93 Civic, mpg not so good.

[liquidzorch]

I have a 93 civic lx, d15b7 engine and s20 manual transmission. The engine has been rebuilt, since it was burning oil. But I still only get around 28-30mpg. It might not sound that bad, but I see people with the same engine getting higher mpg and not using the same driving methods I do.

I never press down on the gas pedal more than 1/8th way down, I mostly only tap it down very little to move. No hard acceleration. I shift at about 2k-2.5k depending on the load. And never go above 65mph. Most of my driving is done on highway like conditions, since in the city we have over 40 overpasses, I hardly ever have to stop.

The ECT, TPS, AIT, sensors are new, which are most effective with gas. The injectors are cleaned and tested. When I am at a stop and it idles for more than 30 seconds, I can hear/feel it entering a lean burn. But I imagine while accelerating it injects more gas. I hooked up a voltmeter to the output of the oxygen sensor, and I can see about .1 volt when in idle for more than 30 seconds (lean burn mode), but when tapping on the accelerator, the voltage goes up to .8-.9 volts. I do not know if that is normal in this car.

My compression does not seem to be that bad, it is about 160psi all across, It should be at 180psi when new, so I say its not that bad.

I have no vacuum leaks. And cat is not clogged.

Any help would be appreciated. Thanks

[cbaber]

The D15B7 doesn't have lean burn, that was reserved for the D15Z1 in the 92-95 generation. Besides, lean burn engages while driving, it doesn't do it while idling (at least in my HX).

1. Have you tried replacing the O2 sensor?
2. What PSI are you running in your tires?
3. Do you have extra stuff in your car?
4. Have you done a full tune up recently? This includes plugs, wires, cap/rotor, fuel filter, full fluid change, etc.
5. Have you checked your ignition timing?

If you rebuilt the motor and still only get 160 PSI compression, the motor wasn't rebuilt well.

If you are idling for 30 seconds, that is an issue. Turn off the car when you know you are going to idle for more than 10 seconds. Start a fuel log. It will help us see your actual results, instead of just listing numbers. We want to see tank by tank results.

[liquidzorch]

El

Originally Posted by cbaber

The D15B7 doesn't have lean burn, that was reserved for the D15Z1 in the 92-95 generation. Besides, lean burn engages while driving, it doesn't do it while idling (at least in my HX).

1. Have you tried replacing the O2 sensor?
2. What PSI are you running in your tires?
3. Do you have extra stuff in your car?
4. Have you done a full tune up recently? This includes plugs, wires, cap/rotor, fuel filter, full fluid change, etc.
5. Have you checked your ignition timing?

If you rebuilt the motor and still only get 160 PSI compression, the motor wasn't rebuilt well.

If you are idling for 30 seconds, that is an issue. Turn off the car when you know you are going to idle for more than 10 seconds. Start a fuel log. It will help us see your actual results, instead of just listing numbers. We want to see tank by tank results.

Ok. O2 sensor has been replaced in last year. Tires are at 32psi. Which is factory spec. No extra stuff. Aluminum rims 13 inch. Tune up done with cap rotor and all filters. The low compression is because i only honed the cylinders since they were still in spec. Oil is 10w30 in both engine and tranny. Fuel consumption has been the same for over a year. With 17 liters of fuel i get 120-140 miles. Depends on how much highway. And not to he rude but i am not going to turn off the car at every traffic light. I shouldnt have to to get the same results of people who dont turn it off. If you think it is due to compressiob is there a way i could verify before rebuilding with overbore?

[cbaber]

Originally Posted by liquidzorch

El

Ok. O2 sensor has been replaced in last year. Tires are at 32psi. Which is factory spec. No extra stuff. Aluminum rims 13 inch. Tune up done with cap rotor and all filters. The low compression is because i only honed the cylinders since they were still in spec. Oil is 10w30 in both engine and tranny. Fuel consumption has been the same for over a year. With 17 liters of fuel i get 120-140 miles. Depends on how much highway. And not to he rude but i am not going to turn off the car at every traffic light. I shouldnt have to to get the same results of people who dont turn it off. If you think it is due to compressiob is there a way i could verify before rebuilding with overbore?

