igw425

I have a 2007 Civic EX with 80k miles that is only getting about 18-19 mpg. I drive it in a very conservative manner. I have changed the spark plugs (replaced with OEM plugs), air filter, and the upstream o2 sensor. No codes are being thrown.

At Idle:
ST Fuel Trim: -5.5
LT Fuel Trim: -13.3
Mass Air Flow: ~2 g/s

I have held it at 1500 and 2500 rpms as well, I don't see any improvement in the fuel trim. It drives fine and feels normal, but it uses a lot more fuel than it used to. I used to get around 350-400 miles to a tank, now it seems like my gas light is on by the time I get to 200.

Any advice would be much appreciated.

Wankenstein

What did the old spark plugs look like when you replaced them? Notice any caked-on, dark soot buildup?
Dark smoke with fuel smell coming from the tailpipe at startup, hard acceleration, hard deceleration or otherwise?

Have you checked for spark quality?
Checked compression?

Do a power balance (rpm's drop) test. With engine running slowly lift each coil one at a time. When coil first "pops" off plug hold it in place and listen and feel for a strong (quality) spark. Afterwards keep lifting slowly and listen for the rpm's to drop..idle drop is normal. If, rpm's don't drop in that particular cylinder then there's an issue. Also, rpm's should drop similarly/comparatively in all four cylinders.

igw425

I haven't done a compression test. The old plugs were in pretty good condition for being 10 years old, not much build up. I put the new plug in about two months ago, I pulled the and checked to make sure that they weren't getting fouled up, and all four still look great. There hasn't been any smoke coming from the tail pipe, and i haven't noticed any strong odors of fuel or anything else for that matter.

I will have to do the power balance test this week, to see if I find any issues there.

When I am driving down the road, the car seems like it is running 100% normal, but it is just using fuel at a much higher rate.

Wankenstein

Keep tires properly inflated?
Rotate and balance tires every 4k miles or so?

Clogged catalytic converter can cause poor fuel economy but, at 80k miles doesn't seem that would be an issue. Though, can't be ruled out either.

igw425

Tires are in good shape, get them rotated and balanced on a fairly regular schedule ~4k miles +/-. I check them fairly often and keep them at 32 psi.

Wankenstein

Leaking fuel injectors (into the cylinders) can cause poor gas mileage and a rich condition.

igw425

If I had leaking fuel injectors wouldn't I have hard starts?

Wankenstein

Here's a list of symptoms: https://www.injectorrx.com/fuel-injector-symptoms/