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I needed some help with a 96 Civic HX. I've done so much repairing, the parts were bad not just throwing parts at it, but the fuel economy isn't what it used to be.

I've even gone as far as trying ethanol free gas but that caused the mpgs to fall to 26! Here I thought ethanol free gas would increase the fuel economy!

Replaced the following:
Fuel injectors and fuel pressure regulator
EGR valve, passages/plenum also cleaned out, intake manifold cleaned
VTEC spool valve
Distributor (set to 12 degrees with service connector attached)
Spark plugs and wires, both gapped and tested to specs (plugs correct heat range)
new timing belt
new exhaust
valve adjustment
new alternator, battery and engine grounds
drive belts adjusted to proper tension
wheel alignment done, with tie rod (front) and toe adjusters (Rear) replaced. All specs for the wheel alignment look good.
Battery disconnected, reconnected for idle relearn procedure
02 Sensors both replaced, correct brands as oem, Denso and NTK, D16Y5 engine requires wideband primary o2 sensor.

All these things have been replaced, and tested out to make sure they're good. But the mpg's will go up slightly and fall again. Highest I've been is 31 only to fall to 28. I'd like to get back to the numbers it used to get 36-37 city and 41+ mpg highway.

 
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Don't mean to bump but does anyone have any ideas of what my mpg issue could be? Thanks..

 
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I saw this earlier and didn't really have much to offer. Has your commute and/or driving patterns changed between then and now?


Changed tires?
Got LRR tires?

Is it cold up there yet?

Does the engine reach and stay at the correct operating temperature?

Got any codes? Got datalist capability/scangauge/etc. to watch live engine data as you drive?

 
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Thanks ezone for the reply. driving patterns have stayed the same, tires are the same, not LRR tires, toyo extensa a/s, standard stock size of 185/65-14, air pressures maintained even when it has gotten colder here in michigan.

Engine does reach and stay at correct temps.

No codes, but my next move was to hook up the obd2 scanner and look at the data, I had before but about to check it out in more detail. Was also going to look at the mode $6 data to see what the sensors are doing, especially the 02, even if they are new.

I really didn't know what else to look at. I even tried ethanol free gas but the mpg's went down with that, which here that was supposed to increase the mileage!

 
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Same tires as when it got better gas mileage? I know tires can affect economy quite a bit even though they are the same sizes.

My own car dropped gas mileage as much as 20% after I removed the stock LRR (Low Rolling Resistance) tires and installed Michelin Pilot Sport A/S




Check this in a scanner datalist, not the dash gauge. Watch temp while driving under all conditions, esp highway speed.

See if it isn't going into lean burn mode at all now.

Mileage appears worse than a regular Civic of the same era.
Does it lack power at all? Any other clues something might be amiss, besides mileage dropping?

Cam timing is correct?
What's the MAP sensor value (voltage) at hot idle, in neutral with no loads? (ideally under 0.9v)
Is there a knock sensor?

No fuel smells? Fuel line isn't rusted out back by the drivers rear wheel? Tank isn't rusted and leaking at the seam?


I'm just kind of tossing ideas out here, this car is before my time at the dealer and I don't think I have ever chased problems like this on one in person.

Weird. Not what I would have expected either.

 
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yes same exact tires when I was getting better mpgs until all these parts began to go bad on the car. I agree I've seen some tires cut 20% or more like you were saying.

I can check the scanner's data list (aka live data) for the engine temp and if it doesn't get to temp then the thermostat isn't working correctly.. That wasn't replaced yet. Also.. how would I see on the OB2-D scanner if it was going into lean burn mode? Would I need mode $6 capable scanner for this?

No lack of power it has a lot of power it's fun to drive since all the parts have been replaced but the lack of gas mileage is the only downside. I'm trying to think if anything else is odd and I mean anything. The idle isn't as smooth as it could be, it's just a very slight roughness, I attributed that to the poly bushings I installed for the shifter rod. But just thinking of anything odd. It does start very quickly since the new distributor is in, 2 cranks and it's started, before it was about 5 cranks.

I did have the dealer replace the timing belt and asked them to check the cam timing, they said that it was correct. If the cam timing was off, wouldn't the distributor be maxed out trying to get the 12 degrees of timing? On the distributor the adjustment for that is in the middle of its range. It's set to 12 degrees. (specification)

I'm out of town writing this on another computer but when I do get back I will check the MAP sensor, it's the only one I've not checked yet. I think the only two sensors that haven't gone bad are the air temp sensor and the MAP sensor, ironic you said to check that because I was just thinking that was the next sensor I was going to be looking at.

