oneheadlight

I have been working on a 96 Civic HX and cannot seem to increase the fuel economy. I have replaced the following and they are new parts that were tested to make sure they're good.

Replaced the following:
Fuel injectors and fuel pressure regulator
EGR valve, passages/plenum also cleaned out, intake manifold cleaned
VTEC spool valve replaced
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
Both 02 sensors replaced, wideband on D16Y5 Engine, correct brand, denso and NTK.

With all these things replaced the mpgs would go up slightly only to fall again. I've gone up to a high of 31 to fall back to 28.

I even went as far as trying ethanol free gas but that actually caused the mpgs to fall! I was down to 26mpg with ethanol free gas. Any help is appreciated as I am frustrated I can't get this back to what it used to be with 36-37 city and 41+ mpg hwy.

EDIT: To update I did forget to put down I replaced the 02 sensors, updated list..

slomofo

iTrader: (1)

Hook a tow rope up to your car and attach it to your neighbor's diesel truck.

Infinite MPGs, guaranteed.

S!K TEG

[QUOTE=oneheadlight;50003026]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
QUOTE]

I almost everything, minus, stock air box filter change?

tony_2018

You clean the EGR passageway on the plenum, what about whats beyond that?

Norlael

have you tested and verified that your base ignition timing is correct?

oneheadlight

Tom- yep the HX is the D16Y5 engine, vtec-e, thanks for reminding me of that, I have updated the original post, forgot to post I did replace both o2 sensors, the scanner shows they're working properly now.

The VTEC spool solenoid was also replaced, since the original wasn't testing correctly.

oneheadlight

yes I connected the service connector for base idle timing and have it at 12 degrees.

Former User

Any CEL codes? Just curious, CVT or manual?

Brand new entire distributor? Brand new plug wires and spark plugs?

Have you compression tested the cylinders?

joey1320

All the money you spent on all of these parts would had gotten you plenty of tanks of fuel ha. What's the spec for your vehicles' mpg? How far off is it?

joey1320

Just curious but have you done a cold/hot compression test? How about a fuel pressure test? Are your injectors spraying evenly? Leaking? Also how are you figuring out your mpg?

cbaber

TomCat the default mode for the D16Y5 is 12V. The VTEC solenoid activates the 2nd intake valve ~3000 rpm depending on load.

Do a compression test. You can have all new parts but if your combustion is not efficient it won't matter. Does the car use oil between changes?

oneheadlight

ron- no cel codes, had one earlier but that was for the idle air control valve, replaced it, fixed that problem.

it's a manual, the distributor is brand new but not a Honda, since Honda doesn't make the Hitachi distributor for the HX, been discontinued. I've tested the three sensors for the distributor (resistance tests) and they are close to spec, hey can't complain when it's the only distributor that actually was close to spec. The others from china were way off in spec, I returned those. The plug wires and plugs are new. I've tested the wires, very low in resistance (NGK plug wires), and NGK spark plugs. The plugs are the correct heat range and gap.

I haven't compression tested the engine, but what is odd about all this is that I'll have a part go bad, mpg's go down, I replace that bad part, the mpg's go back up, then drift back down. very odd

I had said earlier, but it's ok I'll repeat, I had tried ethanol free gas and that made the mpg's go from 31 down to 26! Here I thought ethanol free gas was supposed to give really good mpgs.

oneheadlight

no compression test yet and no fuel pressure test, as I was saying to Ron in the previous post it's odd that a part goes bad, I replace it and the mpg's go back up. Only to drift back down later to around 24-26 mpgs.

The injectors are remanufactured by GB Remanufacturing and are not leaking, and appear to be spraying correctly. The fuel pressure regulator was leaking, has since been repaired, no leaks.

MPG calculation the standard way. Start odometer once fuel is topped off (fuel nozzle clicks off due to vapor in tank build up), let odometer run until next fill up. Mileage divided by gallons pumps (At click off).

oneheadlight

The car doesn't use any oil between changes, no oil on catalytic converter (looked at it when a-pipe was replaced, catalyst is clean).

The problem is replace a bad part, mpg's go up initially, then drift back down. Very odd. If it was compression being bad, how could mpg's go up at all? It shouldn't be able to do that.

