Well CNG , technically speaking should not make car consume more fuel if installed properly.
WHY is it causing so?
The answer lies in the depths of mysterious logics that most mechanics dont have.
When they install CNG they use ordinary venturi. it has two problems
- Bad Design hence requires you to block air intake to reduce air flowing through intake and cng can work its magic
- It almost always introduces a vac leak in cars whos MAP / MAF sensor is before ventury
in case 1 you have same effect as of blocked air filter you can google it up that way, why clogged air filter cause bad milage
in case 2 your car gets extra air un noticed by MAF sensor and Air to Fuel Ratio gets disturb so your o2 kicks in and adjusts it, as more than required air is flowing un noticed by MAF ecu increases the injector pulse width to maintain AFR hence your short term fuel trim would not be in normal range
Why would one install CNG in that civic. Its not worth it, IMHO sell the kit and spend that 30-40k on something you like