Can you post fuel trims and oxygen sensor voltage data at idle RPM. It is a must that thermostat should be installed with fan on auto.
No CEL means that fuel trims and O2 voltage etc are not out of limits yet. What was the color of spark plugs when they were removed ?
Please ask your mechanic to check intake manifold pressure at idle. If above 22-23 hg then you may have an exhaust restriction likely due to blocked exhaust. Cat Cons are likely culprits.
I faced a similar problem a month ago. I ensured following and now, I get 12+ KMPL even in rush hour traffic.
OW-30 oil, Ceratec, cleaned Oxygen, TPS, Throttle body, IAC valve, bypassed EGR valve, Cat Con removed, all grounds cleaned, good suspension and transmission, transmission additive, AC condenser clean plus gas not overcharged, 30 PSI Tyre pressure and wheels aligned, .19mm intake and .29mm exhaust valve clearence, 1.0mm spark plug space, ignition coil and chords resistance within limits, good battery, rust free radiator and Zic coolant, good ECT and IAT sensor and new fuel filter.