No knock sensor on the D16Y5 engine as far as I can see from the service manual.

There was the issue of the tank rusted plus the fuel lines were beginning to rot/leak, that has all been replaced now (honda parts). That was last year, and the fuel filter was replaced at the time, but not this year. Would a fuel filter somehow get clogged causing this? I'm also thinking about getting the fuel pressure tested. The fuel regulator went bad, replaced, but the fuel pump wasn't tested for the pressure. If the fuel pump was bad, wouldn't I have other symptoms (rough idle, lack of power, stumbling at high speeds) than just a fuel economy drop?


Hey I do appreciate the help, ezone! Anything thrown out is good for me to look into as I was at the end of my ability and didn't know what else to check, decided to post it up. You're right the drop in fuel economy from ethanol free gas was odd.

I will check the MAP sensor when I get back home in about 2 weeks and post that up, but definitely let me know how I can check the lean burn mode via the obd2 scanner. Thanks again!

 
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I'd look for reaching and staying at or above at least 175*F, (hopefully stays in the 180-185 range after warmup while at highway speed), and up to whatever temp the fan kicks on as the max.

I'm not sure that any generic unit can give that data.

Mode6 isnt.........Um, check out this OBD2 Mode # info here: http://www.motor.com/magazine/pdfs/062002_06.pdf

The usual air/fuel ratio is 14.7:1, lean burn mode would climb well above that, I think up near 20:1 or maybe even higher. IDK if even better basic scanners can give that info.

Like I said earlier, I haven't really messed with anything on an HX that I remember.....
I asked one of my coworkers who drives a 97 HX (that has some issues) he drives about 40 miles each way to work, he said he can feel when his goes into lean mode, he said it bogs down a little and the VTEC solenoid kicks on when it does.
He said his will randomly get good and bad mileage but his is still quite a bit higher than what you posted. IDK if he has ever driven around with a scanner on it for any length of time to see what's going on though.


Brake drag?
Aerodynamic drag (bike rack on the roof?)?

Manual trans, right? I doubt shifter bushings have anything to do with engine vibration (except as felt through the shifter), but that vibration might be a clue to keep in mind.

You're correct, if the timing belt was a tooth off you'd be trying to crank the dist about 30 extra degrees LOL
I've seen it happen though, so I threw it out there.
I was thinking of using it to read manifold vacuum at hot idle with no loads, looking for a reason there would be extra fuel added, like tight valves cause manifold vacuum to drop (MAP voltage raises), and can cause richer than normal running and sometimes set rich codes.
(I read MAP voltage far more often than a real analog vacuum gauge)

MAP is the main load sensor and had a high amount of influence over base fuel injection quantity (fuel table) so it's worth a looksee.

Also, verify the accuracy of the coolant temp sensor.
An overnight sit should leave the ECT and IAT reading almost exactly the same before you fire the engine up.


Didn't think so, but worth asking.
No, you'd have a problem of lacking power under high fuel flow demand. (acceleration at WOT +redline would be anemic)
I'd be looking at O2 sensor and fuel trim values with a scanner first.... I can tell with a 2 minute test drive if a pressure test is warranted or not.

NOTE: I like to diagnose as much as I can from the comfort of the drivers seat. I'm lazy.



Sure, test fuel pressure (don't forget to check both with and without vacuum on it) but I'd certainly hope the new regulator is doing its job correctly.

 
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UPDATE: Thanks ezone for the help!

I was able to get back home and check the MAP, ect, IAT. They all checked out normal. The MAP was reading normal outside barometric pressure before I started it, and the ect and IAT where the same temps before starting it.

What I did find out is the brand new NTK oxygen sensor (on the HX it's a wideband a/f ratio sensor) is defective. The sensor is intermittently sending the signal to the ECU. I confirmed this with a multimeter and with the OBD2 scanner. That's why some days it has power, other days it's lacking.

Here's the "best" part (sarcasm), turns out NTK does NOT offer any warranty on their sensors at all! Here I read the forums where people claimed only NTK 24300 works on an HX primary sensor and only to buy that one. But now I'm out $250 with a company who doesn't even give a warranty on the products they manufacture. I didn't check that before I bought the sensor thinking that they all had some warranty. Big lesson to learn indeed!

Now I found out NTK has no warranty it's not like I want to take a chance again on them, but everyone says that's the only sensor that works.

Is it possible that Denso would work? They do sell an A/F ratio wideband o2 sensor for the HX. And Denso is decent enough to stand behind their products with a warranty. Could you ask your co-worker who has that HX if the Denso brand would work for the primary o2 on the HX?

