scooty

I've seen a few poor MPG posts on here and most remain unresolved except for one guy who replaced his cat and got his MPG back.

I have an 03 civic LX 5spd with a 115K miles.

My car used to get 30 MPG city consistently (Summer time gas) until like 2 years ago when I slowly saw it degrading to 29, 28, then 27. During this period I also developed a crack in the intake manifold accompanied by an exhaust tick. So attributed the poor mpg to leaking exhaust and didn't think much of it.

Fast forward to 2 months ago, I had the exhaust manifold welded, the ticking sound went away but I never got my MPG back. At this time I was getting around 27.5

A few weeks ago I cleaned the throttle body and IAC valve as per the forgotten tune ups mods and did the idle re-learn procedure.....The MPG immediately dropped to 24 from 27.5.

Three tank fulls of gas later to give the ECU time to re-adjust and there's no improvement. I filled up today and got only 23.

I've not had a check engine light ever. Here is what I have done so far:

1) Spark plugs were changed with the OEM NGK Platinums about 6 months ago. No change in MPG.

2) Valves were adjusted a year ago. No change in MPG but a little quieter.

3) Tires have always been inflated to 32. I keep my car very clean and never carry excess junk in it. No external roof racks, carriers, spoilers etc added.

4) CAI intake filter gets cleaned and lubricated several times a year. Never has any noticeable effect on MPG.

5) I removed the cat, o2 sensors and shined a flash light through the holes. The mesh looks intact and in good condition. Plus I've not noticed any decrease in performance so I doubt the cat is clogged.

6) The car runs fine and never over heats. Oil consumption is about have a quart every 2000 miles which I know is due to a bad Oil pump o-ring gasket.

7) I bench tested the primary o2 sensor and the voltage oscillated within specs. New one is on order just in case.

Any ideas guys? does any one know why the MPG has been gradually eroding over the past 2 years? And why did cleaning the throttle body and IAC valve cause an immediate 3 MPG drop??

lowlife9

you never changed the primary oxygen sensor ??? that would have been the first thing i would have done there only good for around 60,000 miles.after that they just give you progressively worse gas mileage and they won't give you a cell light unless it completely malfunctions.

scooty

Primary O2 senor should be in on Wednesday.

I replaced the PCV valve today and immediately noticed the engine feels much peppier. The butt-dyno reveals I'm getting more HP to the wheels as I can squawk the wheels in 2nd a lot easier now. Hopefully this translates to better MPG as well.

Didn't know a PCV valve can make such a big difference. The old one looked dirty as heck and did not quite rattle the same way as the new one.

lowlife9

a faulty pcv valve will kill oxygen sensors even faster but you could have just cleaned the stock one with carb cleaner because its metal. what brand of oxygen sensor did you order ???

gearbox

iTrader: (95)

not to mention kill the engine. pcv is a really important system

scooty

Yeah, I'm wondering if excess crank case pressure has something to do with the oil pump o-ring leak.

I ordered an OEM Honda O2 sensor from HondaPartsNow. It was $53 plus shipping. None of the O2 sensors at the auto parts store around here looked right.

gearbox

iTrader: (95)

more than likely yes. along with corrosive acids eating away at your engine internals. about the o2 sensor, i used to be able to get the bosch sensors at the store (with ntk inside) but recently they seem to have the wrong sensor part in there. so i would recommend just getting one from honda. you will know if its the wrong one when the engine light comes on again. the primary is the most important for a/f ratio. secondary is just for telling the ecu your cat is working.

scooty

I replaced the primary O2 sensor with a genuine OEM Honda part today and filled up the tank along with "Tune up in Bottle" fuel injector cleaner.

Hopefully I will get closer to 30 MPG on this tank now. I really can't think of anything else that could be affecting the MPG at this point.


I have a narrow band air fuel ratio gauge, does anyone one know if the OBDII port has an O2 sensor signal output?

It would be nice to know if the PCM is maintaining the correct Air fuel ratio...

gearbox

iTrader: (95)

that a/f gauge is completely useless. nevermind that it will screw up the ecu readings if you hook it up and make the car run even worse. even if you were to get another o2 sensor welded in the exhaust, you would need a wideband sensor AND wideband gauge to actually see what was going on. and on a stock car it will be reading rich all the time anyway. if you have an overly rich or lean condition, engine light will come on for sure.

Kennykid2002

iTrader: (1)

I've been consistantly getting worse mileage as you're stating as well.
Started with average of 30mpg, and dropped to occasional ~25 mpg.

Changed my driving style, and I got back up to 31mpg for 2 tanks.

Bought a bottle of seafoam to use when next oil change comes up

lowlife9

i think its an oil company conspiracy.

gearbox

iTrader: (95)

well, everything a company does is a conspiracy then. they are in it to make profits. just like battery companies purposefully design them to fail early. yet over 20 yrs ago technology existed to almost double the usable life of lead acid batteries just by changing a small component of the electrolyte. but it would have lost the battery companies millions (if not billions) in profits. the real conspiracy is why my 2002 civic with old technology gets 42mpg and the brand new 2012 civics with all sorts of eco buttons and hi tech aero/mpg mods can barely get that same number. or why civics from the 1980s got 45-55mpg like it was nothing special. or why there are no diesel civics in the usa that get 45-65mpg like in europe. i guess having double the torque and double the gas mileage doesnt appeal to us wasteful amercians lol.

lowlife9

its funny that there is a vw diesel that gets almost 80mpg but its not allowed in the states because of emissions yet the engine is built here.
http://www.naturalnews.com/036183_fu...overnment.html

scooty

Completely agree with you about the diesel civic, such a shame we can't get that here. I'm pretty sure it's the oil company's lobbying against it.

