Gregorys manual is well worth it, this for an AE92, 102 or 112?
Diagnostics check would be the first thing I'd do, bridge the T and E1 connectors in the diagnostics terminal with a paperclip, then see what code the Check Engine Light flashes up. If nothing comes up from that, check for things like leaking injectors, spark plug condition, O2 sensor condition, dirty air filter, blocked throttle body, MAP sensor, etc, basically anything which has to do with controlling the amount of air or fuel going into the engine.
Out of interest, what kind of economy are you getting now? Does it feel like it has lost power as well?