it's a 1994 Civic Si with 182,000 miles. Bone Stock. Literally.

Ok, I know this probably sounds crazy, but im pretty confident that I burn gas while the car is parked, and the engine is off..... ? over the past year or so, I have been getting worse and worse gas milaleage, and I notice that the biggest loss is noticed within the first few miles of the day and at the end of the work day (after the car has sat for 8+ hours). Like it seems lower than when I parked it before, I just thought I was crazy.

I just figured the engine was getting worn out...however, this weekend I put near 400 miles on it in a 24hr period driving to the 100AW Rally, all over the stages, and back home. All with 4people in the car and driving fairly agressively at high rpms...i averaged 36+mpg on this trip. This is the best I've seen in a long time, especially considering the variables...someone please give me some insight. Also, when I put the car on a lift, it smells a bit like fuel around the back of the car outside. No smell inside. I think it must be leaking, but I dont know these cars very well, so I dont know where to begin checking.

I got like 420 miles to my last tank that I burned in about 24 hours, and this tank thats taken me all week to get through, I'll be lucky to get 320 miles out of....IDK WTF!!!!

Someone please offer some experience insight.

 

u have a fuel leak.

 

look at ur hardlines by the tank. or the tank its self. does the car have a lot of rust?

 

you have a leaky tank or lines.

 

Not alot of rust, normal fender stuff like every honda from the mid-90's. It's apparent that a leak is probable, but I have no idea where to begin looking for stuff like that.

To be honest, I dont know my way around these cars that well, it's just a daily for me, what lines should I check?

 

Also my running temp never reaches the half-way point either, it hangs out below the half way point all the time. takes like 10min of highway driving to even begin to warm up....

 

I looked today, and noticed the gasket sealing the filler neck is leaking pretty bad, so it might be evaporating out of there! Im going to replace that, the fuel cap, and the thermostat, and see what I get!

 

fuel lines underneath may be rusted as well

 

Nothing that I can see, I pulled all of the covers off, and looked at everything. Nothing I saw down there looked to be rusted at all, or leaking. It just looks like its leaking from the filler neck gasket, so hopefully thats it!

 

Is suggest lighting a match under the car. Lol. I had a very simular problem but mine turned out to be a nice hole in the tank from a rock bouncing up and down my under chassis

 

Ok, what else could cause this gas mileage drop?

Where else could fuel be escaping? none of hte lines under the car are rusted at all, or leaking....

 

What kind of loss are we even talking about? I'm finding it extremely hard to believe that yoou could be leaking that kind of fuel as reported in your first post and you NOT find a large leak. Roll a big piece of carboard under the car while it's parked and see if anything deposits itself on it.

 

is it possible that your fuel filler neck is leaking, and the gas is simply evaporating?

 

Well I just replaced the filler neck gasket and gas cap with OEM Honda parts today. I'll see if that gets be back to square. But I would really like to see 40+mpg again

 

You won't be able to do that without an extensive tune up.

 

it probably is evaporating since you can't see a leak. Like the spot above said if you are losing that much gas then the leak should be very easy to find.

 

Within the past 6 monthes I have replaced:
*NGK Spark Plugs
*OEM Wires
*Cap&Rotor
*o2 sensor
*fuel filter
*oil and filter
*tires
*Brakes
*K&N air filter in OEM box
*Full timing belt/water pump service

The tune up was done, but that was the problem, is that I never saw a significant MPG improvement. So I just thought the engine was getting old, and thats all it is.

But then on that road trip I got 40+mpg a few weekends ago, with heavy load and aggressive driving....

 

There's a big difference in highway and city miles. According to this, sounds a little over average. http://www.fueleconomy.gov/feg/noframes/10746.shtml
That's a combined average of 28mpg. You're reporting 33.5mpg. I think you've manufactured a condition in your head that doesn't exist in reality.

 

Have you checked your float in the tank? I had an old ford which the fuel gauge wouldn't work. Turns out I had a float that leaked and let fuel fill it up! I would let the tank get low on fuel and the float would actually drain and would work for the day.

This ethanol crap also causes the contacts to gum up and give an inaccurate fuel reading.