fuel return lines and fuel economy? - PeachParts Mercedes-Benz Forum

ramtony · #1 05-06-2008, 11:08 AM

I have changed my fuel return lines 3 times in the last 4 months, the first time each line was cut in the middle and was attach with a plastic tube that looks something like a vacuum line attachment, I made the mistake not to keep does and now a dont have does, and not sure if the regulated the pressure or serve any specific purpose?. well the last time a changed the return lines I ended up with Viton return lines and it seems that every time a changed the return lines the efficiency on the fuel changed as well. now I'm down to the lows 20 mpg from 27 city to 34 highway.

not sure if it is fuel quality played a major role on the poor mileage or just the return lines. also the engine vibrates or stumble close to 1000 to 1200 RPMS, and when you shut down the engine one can see air in the plastic line from the pre filter down to shutoff pump already changed the oring from the prefilter and no joy. it's the fuel drop related to the air in the lines? or could it be, the return lines.

also the car nails all the time, even when warm. i have check all the injectors and they where all withing specs...

any help you be really appreciated.

1999 E300 152K miles
Va Beach, VA

vstech · #2 05-06-2008, 11:29 AM

unless the return lines are leaking fuel, economy will not be affected by them, no matter what they are made of...
there are many many other things under your hood that contribute to fuel economy.
my bet is on the injector o rings... search for delivery valve leaks...

TMAllison · #3 05-06-2008, 12:19 PM

If you see air in the line from the bottom of the pre-filter to the SOV the leak must preceed that point.

Check the seals of the fuel lines: exiting the prefilter, attaching to the top of the pre-filter, the pre-filter oring, the pre-filter itself (sometimes they crack) and the other end of the prefilter fuel line where it attaches to the fuel thermostat in the cyl head.