hey welcome
my 1998 1.3 slx in standard form used to do upto 320miles from full till the light comes on during long motorway trips.
first check all tyres r inflated to spec and that all the wheels turn freely without much brake binding.
if you also have an engine light during ignition you could check the ecu for any faults and if the exhaust O2 sensor is working cos that could also drastically affect economy and performance.
warm up the engine then turn off.
turn ignition back on, open the fusebox cover by ur right knee to find the ecu consult port down below. use a paperclip to short the lower left pins for 2sec (labelled 12V OGN and Check in pic below)
the engine light should flash 5 long bursts then 5 short bursts to give a code 55 which means no faults logged.
then start the engine to check the O2 sensors, hold rev at 2000rpm and the light should flash on/off every 1/2sec. any slower and it'll need replacing.
the premature fuel clunk issue seems to be a common fault.
The fuel tank uses a soft rubber breather pipe going from the top of the tank, through the chassis beam and has to take a 90deg turn before joining the solid breather pipe exit. The rubber pipe tends to kink at this bend therefore restricting the tank from evacuating out trapped air fast enough, and so fuel fills up the filler pipes till it triggers the cutoff while air is still in the tank. Air keeps slowly escaping from the tank and the fuel level drops back down to continue filling.
To resolve the kinked breather hose, I’ll replace it with a reinforced conduit pipe so its flexible but wont collapse to block the airway
