and thus have more fuel available for your injectors vs a return-less setup, raising the limits of your fuel system and, thus, your power.

    Important Points and Service Notes
    Fuel pressure checks on return-less systems can be done in the usual way by attaching a gauge to the service valve fitting on the fuel supply rail, or you can hook up a scan tool and look at the fuel pressure PID to measure fuel pressure. You can also use a fuel pressure gauge to cross-check the accuracy of the scan tool fuel pressure reading (which will tell you if the fuel pressure sensor is out of calibration). On a return setup, the fuel pressure gauge is a part of the fuel pressure regulator.

    As a side note, fuel volume should be measured if you suspect a weak pump is starving the engine for fuel at higher speeds and loads. A weak pump can often generate enough pressure at idle to meet specifications, but runs short when the demand for fuel goes up.

    Low fuel pressure and/or flow can also be caused by a clogged filter, by restrictions in the fuel line, or low voltage to the pump itself. The filters on most returnless EFI applications are “lifetime” filters with no scheduled replacement interval. Under normal conditions, most should last upwards of 100,000 miles, but they won’t last that long if the vehicle has rust or dirt in the gas tank. If an in-tank filter has become clogged, or the fuel pump has quit, be sure to inspect the inside of the fuel tank after you pull out the fuel pump module. Dirt and sediment can be removed by steam cleaning the tank. But if a metal tank is rusty, replace the tank to prevent a repeat failure of the new pump module.

    Something else to watch out for on return-less EFI systems is contaminants in the fuel rail on the engine. The rail is a dead end as far as fuel circulation is concerned. Fuel flows one-way into the rail and does not return to the tank, so any junk that gets past the fuel filter will end up in the rail or injectors. The injector(s) at the end of the fuel rail(s) furthest from the inlet are the ones most likely to become clogged with gunk.

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  4. #2
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    Return-less Fuel vs Return Fuel Setups 101

    Great write up Dan. As usual my friend!
    Tire slayer
    Breaking everything since 2011

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    Nice skinny guy.

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    Senior Member Spoolin_VTEC's Avatar
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    Hell of a write-up! Great information right here. This guy just gave more knowledge than most people have at car shops! Time for those dumbasses to take out a pen/paper and take notes!

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