My 2003 V6 used to deliver good and consistent mileage, 28-31 mpg mostly highway. Approx 1 year ago (at about 70K miles), there was an apparent step change down in mileage to 25-27 for the same driving pattern. In the past 1-2 months it has dropped to the 23-25 range. Any city driving seriously kills any gain from highway cruising. Now at ~90K.
Correlations and Fixes:
- no check engine codes ever
- odd correlation is that mileage dropped after an oil change by Walmart "pros", in which they used 5w30 and the wrong filter, and I instructed them to put in proper oil and filter.
- Honda "pros" at the dealership response to question about mileage: "duh, the only thing that can affect your mileage is your tire pressure and air filter, so we can check those for you". I asked them to put it on a scanner and check for pending codes or individual sensor checks, but the service manager said, no, they can't do that if there is no check engine light. I immediately left.
- I suspected dragging brakes. Still not sure on this one. Pads have been changed, rotors resurfaced, fluid changes.
- I looked into cleaning the fuel injectors, but could not determine the connection for direct connect of injector cleaner as on my other Hondas on the fuel rail. Have tried several bottles of injector cleaner in the gas tank.
- I had 2 separate cases in which the mileage re-appeared to "normal" level for one tank of gas each occassion. This made me suspect a sensor issue.
- I have not purchased a scanner yet, to monitor all of the sensors in real-time. Any experience with real-time monitoring and troubleshooting mileage issues?

 

HAS THE CITY YOU LIVE IN CHANGED THE TYPE OF GAS THEY ARE SELLING?
SOME OF THE NEW GAS HAS CAUSED A DECREASE IN FUEL MILEAGE.
I WOULD CHECK IN TO THIS BUT ON JUNE 1ST A LARGE NUMBER OF CITY'S CHANGED I KNOW HOUSTON WAS
ONE OF THEM IN MY AREA. YOU SHOULD HAVE HEARD SOMETHING.
I DO NOT REMEMBER WHAT IT IS CALLED I MAY REPOST TOMORROW WITH MORE INFO. FOR YOU.

GOOD LUCK

 

This is common problem with Honda's, they are very sensitive to oil changes and fuel quality. Obviously, if they damaged the engine via an Walmart style oil change, you would hear it and feel it through a clicking, tapping, or groan. If not, lets focus on Fuel, I was told a while back that Lucas fuel injector (Autoparts Stores) cleaner not only cleaned the injectors but improved gas mileage. I was a skeptic until I tried it. My 99 accord 4 cylinder now gets a consistant 32-34 MPG freeway with automatic tranny. I was getting 25-27 before. Lucas is the best and one bottle will last a year because you only need 4oz. at a time. To those who don't believe, try it and tell me I'm lying.

GOOD LUCK

 

i use lucas exclusively. very oil change

 

02 sensors may not be as efficient as they were when new. Test all of them for proper operation.

 

LoL my reply was a bit late and i'm sure it does NOT matter anymore to you, but others may stumble upon this post just as I have. Your problem was due to the introduction on CORN i.e ethanol into your fuel supply. Thank this 'great' nation for benefiting the'few' over the 'many'

 

Yeah, well nasty words won't help anything.

But I agree that ethanol isn't really a good thing. Improves oxygenation a bit, for better emissions during the first few seconds after startup. But after that it reduces your MPG. And I hear talk about "sometime" in the future requiring more than 10% which is going into the range where you have to worry about corrosion and deterioration of hoses & seals.

 

An EGR issue could give you bad mileage but should throw a code. I don't think a Walmart oil change is related.