advice needed regarding recent dip in mpg

[prophetofsin]

I have a 98 dx and recently replaced the header with another d16y7 header which has the cat attached. It came with a used lower o2 sensor which i did not bother replacing. I reused my upper o2 sensor.

Swap was successful, no codes no nothing. I also changed my tires to 205/50/15s.

After that, I noticed my mpg dipped drastically from 36 highway to 30. Nothing else was changed and I recently went to Autozone to get the codes checked, none.

I have been driving the same way in terms of shifting and acceleration with no use of AC. Do you guys believe it could be one of the o2 sensors? I am contemplating replacing both of them as I read that they should be replaced every 100,000 miles. My car currently has 165,000.

I read also that it could be the fuel filter, but I am assuming otherwise due to this happening after my header swap.

What do you think?

Appreciate any feedback. Thanks.

 

[typeR[ivi]]

it might be the faulty o2 sensor. Because the o2 sensor would send signals to your cars ECu from there, it would send a messgae to your cars fuel system to add or decrease fuel, to make ti leaner or less. Stuff like that. Poor gas mileage might be because of this sensor, but it might also be the tires you replaced, where they bigger??? Smaller?? If it was bigger, this means more unsprung weight which means, it would take more the move something the same distance.

 

[prophetofsin]

yep, i think it might be the sensor as well. the tires are supposedly correct for my car. 205/50/15s. I think i had 175/65/14s before. I was even getting 33 mpg on the highway with 205/40/17s before. I was assuming any change to mpg due to 15s would be minimal at best. Wheels are not that heavy either, kosei k1s.

That;s why I'm leaning towards the o2 sensor. I just want other people's opinion before i go and buy the new ones and find out it does not do anything.

 

[RonJ]

Information from the primary O2 sensor -- not the secondary O2 sensor -- is used by the ECU to determine A/F ratios, so the secondary sensor can be eliminated from this discussion. In addition, your primary O2 sensor is not throwing a code, and it was working fine prior to the header swap. Therefore, I don't think your lower mpg is O2 sensor related. I would look for an exhaust leak from the newly installed header as this would definitely cause the engine to run rich. How old are the plugs, wires, cap, and rotor? Is the ignition timing set to spec?