You're probably familiar with "return-type" fuel injection systems, but have you worked on a "returnless" system?

Returnless systems are found on many late model cars and trucks. The first ones appeared back in 1993 on certain Chrysler V6 and V8 truck engines. By 1998, all Chrysler cars and light trucks had them. In 1996, Toyota introduced its first returnless system, followed by General Motors and Ford in 1999. Honda went "returnless" in 2001, and today you'll find returnless fuel injection systems on almost all new vehicles.

In the older return-type systems, the fuel pump delivers more fuel to the engine than it actually needs. The excess fuel is then routed back to the fuel tank through a pressure regulator and return line. But in a returnless system, there is no pressure regulator on the fuel supply rail and no return line back to the tank. The regulator is mounted in the fuel tank and is usually part of the fuel pump module.

A returnless fuel injection system, by comparison, manages fuel pressure a little differently. Instead of using a spring-loaded vacuum diaphragm in the regulator to change fuel delivery when throttle opening and intake vacuum change, the regulator in a returnless system operates at a constant pressure. The older return-type systems need to vary fuel pressure to maintain the same pressure differential across the injectors when intake vacuum drops. When vacuum drops, the regulator increases pressure to compensate. But in a returnless system, this isn't necessary because the line pressure is always the same.

So how does the system compensate for changes in engine load and vacuum? A returnless system uses the Powertrain Control Module (PCM) to regulate fuel delivery. A fuel pressure sensor mounted on the supply rail allows the PCM to monitor fuel pressure. When pressure in the supply rail drops as engine load or speed increase, the PCM compensates by increasing injector duration (on time) and/or the operating speed of the fuel pump.

Some systems (Ford, for example), vary the fuel pump's output by changing the voltage supply to the fuel pump module. When more fuel is needed, pump speed is increased by increasing the pulse-width (on-time) of the pump's voltage signal (pulse-width modulation).

WHY RETURNLESS?

The older return-type fuel injection systems circulate a lot of fuel between the engine and tank. This keeps the fuel from getting too hot and boiling as it passes through the fuel rail on the engine (which can cause vapor lock and hard starting or stalling on hot days), but it also carries a lot of heat back to the fuel tank. Heat increases fuel vaporization inside the fuel tank, and puts more of a strain on the Evaporative Emissions (EVAP) control system.

The EVAP system's job is to contain fuel vapors so they do not escape from the fuel tank and pollute the atmosphere. Fuel vapors pass through a vent hose to a charcoal-filled canister which temporarily traps and stores the vapors. Later, the vapors are purged from the canister via a control valve and routed into the engine while the vehicle is being driven.

The trouble is, EVAP systems have limited capacity and can only store so much fuel vapor. If the fuel is getting hot and vapor pressure is building up inside the tank, it can saturate the charcoal canister and overload the EVAP system's ability to contain the vapors creating a potential emissions problem.

On newer vehicles with OBD II, the onboard diagnostic system is required to monitor the fuel system for vapor leaks. If the fuel in the tank gets too hot and builds up excessive pressure, it may cause a leak that will turn on the Malfunction Indicator Lamp (MIL) and set a diagnostic trouble code (DTC). What's more, the U.S. Environmental Protection Agency tightened the limits for evaporative emissions, making it even more important to control fuel vapor pressure inside the tank.

With returnless systems, there is no return line and no circulation of fuel back to the fuel tank from the engine. Consequently, there is no heating of the fuel in the tank and no increase in fuel vapor pressure from driving the vehicle. This reduces the risk of excessive pressure build up inside the fuel tank, vapor leaks, and triggering an OBD II EVAP monitor DTC.

OTHER DIFFERENCES

Another difference is that returnless systems typically operate at a higher pressure than return-type systems. This is necessary to reduce the risk of fuel boiling and vapor lock in the injector supply rail during hot weather (since there is no recirculation of fuel from the engine back to the tank to keep the supply rail cool).

Returnless systems are very sensitive to fuel pressure, and if pressure is more than a few pounds out of specifications, it may be enough to cause a driveability or emissions problem.

Fuel pressure checks on returnless 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 read the pressure value via the pressure sensor. Using a fuel pressure gauge to cross-check the accuracy of the electronic reading is a good way to check for a fuel pressure sensor that is out of calibration.

Remember, returnless systems are designed to operate at a constant pressure. A simple pressure check with a gauge or scan tool will tell you if the system is within specifications. Pressure should also be monitored with a scan tool while driving the vehicle to check for pressure loss under load.

If the operating pressure is out of range, the PCM will compensate by increasing or decreasing the short term fuel trim (STFT) and long term fuel trim (LTFT) values. As a rule, these numbers should usually be within plus or minus 10 points. If you see a higher or lower value on your scan tool, it may indicate a fuel mixture problem due to incorrect fuel pressure (bad fuel pressure sensor, bad fuel pressure regulator, a weak pump or low pump voltage), or an air leak, or dirty fuel injectors.

