I have a 98 honda civic. I've had the car since 02' and in general it has been very good. The mpg used to be between 35-40 depending on driving conditions. The gas millage has deteriorated to around 25 mpg.

I have changed the fuel/air filter, the plugs, the plug wires, the o2 sensor before the cat, the exhaust manifold/cat (cracked@ 130k), the distributor cap. I also have the second o2 sensor bypassed (though I was under the impression that this was only used for emission testing).

The car runs very well on a full tank of fuel. The first half tank gets me about 180-200 miles (5 gallons). It seems like the less fuel I have in the tank, the more sluggish the car becomes. The second half of the tank normally is only good for another 80-90 miles, and I find myself flooring the gas pedal to get any kind of acceleration (not fast acceleration , just any).

What would explain this kind of behavior?

 

The fuel level sensor is a sensor called a rheostat potentiometer. And long story short they are not as accurate at higher voltages. (full tank) as they are at lower voltages (near empty) So the "better fuel mileage" your seeing at full tank is really just the inaccuracy of the sensor dynamics themselves. Your bypassed O2 will also contribute to the lower mileage. The PCM controls the fuel mixture and if it sees that the imputs that it's giving the engine, (rich, lean, rich, lean....) are not accurate with the O2 sensor readings, than it will go to a default rich mode so as to preserve the engine and prevent leaning out, also causing higher fuel consumption and lower MPG.

 

Is the CEL ON?

You need to calculate your true gas mileage. Did you replace the distributor rotor? What type of plugs did you use? Are they properly gapped? Is the engine idling too low or high? Have you checked for an exhaust leak around the exhaust manifold? You should measure the fuel pressure.

The bypassed secondary O2 sensor won't affect mileage as its readings don't influence the air-fuel ratio.

 

I thought OBD2 vehicles used a combonation of both sensors. The secondary is just mainly used to check that the converter is doing it's job.

 

You're both right, primary O2 does all the "work", secondary ensures that the cat is doing it's work, but won't adjust fuel mixes.

 

Yeah, the secondary O2 sensor serves exclusively an emissions monitoring function. Readings from the primary O2 sensor, on the other hand, are used by the ECU to determine the optimal amount of fuel to burn in the cylinders.

 

I didn't use any kind of name brand spark plugs when I replaced them. I did gap them according to the repair manual.

Within the last year everything from the exhaust manifold to the muffler has been replaced.

The check engine light is no longer on. Before I had the exhaust manifold/cat replaced it was on, reading 'fuel mixture too lean'. I haven't seen this again since replacing the exhaust manifold/cat.

With the exhaust manifold/cat replaced I can remove the bypass on the second o2 censor ( after the cat ), but it sounds like this will not effect the mixture.

I am aware that the fuel gauge is less accurate below half a tank, but that doesn't account for the sluggishness. Which seems to get worse as my tank gets lower. With a fuller tank the gas pedal is more responsive, by a lot.

I did not replace the distributor rotor.

I'll have to check my repair manual for how to measure the fuel pressure. I'll do this and get back to you guys.