I have chked the threads and have,

changed spark plugs to NGK platinum,
changed fuel filter
replaced knock sensor
added KN air filter
replaced a very bad pcv valve
use only prem gas
used sea foam in brake booster
currently have auto rx in for a cleaning
cleaned throttle body and IAC valve
tires inflated to 31psi all round


SO why only 24.5 mpg on the highway. Just what could be wrong? I dont have a heavy foot and I read of many of you getting 30 mpg. I am well below the 28 mpg rating. I know for some this is tedius to see again, so ignore it, but for me It is almost a matter of pride. I have done everything I have seen recommended to do and should be in tip top shape. still...........

just cant figure it out

I have about 93,000 miles

 

I suspect my o2 sensor is dying on me, I can smell gas.. and when I look at the o2 sensor readings with consult it seems to flucate alot.. no CheckEngine light though.. might be worth checking you o2 sensors - diags should be in stickies or Haynes manual

 

So is this all that could be happening? Just the O2 sensor that doesnt throw a code?

 

Give us some information;
What year is your car?
Auto or manual?
How many miles were on it when you bought it? When did you buy it?
How often has the oil been changed? What kind of oil?
You get the idea.

HoldTight...

 

Well, its an Auto, I just got it with about 92,000, it has 93,ooo now supposedly oil has been changed religiously although of course you never know. Its a 98 and regular oil, not synthetic. It seems to run well and as I said I think I have changed everything I know of including aligning it, and chking inflation etc to be sure I did my part.

 

I would do the diag on your o2 sensor... can't hurt.. and you seemed to have fixed a lot already.. if your o2 is bad then you its probably wasting fuel by feeding more than it needs

whats your idle like .. should be 600 when in D with Brake On.. should be 700rpm in Neutral

 

Is 31 the recommeneded PSI? Look in you're arm rest under the cover, check what the psi rating is IIRC its 32 front 29 rear for SE's. What percent are you driving highway and how much city driving are you doing on these fill ups? Also what mods do you have if any?

 

I actually have it about 1.5 over recommended. Ride will suffer, mileage will be better. No mods on it at all. Driving varies city to hwy but, I am never all city and I have been all highway. The best on Hghway though is 24.5, combo city/hwy is usually 20-22.

I wonder if auto zone or someone like that can tell me if my O2 is bad. It does not throw a code in the car though.

My idle is right at 600 after if warms up a bit in drive and about 750 or so in neutral, no a/c on. It his higher when I start it for a couple minutes.

 

I find it hard to believe that recommended psi for the front even on a gle/gxe is 29.5. regardless, how long are the trips you take as far as commute/pleasure etc. At what speed do you cruise?

 

I have it at about 1.5 over recommended and it is different front to back, dont remember off hand but 31/29 is about right. My trips really do vary, but on a highway trip about 60-65 I got the 24.5. If I am all city I will be 19.5 or so. I know many claim to get 30 on highway, and I think I have done everything I know of to do to make sure I am running perfectly, maybe its just too old

 

It sounds like your doing everything right.
When you ran your Seafoam did you get allot of smoke? Because your pcv valve was so dirty it sounds like the previous owner was negligent on maintenance.
You may want to run some Chevron Techtron through two tanks of gas then do an oil change. I found this to make a difference in my 100k civic. I went from 35 mpg to 40mpg after using Techtron. It should clean up those fuel injectors .
Let us know if you find what works.

HoldTight...

 

Maybe I forgot to mention, I did the techron deal, but just once. Is it often necesssary to do it twice? I do have Auto RX in it now in te oil. I did have some pretty good smoke with sea foam but of course who can say what is alot or excessive? , is that something that should be done twice? The Auto RX has to stay in for 1500 miles and I just put it in, then the oil gets changed. If I can figure it out I will post so everyone can see.

 

Well I only get 30 ish mpg if I go 55mph on the highway. If I go 60-65 + some spirited driving I get 26 ish.