If the wear in the cylinder walls was not enough to warrant a bore, the hone should have sealed the cylinder up like new, assuming the rings are new and that the headgasket + valves are sealing properly. So you could have valve issues, causing lower compression. You could verify that with a leakdown test if you really wanted to.

You said you get 30 MPG, which is actually normal for this car. 29 combined is what the EPA estimates. Fuel Economy of the 1993 Honda Civic

So everything appears to be normal if the car is stock and you drive it like a normal person. The only way to push it beyond EPA ratings is to follow the techniques that are on this site, and you already stated you are not open to one of the easiest and most common.

[Gasoline Fumes]

Check your timing belt. Being off a tooth will affect compression.

[California98Civic]

You can check compression and the timing belt, and it might help you pick up a few more MPGs. But I think cbaber is right. The car seems to be functioning close to normal, except for low compression, which could be significant, I realize. Still, you'll need to become willing to mod it and your driving technique to realize really significant gains. Pumped-up tires, grill blocking, airdams, and especially engine off techniques and acceleration in optimum load ranges are the biggest, cheapest, simplest winners. The amazing thing about the acceleration is how without instrumentation, it is really about impossible to hit the right level of engine load. For your OBD1 car, you'll need a vacuum gauge at least, and preferably an MPGuino and tachometer (in that order of importance).

Good luck. Start a fuel log here. Monitoring the numbers over time is a great way to adapt and adjust technique and mods.

[liquidzorch]

Ok. Thank you all for suggestions. I have checked timing, it is in perfect place. And yes, piston rings and everything was new. I know 30mpg is in specs, but that is what I get when short shifting, never accelerating hard, and not going very fast, moving in neutral whenever possible (down hill). And my max has been 30mpg with a tank. I will check on how to start a fuel log like you all say. And I am looking to buy an arduino to build a mpguino. It just is frustrating that a friend of mine that drives like crazy, hard accelerating, and in a less fuel efficient car gets better mileage than me. With my driving tequniques I should be getting at least a little over 30mpg (which is the stock mileage for this car). Like this week I got 28mpg. Without going downtown (where there are stop signs and lights).

The only thing I would not like to do is turn off car at red light. Its only 30 seconds of idling and like I had mentioned, it goes into some kind of mode where almost no gas is being burnt in idle after a couple seconds. It might not be what you call lean burn mode, but it is definitely lean. (veryfied by getting only .1v off of oxygen sensor when in idle, and it does work since once I move the accelerator it goes up). Plus the starter will end up costing more to replace, than the fuel saved.

[Cd]

Originally Posted by cbaber

You said you get 30 MPG, which is actually normal for this car. 29 combined is what the EPA estimates. Fuel Economy of the 1993 Honda Civic

cbaber, did you notice that your estimate was for an automatic ?

The manual version gets 33 / 36 / 41 : Fuel Economy of the 1993 Honda Civic

[California98Civic]

I wish I had double-checked cbaber's citation of the EPA estimate, as Cd did above, but cbaber knows what he's doing and I felt lazy. Since it should be performing better than 29 and by quite a lot, given how you drive, I think the questions about mechanical repair are obviously more important than I realized. It is still good advice, however, to work on technique.

[liquidzorch]

Well thank you all for posting. I did not know I was off by that much. And it sucks since I never put the pedal to the metal. I think I might of found a store in my town that has an arduino. If they do I will buy it to start making a mpguino and be able to monitor a bit better.

I do believe compression might be a factor but I dont know of any place that can do a leak down test. And I live in a small city in mexico where they dont really use that kind of tests lol. But when it was burning tons of oil and the compression was even lower it still got the same mileage. And I used to drive the same way.

I wish there was an easier way to monitor gas, and see what the computer is thinking.