LS1 returnless fuel system info from GM Techlink.

http://www.gmtechlink.com/images/issues/cnt_mo/TLcme.html
Increasingly stringent evaporative emission regulations have caused GM to introduce returnless fuel systems. In a returnless design, no fuel is
sent back to the fuel tank from the engine. All fuel leaving the tank flows through the fuel injectors.
Returnless systems have been in production since 1998 (Trans Am, Camaro and Corvette). A significant migration toward returnless fuel systems began in the 2004 model year. By 2007, the majority of GM vehicles will be converted to the returnless design.
Any contamination (including rust) that is built into or forms in the fuel line will find its way to the fuel injectors. A filter at the inlet to the fuel injector filters out small contaminants, including rust (fig. 11). If enough debris accumulates on the injector filter, it will start to restrict flow. This will have a negative effect on performance and driveabilty.
For the 2004 model year, there has been an increase in the number of injectors returned with rust in the injector filter. The rust appears to be coming from several sources: - the chassis fuel line - the crossover pipe on the Gen III fuel rail.
Both pipes are made of low-carbon steel, and the inside of the pipe is not coated. If water gets into the fuel system, it has the potential to rust the line.
When diagnosing driveability symptoms due to clogged fuel injectors, follow the procedures outlined in bulletin 03-06-04-030A. If the bulletin procedures lead to replacing an injector, inspect the removed injector for evidence of rust contamination. If rust is plugging the fuel injector, replace it, because a procedure for removing rust from the injector filter has not been developed.
If rust remains in the fuel line, there is a chance that the injector filter will plug up again. Whenever rust is observed in an injector, flush the fuel lines. If the fuel line cannot be completely flushed of rust, replace it.
A small amount of rust in the fuel tank will typically not cause a problem. The fuel filter will prevent the rust from getting to the injectors. The capacity of the filter is large enough to keep it from plugging up. There is also a strainer (sock) at the inlet to the fuel pump. The strainer prevents contamination from getting into the fuel pump and causing problems. However, rust in the fuel tank is a sign that water may be in the tank. If rust is observed in the fuel tank, empty it and flush with hot water, according to SI procedures.
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