But the good news is that I found what was causing the fuel economy drop, bad news is, I get to spend more money!

 
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Last I knew he had a header on his and a cat from something else under the car and extended wiring and relocated his sensors... I'll ask if I can remember.

 
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Thanks for asking your co-worker! I re-checked the primary (A/F sensor) oxygen sensor today while driving and a passenger was reading off the live data. When accelerating the voltages were not consistent, where you'd expect it to be rich, it was pegged at 0.500volts and from time to time vary around that voltage. Even at idle when I would blip the throttle, no response from the primary, the secondary o2 would respond though. My plan if all goes well is to replace this NTK sensor with a denso and see what happens.

My mpg's are dropping, about 21mpg now. Let me know if my diagnostic skills are not correct in pointing the finger at the primary o2 sensor as the problem.

 
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Finally remembered to ask, he said the original front AF sensor was NTK and didn't know what brand the rear O2 sensor was.








Yeah, I'm slow.

 
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hi ezone.. well update to the issue, was able to get the NTK brand (like you had indicated from your co-worker) for the primary o2 sensor. The rear (secondary) o2 is a denso brand. I installed the new primary one after testing out the resistance (according to the service manual) and it was within the range. The old one was bouncing all over when testing the resistance, a good sign it was bad.

Happy to report that reading the live data on my obd2 scanner the primary o2 voltages are much quicker to respond to any change in pressing the gas pedal. Was not like that before, was always locked at 0.500v, once in awhile it would go down to 0.08volts (rich for a wideband a/f sensor) but most of the time around 0.500v never higher than that. Not anymore, it's much more responsive, and quick to respond to voltage changes. It will come back to 0.500v as a "baseline" it appears, but will react quickly to any input from the gas pedal.

The secondary o2 when warming up the engine was at 1.275v, where before it was at 0.600v and nothing higher. I'll have to do some digging about the secondary. I understand that with a wideband o2 (primary in this case) the voltages will go towards zero the richer the mixture, and as it leans out voltage increases.

With a regular o2 sensor the voltages go higher for rich, and lower voltages for lean mixtures. From what I can see, the primary o2 was bad, reporting the wrong a/f mixture and that resulted in the secondary not seeing a true correct mixture for warming up the engine. Correct me if I have that wrong, appreciate your input.

I have been able to drive it and no more bogging down in 2nd gear, no odd pulsation when in 5th gear. Now the HX isn't a sports car, but on the highway in 5th gear, it will accelerate now, no need to downshift like before. I've driven it around town and the gas needle has stopped moving, was really drinking down the gas before, will take time for me to see what fuel economy i'm getting, either way it's a lot better now, and actually enjoyable to drive again

Let me know your thoughts on the secondary o2 but I'd like to say thanks for your help it definitely steered me in the right direction. Looks like that new o2 from back earlier this year was a bad one, and it's fixed now! So good deal!

 
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Cool! So glad to hear that!

 
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UPDATE 3/2015.. Replaced the primary O2 sensor back in November. Car is getting 32-35mpg now, regardless of brand of gas being filled with. The "vtec surge" is gone, it's seamless when entering vtec mode from lean burn. I can feel the extra horsepower but it's not a definite surge anymore. Which is how it's supposed to work.

So the conclusion is.. Don't believe that your "new" part is a GOOD part. I definitely learned that lesson here. Always diagnose after replacing the bad part with a new one, to make sure the new part is working well. And for anyone who had my issues, take a look at the o2 voltage levels on an OBD2 scanner, one that can graph would be perfect. I got mine at Harbor Freight for about $83 (coupons are all over the place).

 
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I have wondered how well stuff like that works!

 
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yeah it wasn't bad at all. color screen, and does the important (to me) mode 6 of the OBD2 modes. It also graphs the live data, it's how I was able to see the o2 sensor and it's reaction to giving it gas. The unit is actually made by Autel, and I haven't had to update it. It's working well, no need to do anything, why mess with it if it's not broken? lol

I had a crazy coupon for Harbor Freight which you can always find them online and print them up. It was for something like $25% off. So I got the unit for about $89. sure, the one the dealership has is the best, but those things are thousands of dollars, that's IF they kept the modules for the older 96 models. I've heard of dealerships getting rid of the older stuff, no need to pay for it to keep it updated.

Either way the one diagnosing of the o2 sensor with the mode 6 paid for itself! Since the dealer would want at least $90 just to plug in the scanner. Now I have a nice tool for my tool box.