yes its there in nearly every pakistani assembled civic and corolla too that has no oxygen sensor, DO NOT turn that screw unless you have solid knowledge and have a real good long experience under your belt, Its properly set by sticking an EGA probe into the exhaust. If you turn it without any measuring, you may destroy your engine by going lean or rich.
An engine that suddenly starts drinking fuel is not a trim pot problem, its something else, like leaky injectors, bad timing, fuel leaks, compression etc. Sort those out first, get everything in proper working order - if you cannot do it yourself get/find someone to do it (come to Karachi for this very same purpose) as you may end up with a severe headache and a problematic car if you do it without any precautions.
The ECU you have is OBD-1 btw.
before anything - check and verify that the check light is actually wired to the ECU and not something else, if its on check for fault codes.
Most high fuel consumption issues are due to a faulty coolant temp sensor or missing thermostat valve, by running cold all the time your ECU will overfuel the engine as its setup just like that. Once the engine reaches any temperature between 80 and 95 degrees centigrade it will get to correct economical fuelling.