I've been noticing for the past few months that my MPG has slowly dwindled from 22-25 in the city to now 10-12 mpg. Another thing I noticed for the past month is that whenever I fill my tank all the way, it idles rough for about a mile. The CEL light came on a few days ago, and threw P0141. IIRC, that O2 sensor only sends info to the other, which does the actual air/fuel corrections. I know that during the winter, mpg could dwindle a little due to running the heat, but I besides warming the car up for five minutes before I drive, I keep the heat at a minimum.

Here are the things I've done:

• Regular fluid changes
• Tires are two months old and inflated to factory spec (checked every week)
• Brand new spark plugs
• Replaced the air filters
• Replaced the brakes
• Replaced the front O2 sensor about a year ago (maybe that could be the culprit?)

There aren't any visible leaks, and no off smells, so I'm really at a loss as to what it could be. Is there anything else I should check?

 

How are you calculating the gas mileage? Distance between filling divided by gas needed fill tank is best, in case you fuel gauge isn't accurate.

Why did you replace the upstream air/fuel sensor? What brand sensor did you use?

P0141 is for the heater circuit in the catalytic converter O2 sensor. The code can be caused by a bad heating element in the sensor, a wiring problem or a problem with the PCM. You should do some electrical testing to determine the issue.

The secondary O2 sensor's main purpose is to monitor and protect the catalytic converter and reduce emissions. I think the sensor input has an influence on the air/fuel ratio, but I do not know how much impact. I always learned the primary O2 (called a/f sensors) had the largest impact on air/fuel ratios.

Do you have a scanner that can measure fuel trims? This may help you identify the problem.

 

Thanks for the reply!

I have been going by the gauge, but I can do the distance by amount division the next time I fill the tank. I had to replace the upstream sensor (used the Denso brand) because I was getting a P0139 code. My OBD2 scanner does measure fuel trims, so I can check that later today and update.

 

Just an update. I did the division test today when my fuel light came on, and it was 13.2 mpg. Here are a few things I got from my OBDII scanner. While the P0141 code was the only one flagged last week, this week I got a p2195 and p0171 as well. I did do a snapshot of the OBD stats and came up with the following:

• Fuel SYS1 CL
• Fuel SYS2 N/A

I assume it'd probably be best to change both O2 sensors, but besides that, could it potentially be something like the air intake manifold?

 

Take a look at fuel trims, because it may indicate an issue with the PCM adding too much fuel. You can determine if you have a vacuum leak causing unmetered air into the engine, or a fuel delivery issue.

 

Wouldn't a vacuum leak cause the exact opposite problem? By that I mean, wouldn't a vacuum leak cause the engine to run lean?

 

If you have enough un-metered air entering the engine, the PCM will max fuel trim, but the O2 sensor will still read lean.

 

So I checked the fuel trim, both while idling and revving to 2000 RPM. The numbers stayed consistent except for when I took my foot off the gas. The LONGFT remained constant, but the SHRTFT dropped to 0 and slowly climbed back up. IIRC, the percentages shouldn't be more than 10%, so something is definitely wrong. (I attached pictures of the scanner just in case I'm missing something)

• STFT 1: 46.9%
• LTFT 1: 19.5%

 

The PCM is programmed to have an ideal (0%) fuel trim over different loads and rpms. Think of it as a grid. Long term fuel trim is stored in that grid if the engine conditions change, like small vacuum leaks, normal engine wear, etc. The short term trim is an immediate correction based on O2 sensor readings. The total of the short and long term fuel trims is you fuel trim. You want the sum ideally be zero, but <10 is reasonable.

If you fix a problem, then the long term fuel trim will eventually be adjusted in that grid and ideally will approach zero.

I see that you have 3 cel codes. I suggest that you clear the codes (EDIT) with your scanner after you warmed up the engine just to make sure the check engine code is not blocking the PCM from using the fuel trims. The edit is because I confused your post with a 97 accord in another post.

Repeat your test at idle and give us the numbers. Raise the rpm and hold at 2000-3000 and report both numbers.

If your scanner can read the change in the primary O2 sensor voltage, look to see if your primary O2 sensor is oscillating between 200 and 800 mV. The oscillation may not look perfect on a scanner, but you would see it moving up and down. Let us know if it is pegged high or pegged low, or oscillating.

The low/high rpm can help determine if this is a vacuum leak issue or a fuel delivery/pressure issue. The O2 sensor plays a big role in the fuel trims, so you want to see if it giving a signal.

 

I just checked again, but scanner kept failing to clear the codes so I'm unable to provide those numbers. The scanner does show 1.275 volts (O2B1S2 V) that holds steady. If this helps, the O2S11 (mA) stays steady at -0.180.

Not sure if this is important, but I forgot to mention that when I do fill my tank, it always idles rough for about five minutes afterward.