is the oil burnt (black?), how long since last oil change, cos if the oil is burnt, maybe it just needs a tune.
Also try reseting the computer, my mates was doing the same thing (fuel economy), and after we reset the computer it fixed it up.
Head gasket - undo the oil cap, if there is a white residue on the bottom of the cap, theres a prob with the gasket
Good Luck, hope that helps.
91' EA II Sapphire
JMM DEV1, MPFI, Lowered King Springs, Monroe Shocks, 16" ROH Rims,CD Player, Kenwood Speakers.