Hello. I have a 93 civic with a d15b7 engine that is getting low mpg. The engine has been rebuilt and seem to run perfectly. It has good compression, although it was done wrong (with throttle closed) it still got 130psi in each cylinder. Not getting any cel error codes.

I am getting about 25mpg, when it should be around 38mpg. Most of the streets I go on have no stop signs or traffic lights, almost always highway. I do not rev the engine much, Usualy shift at 2500rpms.

There is no vaccum leaks. Oil change every 3000 miles aprox. And engine has been rebuilt about 6 months ago. The problem of bad mpg has been also before the engine rebuild.

Could a "bad" oxygen sensor cause this much of a drop? Or could clogged injectors cause this?

 

did you ever replace your air filter / fuel filter and are you running correct cc fuel injectors for that engine

 

Yes, new filters and the injectors are the original injectors for this engine.

 

I have sent the injectors for cleaning and got them put back on. It seems to be the same. I have wired a volt meter to the oxygen sensor to check the voltage (the sensor is working correctly), at Idle I get around .8 volts for awhile then get around .1 volts for a long time. When I accelerate it goes back up to .8 for awhile. What I seem to notice is that there is almost never any time when it is close to .5v.

(.8v means rich, and .1v means lean).

How can I find out what is failing? what is making the computer inject more gas?

 

What trans are you running? Any breaks dragging? Why do you think you should be getting 38 mpg. What's the air temp? In winter I loose like 10% mpg.

 

Im not sure of the trans model but its the original in 92-95 civic lx. No dragging at all, I can go for miles in neutral, the 38mpg is what it says on the government mpg page, plus about 2 years ago I was getting great mileage. The air temp is normally around 25-35c (Mexico).

I have in mind that gas prices have been going up but I am calculating with liters/gallons and it is not like it was befor, I am getting around 25mpg and I never accelerate quickly of go over 60mph, I press on the gas pedal about 1 inch down at the most.

 

How many miles on the motor?

 

It has just been completely rebuilt. New piston rings, bearings. It had about 200k on it

 

Have you checked the distributor cap and rotor? Ignition can have a big impact on fuel economy.

How do the spark plugs look?

 

What "Government page" are you looking at?

http://www.fueleconomy.gov/feg/noframes/9770.shtml

29 combined. Are you letting the car fully warm up before driving it? How long is your average commute?

 

might seem a little simple, but check to make sure you are running ngk plugs have heard about mpg issues if running other brands in hondas.

 

130psi? that not good compression for a motor, even more so if it was just rebuilt. How did you break in the motor after the rebuild?

 

I have been absent a few months due to health issues sorry. I redid the compression test wot and got 160 +/- 5 at all cylinders. I should change my spark plugs, I am using motorcraft, I want to get ngk. I see everybody posting about how the minimun they get i around 36mpg with this car and I am still getting around 26mpg. Mine is a standard transmission so I should get more than automatic.

Is there any test I can do to see if it is burning more fuel than needed? Or something that can help me out so I can change things and not have to wait to refill tank to check mpg?

 

No one has mentioned driving habits or if this is mostly city/highway mileage. I get better mileage than that on a boosted engine.

 

All those things were mentioned in pretty good detail. Mostly highway. No ******* on it.

OP...(just spit ballinghere)

What about the catalytic converter? Maybe it's clogged.

Or maybe the muffler?

Didnt read you answering about the cap and rotor. (but I'm guessing you changed these)

Spark plug wires?

Spark Plugs and their gap is something you could look into too.

 

Thas right, I never press the gas pedal more than 1/4 way down. And for most time it is on highway. very little stops.

I have not thought about the muffler or cc. Any way to check these without special tools?

The spark plug gaps are at 1mm just like it says under hood. The wires arent that old, the cap and rotor seem fine, they dont seem very used, changes about 2 years ago (they shouldn't go out that fast im guessing).

 

I guess I missed those details. Yet, I don't see how clogged cat or faulty O2 would affect mileage to this extent and not throw a CEL. The fact that no maintenance has been performed outside of an oil/air filter change should be a red flag and should have been done before posting in the first place and before bumping the thread back up. And he also mentions that he is not calculating mileage as he did 2 years ago (don't know any other way to calculate mileage other than to record gallons used to miles traveled over a period of a few fillups). That might have something to do with it as well.

 

I think it should thow a cel also. I dont know why you say no maintenance. The engine was rebuilt 6 months ago, new belts, oil pump, new filters, spark plugs, wires and distrib cap were changed not too long ago, cleaned and tested injectors, cylinder valves polished and checked. I dont see how that is no maintenance. Also all internal engine parts have been changed.

Also you might have misunderstood with the mpg calculation. I mentioned that I am calculating it the same way as I had years ago, with quantity (liters) instead of calculating with price (like some people mistakenly do).

After rebuild fuel consumption is the same as before. Thats why I am thinking it could be a sensor. I have changed the ect, air charge temp, and recently the tps.

 

When asked about maintenance like cap and rotor you never responded. Is the fuel filter new or old?

And again, I misread your post, you did say you were calculating the same. What was not the same were the results.

 

Sorry I never responded, I have been away. cap and wires were changed about 2 years ago (if im correct they should have to be changed that often). The fuel filter was changed a couple months ago when I sent the injectors to be cleaned, with no improvement).

 

You can get a gauge like a compression tester that screws into the o2 port and will read back pressure in the exhaust. Anytime Ive seen a cc fail the engine would not accelerate past a few thousand rpm so I doubt that's the problem. However I've only seen them really messed up so I suppose a slightly clogged cc might be the cause.

 

So is it running rich? Soot on the back bumper maybe?

 

Well it does accelerate fine, at least to 5k revs. But its worth checking with a meter just to be sure. How low of psi should it be? Im guessing little to none.

 

it seems like it. There is never a black cloud coming out of tailpipe, but a friend behind me one day said that when I sarted the car a tiny bit of black smoke came out, You could barely see it. Also if I rev it up quickly when parked I can see a tiny bit of black smoke, you have to be really close to exhaust, and can barely notice.

 

Pretty sure you don't have a O2 after the cat. (OBD2 thing). So a clogged cat would not throw a code. Bad O2? Sure it "should" throw a code. But the premise of this topic is based on a slight problem causing lower gas mileage which "could" never make a code pop.


PCV clogged? How about a smog emissions test to see how it smells?