the ecu should have on the cover the code wd48.08/ this is a level 8 ecu for the j reg, the numbers after the / basically let you know if it can run a lambda sensor or not. l3e.31 is not lambda capable. and i think l2e.31 is
at that level (stage 3) it would be worth bringing it to a good cozzy tuner to sort out the fueling as its more than likely a sensor or just a basic set up.