The engine is a brand new replacement engine put in back in 04, so it only has about 80,000 miles on it. I know that doesn't make a difference but thought that would be early to have any issues with compression, as it hasn't had any other owners than my family. (not abused, etc..)

Former User

Is the temporarily improved gas mileage here^ associated with resetting the ECU?

oneheadlight

I have reset the ecu (computer) but these temporarily improved mpg's came when the computer was not reset.

What about ethanol free gas? here I thought that it was the holy grail and would improve mpg's, didn't happen here, it actually dropped 5 mpg using it.

oneheadlight

So you're saying that somehow the ECU (computer) will retain "memory" even if the battery is disconnected for 20-30 minutes? Does the ECU have a capacitor to remember it's settings?

oneheadlight

tom, hey I appreciate any and all ideas of why this is happening! I'm wondering if Ron might have input as to what it would mean if I reset and the mpgs went up but then drifted back down. It hasn't done that but I wondered if that's what he was getting at.

but I do thank you for all your input!!

Former User

Pulling the Back Up fuse or disconnecting the battery will reset the ECU.

I asked you about resetting the ECU because years ago a member documented clearly that resetting the ECU on I think a 94 Civic led to temporarily higher gas mileage.

tony_2018

You're probably not maintaining lean burn mode therefore your mpg is not consistent. How are gauging your mpg anyways? It needs to be solid proof and not some guestimate.

slogfilet

iTrader: (1)

This seems likely... the HX/VX get very, very good mileage when lean burn is working. They get on the order of a 25% drop when it's not. A lean burn monitor, Scangauge, Ultragauge, or mpguino would be a good investment. They can actually pay for themselves in fuel savings.

It does sound like you're calculating mileage correctly... keep in mind that a single fill is wildly variable. I would recommend logging mileage in Excel or on a site like fuelly.com. Fuelly is nice, you can just text your fuel-up details to the website and it tracks it for you

cbaber

No, the HX/VX still get good gas mileage without the leanburn. Besides 12 valve operation and lean burn, the HX and VX have taller gears which put the cruising RPM much lower than the EX/DX/LX (coupe) transmissions. Even without lean burn you should expect MPG better than the D16Y7.

MPG calculations are most accurate when doing it from a tank fillup. You should fill up the tank, reset the trip, drive the car at least 3/4 of the tank, and then fill it to the top again. Your MPG is your trip distance / gallons filled. The scangauge and other gauges are reliant on these manual calculations to make sure they are calibrated, so they shouldn't be used by themselves for accurate MPG numbers. Also, the HX doesn't work correctly with the OBDII MPG computers because they don't factor in the changing A/F ratios in their MPG calculation. Like slogfilet said, it's best to manually track tank MPG over weeks and months rather than just look at one fill.

I would borrow a wideband O2 sensor and check your exhaust. Maybe the ECU is running the motor rich and you don't know it. But you need to start checking the obvious things like compression before you buy more parts. It doesn't make sense that these single parts would cause the MPG to jump up and then go back down. I'll mention slogfilet's comments again, make sure you are reporting your MPG correctly and over a period of a few fills.

One other thing you can try that is free and easy to do: Unplug the primary O2 sensor. I know you said it's a new NTK unit, but perhaps it is faulty causing the motor to run rich. By unplugging it you run the car in open loop mode which runs it at a constant A/F ratio. You should still get good MPG in open loop mode. I had to drive like this for a month or two while waiting on my new O2 sensor. I was still able to average 40 mpg in open loop mode. If your MPG improves significantly over the course of a few tanks, you know your O2 sensor or another sensor in the closed loop is bad.

baller status

What about your tires?

Just because the engine has 80k miles doesn't mean you shouldn't do a compression check. It's still the best lead you have.

tony_2018

He still hasn't described his driving habits, how much city driving he does, highway, what tires, what tire pressure, ANY ACTUAL METHOD of figuring out his mpg.

Actually these vtec-e civics are mainly bought for the fact that the lean burn does effectively gets you mpg, if it weren't for the vtec-e anybody would've bought a y7 instead and would be saving gas at the pump.

Thor294

so... what kind of tires do you have? pressure?

and do a compression test please.. lol