05 Accord V6 running lean, awful fuel economy.

2005 Honda Accord V6
162,782 miles

Friend noticed he’s been getting awful fuel economy. About 14MPG which is about 180-200 miles per tank.

I ran a test on his fuel trim with my BlueDriver OBD2 scanner and noticed the engine is running lean about 10-17% per each bank. No check engine light or trouble codes.

I’m very new at this and not sure where to begin.

I recorded some live data of the fuel trim (short and long) for each bank, engine RPM, coolant temp, and vehicle speed which I can share in here. It doesn’t seem to get better or worse while raising the RPM from idle.

Engine has a MAP sensor. BlueDriver is unable to read Upstream Air/Sensor voltage, instead it’s displayed as a current. Only voltage is for the rear sensors. I was able to record that live data as well while driving around.

I was hoping the ScannerDanner community could point me in the right direction to start testing this engine for possible issues.

Here is the fuel trim recording.

30sec @ idle
30sec @1500rpm
30sec @2500rpm
30sec @3000rpm
30sec @idle again

What does your map sensor read when key on engine off?
when key on engine running at idle?
1500?
2000?
2500?
3000?

Also what where the voltage reading of the o2 sensors (all of them) during that video?

I’m gonna get those values recorded tonight. I couldn’t get my scanner to display live data with just the key on, so I couldn’t get the MAP reading with the engine off, but I can get the rest.

I should add some context:

The car was rear ended about five years ago, bought back from insurance, and rebuilt under a salvage title.

The owner says his mechanic found something unplugged at the charcoal canister that was giving him poor fuel economy. This was addressed a few years back.

He also mentioned his air intake boot was torn and was replaced which also resolved a fuel economy issue.

Timing belt has never been replaced. Owner doesn’t care and doesn’t want to do it.

Alright well it took longer than expected to record the MAP and O2 values but here they are.

I also found a potential culprit for some of the lean condition. The air box lid is not fully secured as two out of the four screws have been stripped out. As a result, we ordered a used replacement from eBay to fix that. Not sure if extra air before the MAP sensor would cause a lean condition but it’s a possibility.

You need to check the fuel trim under load, ideally at cruising speed. How is the power and acceleration? Also, on a speed density system like that, any air leaks before the throttle body should not have as great an effect on the fuel trim as a mass air system.

So we ended up replacing the airbox and the long term fuel trim improved significantly. I did the same test while parked with the engine at operating temperature and the long term fuel trim is reading close to zero from idle up to 3000rpm. Short term trim is between 3-8% per bank.

We also had to replace the rear brake calipers, rotors, and pads because the driver rear caliper leaked out all brake fluid.

So between those two repairs, my friend is reporting a significant increase in fuel economy. Averaging about 23MPG and will hit at least 320miles on the current tank gas that was refilled right after the repairs.

So he’s happy and I’m happy we found the culprit.

Just to make sure I understand you correctly, you did nothing else but replace the air box and the rear brake parts, and the fuel economy improved from 14 to 23 MPG? It’s possible that the rear calipers could have been frozen and caused the brakes to drag, which would decrease fuel economy, but not by that much. And that still wouldn’t explain the big drop in fuel trims. This defies all my years of knowledge and experience. Can someone more knowledgeable than me enlighten me as to what’s going on here?

That’s correct, we only replaced those two items. My friend also thinks the rear calipers were sticking because the car has less rolling resistance with the new brakes.

To be fair, he has driven a lot more on the highway than usual so we still need to test it more in town with the next gas tank.

He also noticed his trip computer improved significantly after about 40 miles of driving. Initially it estimated about 178 miles of range on a full tank but then it improved to 240, 280, and more. Last time he checked, he drove 257 miles and still had a quarter tank of gas left with the computer indicating 75mi left to go.

When I used my scanner, I could see the long term fuel trim went down to 0.8% on bank 1 and 0% on Bank 2. Before the airbox repairs you can see it was at 5.5% on Bank 1 and 3.9% on Bank 2 in my YouTube video.

The air box repair is puzzling, but it is what it is. Wideband sensors only show current change in value unlike narrow band sensors so you will see a graph straight line instead of the oscillating line and current readings instead of voltage. See if your scan tool shows an equivalency rate. This will be the stoichiometric rate of 14.7 to1 in voltage. Honda I believe is 2.8v. What I see is your 02 readings on the post cat sensors are concerning. The readings should not be oscillating. Those 02's are narrow band but again, they should not be oscillating. You may have cat problems. Fuel mileage on that car is rated 29 to 37. The car was hit from the back. Check for damage to the cats and exhaust leaks before the #2 sensors. As the problem worsens the car will have loss of power and P0420 codes.