VOLUME JUST AS IMPORTANT AS PRESSURE

Fuel volume is just as important as pressure in all fuel injection systems. The pump has to push enough volume of fuel to keep up with the engine's demands when it is under load, accelerating hard or running at wide open throttle. A weak pump may still produce enough pressure to be within specifications at idle, but may not deliver enough fuel at high rpm and load, causing fuel starvation, lean misfire and a loss of power.

As a rule, a "good" pump will deliver at least 750 ml (3/4 quart) of fuel in 30 seconds.

Sometime a low pressure or volume problem isn't the fuel pump but a clogged filter, plugged fuel inlet filter sock, restricted fuel line or a faulty fuel pressure regulator. A low voltage supply to the fuel pump due to a wiring problem, low charging voltage or bad relay may also prevent the pump from operating at normal speed.

TECH TIPS

* On returnless systems that use pulse-width modulation to vary the operating speed of the fuel pump, you should be able to read the value of the control signal on your scan tool. Look for a change in the number when engine speed/load change.

* The injector on the furthest end of the fuel rail(s) in a returnless system may be more prone to dirt contamination and clogging than injectors further upstream. Because there is no circulation of fuel back to the tank, the end of the fuel rail may become a sewer pipe and collect any debris that gets past the filter. The debris may clog the inlet screen in the injector and starve the injector causing that cylinder to run lean and misfire.

Cleaning the injectors on the engine may not help because the debris may remain trapped in the end of the fuel rail. It may be necessary to remove the injector(s) and fuel rail(s) for cleaning, or to replace the rail if the debris cannot be flushed out.

* For best performance, injector flow rates should not vary more than about 5% from one injector to the next on returnless systems. Injector flow rates can be measured and compared on a test bench. If this is not possible, and one or more injectors are clogged or dirty (and do not respond to cleaning), you should recommend replacing the entire set of injectors. Why? Because if you only replace the "problem" injector(s), the new one(s) will likely flow more fuel than the old ones (unless all have been cleaned and flow tested). This can create an overly rich condition in the cylinders with the new injectors, and cause a driveability or emissions problem you didn't have before.

* Most fuel pump failures are caused by dirt or rust in the fuel tank. So it is very important to inspect the inside of the tank when a pump is replaced. If the tank is dirty, steam clean it. If a metal tank contains rust, replace it.

* When replacing a fuel filter, pour a little fuel through the filter inlet to "pre-wet" the filter element inside. This will reduce the risk of the filter element shredding loose paper fibers into the fuel system when the pump starts up and sends fuel at full force through the filter.

FUEL FILTERS

Another difference between some return-type and returnless fuel injection system is the location of the fuel filter. In most return-type systems, an in-line filter is positioned somewhere between the fuel tank and engine. The filter may be located under the vehicle in the fuel line that carries fuel from the tank to the engine, or in the engine compartment on the firewall or fuel rail. The filter typically has an OEM recommended replacement interval of 30,000 to 50,000 miles.

On some returnless systems, an in-line filter is also used. It may be located in the fuel line or engine compartment. On some hybrid "semi-returnless" systems, the filter is located outside the tank and routes fuel back to the tank through a third return port. But on some returnless systems, the fuel filter is located inside the fuel tank and is part of the fuel pump module or regulator.

What's more, on some of these applications (Dodge Neon, for example), there is no OEM recommended replacement interval for the fuel filter! Others say to replace the filter "as needed."

One reason for the extended filter life is because a returnless system pumps much less fuel through the filter. A typical return-type system may circulate up to 30 gallons of fuel per hour through the filter and return line. With a returnless system, the only fuel that passes through the filter is that which the engine burns. On a vehicle that gets 20 miles per gallon, that would only be about 3 gallons of fuel in an hour at 60 mph.

This doesn't mean a filter with no OEM recommended replacement interval will last forever. It won't. Eventually, the filter will become clogged and have to be replaced -- and when it does the fuel tank will have to be drained and lowered to gain access to the filter and pump (unless the vehicle has an access panel under the back seat or in the trunk).

The life of the filter will depend on the cleanliness of the fuel that goes into the tank, driving conditions and corrosion inside the fuel tank (not an issue with plastic fuel tanks but can be with aging metal tanks).

If engine driveability or emissions problems indicate a restricted fuel filter, the fuel filter must be replaced regardless of mileage. It can also be replaced at any mileage interval for preventive maintenance, though in the case of an in-tank filter that could be an expensive and labor-intensive job.

On many of these returnless applications with in-tank filters, the fuel filter probably won't be replaced until the pump fails -- so it is very important to make sure you also install a new filter when you replace the pump.

The pump pickup "filter sock" should also be replaced when the pump is changed. And don't forget to inspect the inside of the tank for dirt or rust!