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Old 13 Jan 2011, 05:46 am
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Default i have a 99 honda civic and my mpg has dropped drastically what could be the problem?

i went from almost 500 miles a tank to less then 250 a tank. my car is mostly stock but i do have new headers and exhaust system and an intake my headers and exhaust r off a 99 civic dx hatch so the exhaust is short and my oxygen sensors r in different places but i ran new o2 sensors and for a month or 2 my mpg didn't change could the new headers since my o2 sensor is reading off only one pipe instead of all 4 be my issue or is it something else and if so what else could it be? i replaced my rotor button distributor cap and put in performance wires and spark plugs. i have also replaced my fuel filter. my check engine light is on but it does?t say anything about my air flow sensor i believe it says something about my oxygen sensor bank one. my check engine light came on shortly after i installed my headers and exhaust system i had my intake for a month or so already.
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Old 13 Jan 2011, 06:01 am
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...Check the cat converter.
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Old 13 Jan 2011, 06:16 am
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Can you give us the code number for the Oxygen Sensor CEL?

Since your throwing an Oxygen Sensor code, the car is staying in what's called Open Loop. Normally it stays in that mode only until the car reaches operating temperature since a cold engine requires a richer (more fuel) air/fuel mixture. In Open Loop the ECU does not use the Oxygen Sensors so it runs rich causing a loss in MPG and increased emissions. Once warmed up, it switches over to Closed Loop where the ECU now takes readings from the Oxygen Sensors.

What type of Exhaust system and Header are you running, or those OEM from the hatch or aftermarkets?

Modifying the exhaust on an OBD II car is very tricky, they are very sensitive to the changes, so anything you do, you must make sure that the aftermarket part retains a near stock setup for things like the O2 Sensor (especially location and wiring) and Catalytic Converter.

I can guarantee one thing, if you change the exhaust back to stock, all your problems will disappear.
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Old 13 Jan 2011, 06:31 am
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post this on or honda-tech,com
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Old 13 Jan 2011, 06:46 am
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Hey Jared, You been dating a fat chick lately?

Seriously you need to have the valves adjusted and test the MAF sensor voltages.
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Old 13 Jan 2011, 07:01 am
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mostly would be the installation issue. try to find out if you installed the headers and the full catback exhaust. sometimes if you arent precise with the washers and might be leaks somewere. also headers. shoudlve got that done by a proffesional. check engine light is on umm..consult dealership or mechanic
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