New O2 sensor but still lower mpg 96 civic hx

[ryanschillinger]

Was getting cel for O2 sensor p1169. Replaced the front sensor (expensive wideband version in this car) and no more cel. My mpg had been up about 41 average and dropped to 38 before replacing the O2 sensor. Last tank was only 36.6 which was surprisingly bad for this car. Internet research said to reset the ecu by pulling the FI and backup fuses before letting it idle 10 minutes. Is this correct? Is there anything else I should do after replacing the upstream O2 sensor?

Thanks!

 

[Mr Nice]

Is your EGR clogged ?

Some Civic HX will get this as the miles rack up.

 

[ARCOgraphite]

Is low torque + power OK?
Wondering if cat got smoked. That should set a MIL for the rear sensor though in time.

Sometimes relearning takes a while. You could just DISCO the BAT=GND for a bit then re-attach it and relearn from warm. There are extensive relearn procedure for most OBDII engines maybe a honda forum might have it.
Sometime a 3 min undisturbed (NO AC or RADIO) idle and some slow or EASY driving to the highway for a few miles then moderate accelleration up to 60MPH then off-throttle engine braking back to 40 a couple times then SS cruise at 50-60 for a few mins might do it. Then idle for 3 mins and KEY OFF.

 

[zzyzzx]

I think you need more than one tank of gas to re-evaluate the gas mileage.

I also want to know the mileage on this car and/or the previous O2 sensor.

I also want to know what brand of O2 sensor you used and is it a direct fit or did you get a universal one and solder.

 

[ironman_gq]

I would guess that aside from the ECU needing to relearn it's possible that the cat is plugged. They do foul up with oil residue and soot over the years, especially if the engine ran for a while with a bad or lazy O2.

 

[Gasbuggy]

How long has it been since you changed the air filter? Is the tire pressure good? I would check mileage for atleast 2 tanks before worrying about it.

 

[Mr Nice]

Here is info on EGR clogging.

http://ecomodder.com/forum/showthread.php/nasty-egr-passage-completely-clogged-civic-hx-22824.html

 

[ryanschillinger]

Car just turned 150k yesterday. Old O2 may have been original. New one is Denso brand direct replacement.
I've already done all the tune up stuff like plugs, wires, cap rotor, cleaned throttle body, ran fuel system cleaner through the tank, changed engine and manual trans oil, that kind of thing.
I have no idea if the EGR system is clogged, but wouldn't that trip a CEL?
I know a single tank isn't a good judge, but I was expecting to go back up in mpg. I'll give it a couple more tanks and see what happens.
As far as whether it has power, I've only had the car for about 10k miles so I don't have much of a baseline. It's not a fast car, that's for sure. The HX was built for mpg, and that's why I bought this one. Once it passes 3k rpm, the vtec-e switches over, you can definitely feel it. And it pulls strong if you let the rpms build. I think if the cat was clogged, it would really hesitate and bog down at higher rpm, right?

Thanks everyone for the input. I'm no expert, so I really appreciate the help. I drive a lot of miles, so the difference between 36mpg and 41mpg is pretty big.

 

[SatinSilver]

What brand spark plugs were used? Also does a 96 even have a cel?

 

[eljefino]

dragging brake. 96 will for sure have a CEL, he has an OBD2 style code after all.

Go for a long drive, coast to a stop, feel each wheel for warmth.

 

[ryanschillinger]

Denso plugs and wires. The car coasts very nicely and I had it jacked up for tire rotation recently and the rears spun a little more freely than I would've adjusted them to, but it's nearly due for rear brakes anyway so I didn't waste the time adjusting them. I don't think the fronts are dragging. I'm thinking the egr cleaning might be a good start but I've never done that before. Alignment was recently done along with 4 Michelin defender tires that have low rolling resistance. I guess if the mpg doesn't come back up after a few tanks I should clean out those egr passages. I'm open to other ideas also.

 

[PandaBear]

My experience on a 97 Integra is that pulling fuses may not be the easiest way to reset the ECU after an O2 change, it is easy to miss a few fuses here and there.

I'd pull the battery cable and let it sit for 1 hour to be sure.

 

[ryanschillinger]

Alright. I'll give that a shot if it doesn't go back to normal next tank or 2.

 

[circuitsmith]

Original thermostat?
It could be sticky and not warming up the engine fully.
As a rule I change the thermostat and radiator cap every other coolant change.
Use only OEM.

 

[ryanschillinger]

Hmm that's a thought. Certainly wouldn't hurt. Coolant looks pretty clean but still couldn't hurt since I'm not sure about the maintenance history. Timing belt looked to be in good condition when I had the valve cover off. Suppose I could've checked valve clearances but didn't have time while changing the vc gasket. Is there a way to check the temp of coolant with engine warm or just go ahead and replace the thermostat you think?

 

[circuitsmith]

If you don't know that it's less than 10 years old I would just change it.
They're not expensive.