Newer civics have to meet more stringent crash ratings and have more structural reinforcements and thus weigh more then civics from 15-20 years ago. MPG suffers as a result.

If you notice an early 90s civic had a much lower belt line and was overall smaller and lower to the ground. Two things happened:

1) Americans got dumber and started texting in the middle of the street. Cars are now required to have a certain volume of clearance between the top of the engine and the hood, so in case a pedestrian's head smacks the hood it will cave the hood in as opposed to hitting the engine block underneath. As a result cars have higher hood lines, taller belt lines and more metal in the rear fender to keep some semblance of proportions that don't look retarded. Overall cars are now bulkier.

2) Americans got fatter. Cars got bigger to make them more comfortable. I think a 2012 4 door civic now has more interior room than a 92 Accord.

gearbox

iTrader: (95)

about the safety thing, my 02 got the same 5 star crash rating as the new civic and nothing has really changed aside from side curtain airbags (which were an option for my year as well). airbags weigh about 10-15 lbs each. cars really were not that much lighter 10 to 15 yrs ago and the engines are also similar. its only gone up 0.1L every generation which is like nothing. there has to be more going on here. haha +1 to getting fatter. altho the 92 accord seats feel like im sitting on a soft couch compared to my civic. some of this has to be done on purpose, because there are so many things ive come across that got me wondering, what were they thinking? anyone remember when a car's only purpose was to transport you from one place to another? and you had to get off your lazy butt with a towel to clean the windshield because wipers didnt exist?

sdaidoji

iTrader: (1)

i just got a 95 car, no AC, no PS, no power anything - only computer is for the engine and the airbags.
waaaaaaaaayyyy better and light! unfortunately it's not so aerodynamic as the vics, so not so good in mileage...

but back to topic, yeah, the primary O2 was what i did when the mileage started to drop. it came back.

then, not aiming at the mileage, but had a stage 1 crower cam dropped in during my head gasket/timing belt, it raised mileage by 2 MPG - so the theory that engine actually runs richer than it actually should seems to be correct. Stage 1 runs more air but using same ECU, that self-corrects. so the mixture is more lean, more power and better mileage. What actually honda shoulda have done, but did not, and crower rocks for having done that

gearbox

iTrader: (95)

our cars do run too rich in stock form for no reason. you should see the a/f of the 06+ SI civics. you would think the car is turboed.

scooty

MPG problem FIXED!! and some interesting findings on different gas.

City driving : 30 MPG

Highway: 45MPG with AC!!! I averaged 70 mph and was very very light on the gas and skipped gears as well, first to third, and third to fifth etc.

I made a permanent move from South NJ to North NJ about a year ago for my job and that's when I noticed the reduced MPG.

I filled up at Shell in North NJ and set off on a weekend trip to South NJ. When I got there I filled up at Lukoil and calculated 32 highway MPG (NO AC), which was a little disappointing. Here's where it gets interesting.

For the return trip on the exact same route I got 45 MPG using South NJ. Lukoil gas!!! And this was with AC and in much heavier traffic and about 20 miles of crawling along at 20 MPH on the Turnpike.

While I was driving in South NJ I got about 30 MPG in the city, which is what I always got when I lived there regardless of whether i used Lukoil, Shell, Sunoco or BP. (Maybe about 1-2 MPG less with Shell).

Looks like there is something up with the gas in North Jersey, I'm going to see if I can find a Lukoil near by and see if that makes a difference. I just filled up at Sunoco and I'm pretty sure the MPG is going to drop to 24 again.

I don't think this had much to do with the O2 sensor or the bad pcv valve because I still got 24-25 mpg after replacing those. (verified over 2 tanks of gas, one at Shell and the other at Hess).

Now I'm really curious to know what the heck they're putting in North NJ gas??

lowlife9

told you its an oil company conspiracy.

sdaidoji

iTrader: (1)

thought that only happened in my home country and other 3rd world places... where they put some solvents to make more for the gallon...
seems that my theory of people is people wherever you go is still valid...
good catch!

04 Honda Civic

iTrader: (2)

Just a side note, its not good to skip gears on a manual.

Read it on the S2k forums, syncros designed in such a way that they should be shifted to next gear even if you dont let the clutch out.

scooty

It's ok if you rev-match, which I always do. Plus the way I see it there is less overall wear in the tranny by using less synchros in a gear shift there is zero wear on the synchros that got skipped. Also less wear on the throw out bearing since the clutch is used only once to change gears.

The six speed trans in my Firebird has a solenoid that forces the shifter from 1st to 4rth when the load is light. Apparently GM is not too concerned about synchro wear, but that's GM for you. Of course I unplugged the solenoid because it's straight up annoying.

scooty

Crash test safety ratings got more stringent for 2011 and newer cars. A 5 star rating in 2002 is not quite the same as a 5 star in 2013.

gearbox

iTrader: (95)

hmm good to know. and i know what you mean about different gas. i once filled up at a small philips66 in a tiny freeway town in IL. one of the few gas stations at the time with NO ethanol at all. the power of this little civic was unreal, and the tank seemed to go on forever. its amazing how well these cars run on pure gas, compared to 10% ethanol. but those days are long over, even tho cars of today are vastly more efficient than even 10 yrs ago and oxygen/additives are not required at all to reduce emissions. most cars on the road are already ultra low emissions even in winter. ethanol is one of the biggest scams ever.

johndeerebones

Got that right...