The fuel rail delivers fuel, either gasoline or diesel, to the motor’s injectors. It is fundamentally a pipe equipped with an inlet port at the upstream side and threaded seats downstream for the installation of injectors. Some manufacturers use individual pipes to feed the fuel to each injector, but the vast majority of fuel-injected engines are fed through fuel rails.
In electronic fuel injection, or EFI, systems, each injector is opened and closed by a solenoid, itself controlled by the vehicle’s onboard electronics. Because the gas or diesel in the fuel rail is under pressure, it sprays out from the injector when the valve opens, entering the combustion chamber.
Because there are no moving parts inside a fuel rail, there is no regular service procedure. In common with most parts of the fuel system, however, the fuel rail can become contaminated either by dirt drawn from the fuel tank or by varnish. Varnish is a deposit that forms from decomposing fuel after only a few months of disuse; cheaper grades of gas varnish more quickly than premium grades. That said, because the nozzles in the injectors are much smaller than the diameter of the fuel rail, contamination is more likely to occur there. Because contamination can restrict the flow of fuel, the end result can be lean operation or -- in extremely advanced cases -- stalling. Increasingly poor engine performance is the usual indicator that cleaning is required; idle becomes rough, and fuel economy decreases.
Injectors can be extremely expensive, so cleaning a contaminated system is usually the first option to explore. Introducing a proprietary solvent into the fuel tank can be successful in cases that don't involve advanced contamination. If fuel additives fail, pressurized kits are available that competent home mechanics can use to introduce undiluted solvent directly into the fuel rail while it is still attached to the engine. For the worst contamination, off-engine pressurized cleaning with undiluted solvent, a practice best carried out by professionals, is usually effective.
Because the capacity and efficiency of the fuel rail is so vital to the engine’s operation, many custom car fans and racers uprate their rails. Upgraded injectors typically work at their optimum capacity only if supplied by a likewise incremented fuel rail. Custom units typically provide fuel faster and at a more constant rate. Materials may differ from stock so that higher-octane gasoline at increased supply pressure, or racing fuels such as methanol or nitrous oxide, can be safely used. Designs are often specific to matched manifolds.
