The main pump will lift fuel from the main tank through the stationery tank pump and produce pressure to run .. the in-tank pump helps in hot wetaher when vapourisation in the line would cause hiccups in fuel pressure , it pressurises the line from tank to pump by about .,5 bar to prevent air pockets . In the winter it makes no notcable difference to fuel delivery ...
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 My comments are only based on my opinions and vast experience .
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