Hi guys. I am not an expert on tuning thing but what I have learnt from Pakwheels experts and my experiences with these JDMs (OBD-II vehicles), they are all dependent on engine sensors (O2, MAF, ECT etc) If any of these sensors got faulty you get CEL but if they got lazy or hanged on some value your engine will be fuel guzzler with poor engine output. You will need to check the functionality of MAF and O2 sensor in specific by hooking up some fast acting OBD-II scanner and visualize live graph.
I also own an Nissan AD 2006 with original engine clocked around 250 k kms and it also used to drink fuel as most respected members mentioned. It used to give me some 8 km/lit in city driving with AC. After studying thousands of forum lines, I decided to go for a thorough diagnostic of my car. My car is operating well (with poor mileage) with no CEL but upon investigation, I found both my MAF and O2 sensors lazy. I replaced them with used parts from a junk dealer and now my car used to give average 12 km/lit in city with AC and 14km/lit at highway speeds with ample power and quick acceleration. Following are few things which needs to be checked as they contribute the most in bad fuel consumption.
MAF Sensor : For MAF sensor you need to hook up scanner on OBD port of your vehicle and get MAF readings while engine is running. The reading should change with every change in throttle position. If MAF output seems stable at different throttle position. your MAF sensor has gone and you need to replace with new or scrap item. You can find working sensor in scrap at around 1500 ~ 2000 rupees.
Oxygen Sensor : For O2 sensor, again you need to visualize O2 graph on an scanner. It should be moving between 0.1v to 0.9 volt with a square wave pattern. If you see something like SINE wave or the rising and falling is not sharp or its not moving between 0.1 to 0.9 volts, you need to replace your O2 sensor. The cost of O2 sensor from scrap would be around 1000 rupees and new one would cost around 5000 to 7000.
Spark Plugs: The spark plugs should be in good condition with correct heat range and tightening torques. A bad spark plug or incorrect heat range can induce knocking/pinging and poor engine performance.Recommended spark plug for Nissan AD is BKR5E-11 NGK.
Thermostat valve: Your engine thermostat valve should be in place and in working order. Your car coolant temperature should be between 95 to 100 degrees centigrade for CPU to go into closed loop. If your engine fails to achieve said temperature (due to removal of valve or non-functional/leak valve) your engine will remain in OPEN loop and it does not try to achieve air/fuel ratio of 14.7:1 resulting in bad fuel consumption and performance.
Vacuum Leaks: Your car's engine vacuum pipes should be in good state. Any minor crack in any pipe can let un-metered air into engine resulting in lean mixture and poor engine performance.
Finally, Nissan AD is equipped with CAT CON. you have to visualize the condition of CAT by removing it out of exhaust system. If its chocked you need to clean it using toilet cleaners/acids and compressed air and if its chocked beyond cleaning you ultimately have to remove CAT from your exhaust. A chocked exhaust is also a major reason in high fuel consumption and sluggish engine response.
Experts are welcome to correct me if I made any mistake in above as I am not an pro in engines. All I learnt is from Pakwheels experts like @Xulfiqar bhai, @Capsat, @SER_GTR, @ahmed2396
Regards.
