I have a 2003 Honda Civic EX with an Automatic Transmission with 160K miles
On my last two trips (over 200-300 miles each time mostly small highways at under 65 mph) I averaged 28 mpg and 26 mpg. This is considerably less than I remember getting last fall when I was averaging 34 mpg (32-36 on trips). I did not drive much this spring or over the summer and didn't keep track of mpg (I bought the car last August). I also have had a persistent all cylinder misfire condition (Codes P0300, P0301, P0302, P303, P0304). The other two drivability issue besides the mpg is low power going up hills (foot flat down in 4th can hold steady at 58 miles per hour, my car eventually downshifted which put me in VTEC and I was able to accelerate up the hill. I generally take it easier but I was holding up traffic), and my car not liking to cruise at 20 mph when in towns. The delicate use of the throttle pedal seems to cause a sputter sound at low speed, but not always. I have replaced the whole fuel pump assembly (last August, minor improvements), fuel injectors (minor improvements to drivability), spark plugs (minor improvements to drivability), EGR Valve ( I use to have an EGR code), and Cam Shaft position sensor (this fixed an issue where the revs would drop several times shortly after starting the engine, it did not get rid of the misfire codes).

I'm contemplating the MAP sensor which would be easy and cheap to fix. Does anyone have any ideas on the fuel mileage and misfires? I expect they are connected. I have no problem doing the labor myself on any potential solutions.

I have not noticed anything wrong with coolant, overheating, oil pressure, oil volume, transmission fluid is full and looks good as well (no Idea when it was last changed), the car starts easily, and seems to idle fine.

 

Do a compression test , don't tighten the gauge adapter too tight on the pocket as it can be a P.I.T.A. to get back out , go finger tight , sounds like a worn head gasket

 

yup

 

Have you tried filling up the tank with Premium from a different gas station than you normally use?

I just changed my Cam position sensor today. It was causing intermittent stalls and bucking and misfires. I also sometimes get a random code for TPS voltage high. Maybe my car is telling my not to floor it between the lights. That sensor is next on my hit list.

 

benchwarmer thank you for the suggestion, I'm going to check the compression this coming Wednesday and edit this post when I have the results. JimmyCreedog I haven't tried using better gas, right now I have half a tank but will try that next time I fill up. Great thing about driving my civic is that filling up with premium doesn't break the budget, unlike the truck I drive for my summer job. I also had bought the MAP sensor before any one responded and will be changing that Monday. Will share when I have more information.


Edit 09/16/2020: Turns out my socket set I keep in the car doesnt have a long enough extnesion for the spark plugs, so instead of buying another Im just going to wait two weeks until Im home and have access to the full tool set.

 

If you're worried about bad gas you can use an octane booster , the only reason I use 91 octane is because it doesn't have ethanol otherwise 87 is O.K.