Ok so after i put my new header on i drove the car 2 weeks with the o2 sensor gone. i bought a new one couple days ago and i noticed my gas mileage is alot worse than it was without one. im getn about 25 mpg now and when i first got the car i was getn about 33-35. What am i missing here. It has new wires, plugs, o2 sensor, oil, air filter. any help would be helpful thanks guys. James

 

Was an O2 sensor code thrown prior to installing the O2 sensor? Are all CEL codes now gone since installing the O2 sensor? Has the ECU been reset?

 

ecu was rest and yes cel was on bcuz of no o2 being hooked up and no codes now

 

Post back after you been through a couple tanks of gas. The ECU is still likely in a learning phase. After that, gas mileage should improve.

 

so far its been thru at least 4 full tanks

 

D16Z6 engine and P28 ECU? Did you buy the correct O2 sensor?

 

pretty sure i did lol ill check the part number.

 

says its for a d16z6 so idk what else to do or check on why i would get worse gas mileage with new o2 sensor

 

should be 4-wire o2 sensor, but that is strange, try unhooking the plugs that go into the ecu, count to 10-20 sec then hook it back up, make sure your not leaking any gas, i was at fpr, just dripping, but did not smell nothing, lmk what happens

 

What brand O2 sensor? OEM? Other? A faulty O2 sensor could possibly be worse than no O2 sensor.

What brand wires and plugs? Have you inspected the cap and rotor? Is the ignition timing set to spec?

 

its a bosch o2 sensor. direct fit. plugs are ngk stock spec. wires are from oreillys. im resting the car right now.

 

Answers to the other questions? Bosch O2 sensors don't have a great reputation for Civics.

 

thats all i can get around here tho

 

Try buying a used OEM O2 sensor online or at a junkyard if you can't afford a new OEM one.

 

Would outside temp affect gas mileage that much? its lower 90s here. just a thought

 

Use of A/C in hot weather would reduce mileage only a small amount, which would not account for the large drop you have seen.

 

Unless you have some new information, this is where I recommend you focus:

 

here is wat done to the car. 4-2-1 header, gutted cat, stock exhaust to aftermarket fart can, omnispark wires, ngk plugs, new thermastat, new bosch 02 sensor, new pcv valve, new fuel filter. i have replaced and or added this stuff since i bought the car in feburary 2011.

 

Bosch O2 sensor is a concern.

Have you inspected the cap and rotor?

Is the ignition timing set to spec?

 

just cleaned the the rotor and cap. might be getn a new in morn to see if that help. there is oil just a lil bit in the cap. i did replace the distributor oring too.

 

If there is oil inside the distributor, then the internal oil seal (not O-ring) is bad. You can replace the internal seal, or the entire distributor if it's old (~150K miles).

 

on the rotor is the metal piece on the out side supposed to have some plastic whit stuff on it?

 

Does this help you?

 

well ill just buy a new rotor.

 

well yesterday after noon i was on my way to pick up a new cap and rotor and on my way there i noticed my gas mileage was alot better!!! i left with a half tank and got there with 2 needles below half. i went 110 miles on a quarter of a tank and i never get that good of gas mileage im lucky if i get 100 miles from half to empty. i still bought a new cap and rotor anyway just to be on the safe side. Glade i got my gas mileage back.