Dissapointed with my MPG... any tips?
 

Ok, so about 3 weeks ago i bought my 98 Civic HX, and after about a week or 2 of driving, i had burned about 1/2 a tank of gas, and then filled up to check my MPG. I was saddened to learn that my MPG was only 30.49 MPG, an increase over my old 4runner truck, but no what i was hoping for. Here are a few more specifics.

1998 Civic HX 5s M/T, original engine, original HX wheels.
careful slow acceleration, shifting before 2500 RPM
Tires overinflated to 40 PSI
EOC whenever i go down any hills that are at least 1/4 to 1/2 a mile in length.
engine turned off at any excessive red lights
Replaced air filter
Replaced plugs with NGK iridium .044 gap plugs
Try to be light on the brakes
try to time all my stoplights
mostly city driving, but about 1/5 highway driving
almost no A/C (maybe 10 mins once a week or so, so 20 mins per tank if there's a few hot days)
I also live in San Diego, so my mornings are not cold on the engine.

The only things i think i still need to do are change my oil, and replace my PCV valve. I have no check engine light codes, and i don't have my scangauge yet (should be here late next week). I also need a new battery as the old one is pretty corroded.

First question is that if fully synthetic oil will make a big FE difference? What about if i had a clogged PCV? Enough to rob me of a good amount of MPG? If not, what else do you think it could be? I'd rather not have to drop $300-400 on a new o2 sensor just to find out that it has nothing to do with it, especially since i'm not getting any o2 sensor codes.

I'm about out of ideas. I believe that i should be getting at least another 5 MPG even with a good amount of city driving, and even more honestly with the change in driving habits. Anyone have any helpful input?

I'd say that's probably not too terrible if it's bump'n'grind city driving.

Yep, PCV valves can do wierd stuff, clogged, it can let too much pressure build in the crank case at certain speeds, making your pistons have to work harder to push down against it. Wide open and rotted out, it can give you a vacuum leak that has your fuelling screwed up, and it will be flooding the motor with PCV vapors at the wrong times. Check the breather filter also, if it's oiled up something nuts, your PCV valve is probably screwed, and you could do with a new filter also. Much oil out of the breather can also screw up any downstream sensors, such as MAF sensors.

synthetic motor oil may yield a small increase.

try 2k rpm shifting.

clogged PCV would hurt. a new fuel filter might help as well.

might run a good fuel system cleaner like amsoil PI, chevron, or even acetone and possibly isopropyl alcohol. heard sea foam is good also.

indeed! mine was absolutely filthy.

Thanks for the tips, i'll try all of these, and see what happens. Whatever the case, it's still better FE than the 15 MPG i was getting in my Toyota 4Runner.

my friend has a similar issue with FE. he gets around 38mpg on his 98 hx. he uses a lot more ac though. i get 40+ easily with easy accleration and driving slow. it could be your o2 sensor. that was my friends problem. we switched out the o2 sensor and place it in my car. did not go into lean burn where my original o2 sensor on his car kept it in lean burn for the duration of our "test". lean burn was verified through scan gauge. the last tank i got 48mpg i think i should be getting more. like my friends o2 sensor mine might be "lazy" and his might be pretty much old and tired. im looking into purchasing new o2 sensors (primary and secondary) and also change the ECU to the 49 state ecu. lets see where that takes me.

you mentioned that you're not throwing any o2 codes. well like i said "lazy o2 sesnor" is what you may have. it could be your o2 sensor.
also consider changing your thermostat. both my car and my friends didnt warm up to the operating temp to around 195 degrees. i dont recommend getting anything other than oem spec. people suggest getting a hotter thermostat but i've learned that in the HX the cooling fan turns on between 202 and 204 degrees. stick with the oem 180 degrees thermostat.

My worst city driving was 30 mpg all A/C no hypermiling on my 99 HX M/T. 1.5 mpg tank increase after a synthetic and PCV Valve change. Usual city A/C driving 39.6 mpg. Best hypermiled city 54 mpg. Best 70 mph hwy 50.25 mpg no A/C, 50.17 with A/C.

Put Bridgestone B381 tires on your car, they're the most fuel efficient tire!

I can't make any promises, but if you will look where I introduced myself when I joined you will find out how to make a PCV enhancer. (Cost including hose $5-$10.) I put one on two of my cars and I have consistently been getting from 5-20% better fuel economy. I would say on average I have gotten between 10 and 15% increase. Even if it doesn't help your mileage it will help keep the internal engine cleaner because it filters out the liquid blowby. I just added one to a third car yesterday, but of course I don't know what the results will be on that car yet. I didn't know it when I put them on my first 2 cars, but that is also part of the Water4Gas system. If you try it please let me know what kind of results you get.

I'm assuming you're refering to a PCV catch can? I'll have to look at your intro post.

As for the o2 sensor, i'll know more probably once i get my scangauge here in a week or 2. If nothing else it should tell me if i'm in lean burn when i think i should be. Honestly, it seems that there should just be some overwhelming thing that would be keeping me from getting at least 38 MPG even city. I never use my A/C unless my GF is in the car, which isnt all that much.

I have had oxygen sensors go bad before and it has usually cost me about 25-35% on FE. If you have a service manual it will probably give you details how to check it.