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To
comply with California and Federal exhaust emission standards, in
the 1970's and 1980's vehicle manufacturers  began
replacing carburetors with fuel injectors. These new fuel injection
systems used dual fuel lines -- fuel inlet and fuel return -- to
maintain injector fuel pressure and cool the injectors.
The
two line systems operated by sending more fuel to the fuel rail than
was actually consumed by the engine. The excess fuel was sent back
to the fuel tank via a "return line".
Single
line "return less" fuel injection systems began to appear
on the market in the early 1990's and are now found on almost all
new vehicles.
WHY
SINGLE LINE?
In 1990, the California Air Resources Board adopted zero evaporative
emissions standards for fuel systems. The Board found that up to one
half of all hydrocarbon emissions in urban air was from poorly
designed fuel systems and defective gas caps.
In
the dual line systems, the return fuel line carried excess fuel from
engine back to the gas tank. On many vehicles more than one gallon
of hot fuel and vapors were returned to the fuel tank every minute.
This
strained the capacity of the fuel Evaporative Emissions Control
System (EVAP). The EVAP system circulates fuel vapors through
charcoal- filled canisters to trap and store vapors.
SOLUTION
- With a single line system there is no return line and no
circulation of fuel back to the fuel tank from the engine. This
eliminates hot fuel from being dumped into the gas tank and limits
harmful emissions from being released into the atmosphere.
FUEL
SYSTEM CLEANING
On return less systems, Injectors at the end of the fuel rail are
prone to contamination and clogging. This can cause a lean condition
and cause the engine to misfire. If engine emissions or drivability
problems indicate a clogged injector, a fuel system cleaning is in
order.
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