similar problem i am facing with my 1.3 gli. the problem started with rough idling which disappears on cleaning the fuel injection system like throttle body, plugs, etc. the problem again re-occur in a couple of months time. some times, the gas treatment liquids also help in controlling the problem, like Gumout, STP's fuel injection system cleaners, but the problem can only be minimized for some time but can not be eliminated permanently. initially, when this problem started to appear, the symptoms were mild and resetting the ECU by disconnecting the battery terminals helped restoring the smooth idling. i have consulted many mechanics, toyota service centers, but it has been impossible to diagnose the exact root cause of the problem. the computer scanner does not show any error or faulty readings. in order to carry out a perfect diagnosis, i started replacing fuel injection parts one by one so that i might catch the real culprit in the system. i changed the throttle body, the problem vanished for few days and re-appeared. i changed the plugs, same thing happened, rough idling disappeared for some time and re-appeared. i changed the coils on plugs, the problem vanished for some time and re-appeared. i changed the fuel injectors, the problem still persists. i have also tried to fuel my car with high octane on full time basis but that does not help fixing the problem too.
historical background of my car: initially, an italian landirenzo cng kit was installed in the car which i uninstalled at around 60,000 km. i am doubtful if this problem even existed in the time of cng as well because i used to witness slow throttle response. i tolerated the problem as the symptoms were very mild and thus negligible but the problem was identifiable. after uninstalling the cng kit, with the passage of time, the symptoms became worst and made driving the car impossible. the rough idling becomes worst, the car does not accelerate easily, the rpm drops and turns the engine off.
please advise solutions with technical logic and reasoning. your valuable advice is highly appreciated.