semvhu

I have a stock 2005 Honda Civic LX 4-door with 120k+ miles. I used to get 37+ mpg in this car. Over the last year or so the mileage has slowly dropped. It only gets about 33 mpg these days, occasionally less than 31 mpg on a tank.

In trying to diagnose the problem, I borrowed a friend's scanner. The only thing that stands out is that my LTFT's are -10 to -12. If I understand this correctly, the computer thinks it is running rich and is leaning it out a bit more than average.

I'm a little unsure where to go next. Some potential candidates I've found regarding both the mileage and LTFT include the O2 and MAP sensors, though I don't want to just go throwing parts at the problem.

I've checked the fuel pump and the filter sock was clean (though I have not checked actual fuel pressure). I keep regular maintenance on the car according to the schedule, though I have done nothing to the car beyond what the maintenance schedule calls for.

Suggestions and thoughts are very welcome. TIA!

dailydriver

Have you been using your a/c a lot? I think that might be my problem, as I'm having similar problems with a totally tuned-up '04 Civic coupe.

My air has been on full blast every second I drive lately because it's been 100+ out. I'm trying a tank of non-ethanol fuel this tank too.

civicbyrc

O2 sensors can give bad signals over time, I'm not sure if b1s1 is a wideband on the civic of your year, see what the sensor is doing with the scan tool. If it is switching slowly it could be affecting your fuel economy.

sdaidoji

iTrader: (1)

+1 on the O2 sensor.
03+ are wideband, so expect a higher price around 250

nick060200

02
spark plugs
air filter
wheel alignment
when was the last time you changed any of those or got an alignment?

Nosepicker

This is a hard question to answer because there could be several factors contributing to the problem.

When I got my car I was getting 32-33 MPG.

I upgraded the tires first. MPG increased by a mile or two.

Then I switched to fully syn oil. MPG increased by another mile or two.

Then I changed my driving habits, inflated tires from 33 to 35 psi, switched spark plugs & wires, and completed valve clearance and MPG increased by 2 or 3 more.

See where I'm going with this?

There are several factors that can affect increase or decrease of MPG.

semvhu

Thanks for all the responses. The things I have not changed or done mentioned in the replies since my mileage started dropping are O2 sensors, plug wires (changed the plugs but not the wires ), and valve clearance. I have new O2 sensors on the way. I'll probably change the wires this weekend. I'll take care of the valve clearance as soon as I can after those things. We'll see what the mileage is from there.

Nosepicker

Don't forget to clean/replace air filter.

gudoffer

hi...my cars the same...

btw. 01 to 05 civics have only plugs...no wires...

anyhow...my civic gas sucks...i get 400 km per tank...thats when the light turns on...

once i got the engine light for o2 sensor and some lean back cylinder or something...but it went away itself...and never came back...cars at abour 190 000km ...any help...

its time for me to tune it up again...

how do i fix this

2001 civic ex