97 Civic HX MPG not up to snuff
 

Hello the camp...new member to your forums needing some assistance.

We recently helped my daughter buy a 97 HX specifically for the fuel mileage aspects. She drives the car back and fourth to school which is about 18 miles one way most of which is highway. After driving it for several tanks at various speeds....first tank 75 ...then 70...then 65...then 60 (Which is hard to do when the speed limit is 75 on this TX Hwy...bless her heart she was a trooper to test for the sweet spot on mileage).

I have read you need to keep the RPM at or below 2500 rpm to reap the benefit of the HX's fuel economy which we have done repeatedly.

The problem is that at any of those speeds the best we have done is 33 mpg.

There is no CEL and the car runs and drives just fine. I have replaced the air filter (which was a mess) spark plugs, cap/rotor and just put new Michelin tires on it (not just for economy...it needed tires anyway) as well as an alignment and so far nothing has had an impact...still 30ish mpg.

I am going to try cleaning out the EGR valve next and check the timing as these have been suggestions.

Anybody have some insight as to why this HX is only getting 30-33 MPG with no check engine light? She is not using any fancy driving techniques but she is also not a lead foot so I really don't get it. This car I would think should be getting into the high 30's low 40's easily....:confused:

More specifics miles, transmission, etc. The 1.6 with an automatic is only rated at 32 highway where as the manual is 39 highway.

Many eco-driving techniques are about anticipating what is going to happen and not all that fancy. Many time people accelerate past me on the way to a red light. Please take a look at the 65+ Vehicle modifications for better fuel economy - EcoModder.com and try some of them yourself. Many are just common sense.

Like the post says...its a 97 HX....isn't that one of the holy grail cars for eco-modding?

It has the stock S40 manual transmission and the 1.6L D16Y5 vtec-e engine and has something like a 37/44 estimated mpg from the factory.

It's rated at 34 combined on the new (more realistic) EPA ratings. I would hesitate to recommend replacing parts when you have no CEL.
If it was mine and I knew I was not getting the potenital mileage of that vehicle I would probably do what you are doing, making sure the maintenance is up to date, then I would probably replace the oxygen sensor if it was still falling short of my mileage expectations.
Your mileage quote was the original EPA, which was done at much lower highway speeds than 75 MPH.
EGR blockage will cause the ignition timing to be retarded which will affect mileage.

regards
Mech

Thanks for the info....I am out in the garage right now, just finished adjusting the valves (which were a tad tight) and fixin to disassemble the EGR assembly right now as a matter of fact.

Very timely post as I was debating whether I was actually going to do it....after reading your post....I definitely am....then ignition timing is next.

I have another 5 wire heated O2 sensor I can try if need be. I built a DIY wideband O2 out of the 5 wire NTK L1H1 sensor that came out of my 95 Civic VX. I use it to tune A/F ratio on my Civic....it now has a 200HP 98 Spec Integra type R motor and tranny in it (wish I would have kept the original VX motor and tranny...but they were both whipped with nearly 250K miles on them)....but still...getting nearly 35 MPG out of my 200HP rocket. The car now has over 500K miles on it. :thumbup:

I can both test her sensor in my WBO2 meter as well as test my sensor in her car...should be able to tell straight away if they are both working correctly.

I may add another exhaust bung to her car so I can use my meter on it, that way I can tell for sure if I am in lean burn mode.

The Vout on the WBO2 produces a linear voltage which equates to air/fuel ratio.

How lean does the HX motor burn?...My scale only goes so far....but I guess I can interpolate if need be as the voltages are linear.

Vout: AFR:

1.40 10.08
1.45 10.23
1.50 10.38
1.55 10.53
1.60 10.69
1.65 10.86
1.70 11.03
1.75 11.20
1.80 11.38
1.85 11.57
1.90 11.76
1.95 11.96
2.00 12.17
2.05 12.38
2.10 12.60
2.15 12.83
2.20 13.07 - Max Torque
2.25 13.31 - Max Torque
2.30 13.57 - Max Torque
2.35 13.84
2.40 14.11
2.45 14.40
2.50 14.70 * Stoich
2.55 15.25
2.60 15.84
2.65 16.48
2.70 17.18
2.75 17.93
2.80 18.76
2.85 19.66
2.90 20.66
. .
. .
. .
4.00 Free Air

Having another sensor and the ability to confirm if it is working properly will put you way ahead on proper diagnosis and I think you will see your mileage objectives. If traffic is moving that quickly and your daughter is a good driver, see if she can hook up with a drafting partner. Tell her to leave 3 stripes (minimum) between her and the host vehicle. Feel totally free to ignore this advice if you think it could lead to any trouble of any kind.

I find that on I64 between Richmond and Hampton Va, picking up a drafting partner is as good as lowering your average speed by almost 15 MPH and that's from 65-70 (Interstate limit is 70 MPH here) and 55 (Older US routes). A larger vehicle will also let traffic behind her know she is not driving at higher speeds.

Realistically, if she settles on a 75 MPH (flow with traffic is safest) speed and can maintain a draft SAFELY, then you might see 40 MPG.

Also you want to see if lean burn is working and it sounds like you have the equipment and knowledge to accomplish that.

If she can average a safe speed and manitain lean burn at that speed, then you should get to maybe even mid 40S for mileage, but the limiting factor is going to be the much higher aero drag at that high a speed.

Maybe some subtle aero work, but watch your cooling system temps if you block a lot of the grille and make sure your radiator is not on it's last leg, which if it is original, it must be very close to replacement time.

Minimize AC use, but a gal needs to stay cool.

regards
Mech

Not sure how lean the HX goes. My old VX would hit 24to1. My guess would be 18-19 but someone here knows better than myself.

regards
Mech

Instrumentation like a Scangauge or Ultragauge could help (I personally would recommend the Ultragauge unless there is an Xgauge for lean burn on the Scangauge). My '97 DX would more or less maintain the same speed with different throttle amounts, but the mileage was much better with a light foot. Being able to see what works in real time helps a ton. It would likely help you with diagnosing the issue (if there is one) as well.

UltraGauge lean burn
 

on the UltraGauge Select bank 1 o2 sensor 2 and when it reads out .09 to .00 you will be in lean burn.

I hear that alot of the problem with the civic hx mileage is the egr. cleaning the egr plate above the intake gave me about 3 more mpg.