Yet another poor MPG Civic VX thread..
 

Hi All,
I just got a VX a few weeks ago, it runs wonderfully, but definitely needs some help to get proper VX milage. I thought I would use this thread to log my progress as well as seek advice, if you all aren't sick of VX threads yet :p

The car is a Canadian (5-Wire) VX. It has 108K Miles, very minor rust, and only one modification from the previous owner, a magnaflow muffler. The tires are non-LRR, 175/70/13s. It has a passenger side mirror, but no AC or mud flaps.

I'm still new to eco-modding, but I'm trying to drive as efficiently as possible. I use DFCO all the time with gradual acceleration and rarely go much over the speed limit.

I'm getting 36.7mpg, but I figure it's because my drive is 4km long and i've only been using her in the winter time.

I should note that the second tank, my friend borrowed the car to teach his wife how to drive manual. Either way, there are no excuses for the first tank.

I have no CEL, but I'm still suspecting the O2 sensor, but want to confirm it before I shell out 2-300 bucks.

Still to check:
- replace axleback with stock
- check timing

Already checked:
- check tire pressure - 33PSI on 35PSI tires. Upped them to 37PSI
- check spark plugs for proper NGKs - ZFR5F-11 in use instead of ZFR4F-11 :(
- check grounding wires - not frayed, seem to be relatively good condition
- Drive with O2 unplugged Made MPG worse..
- check rear wheels for alignment/sticking Seem to rotate fine
- Clean EGR cleaned
- lower idle screw adjusted
- check tailpipe for oil. none

How much of the exhaust system did the car have replaced? that would be first on my list of things to fix, after of course making sure that it was otherwise running right.
Add to your list, changing transmission fluid (0w30 motor oil, or Honda MTF), clean points on distributor cap, check timing, check under the hood for other mods done by teenagers.

The first thing that comes to mind, jack up the back end, with the hand brake off, and spin each back wheel. Look for sticking or rubbing brakes, wheel wobble, noise, etc... I've had wheel bearings go bad and sticky parking brakes drag.

Also look over the tires for scallops or cupping that might indicate a bad alignment.

Third- wipe your finger inside the tail pipe. Is it oily? Bad seal/ring or too rich.

These checks should be fairly quick and free.

How long is your commute?
What is the average speed?
Unfortunately the VX loses lean burn capability for a significant amount of time after engine shut down, which eliminates the lean burn advantage, as well as potentially making the 02 sensor cool off and possibly not give you closed loop.
How long does it take to warm up-possibly thermostat not closing properly?
EGR circuit clogged?
Top gear in the original transmission should give you about 30 MPH at 1000 RPM and the car should pull away from that speed even at that low RPM. Short shifting is an essential part of VX mileage, unless your speeds are just to low.
Look at Pale Melanasians mileage on his DX. It has none of the design advantages of the VX except it is also a Honda.
His mileage would probably be lower if he drove a VX, due to the low average speeds and high rates of DFCO. The Vx systems were designed more for prodigious highway mileage. On mine I did well in the city but I did not use DFCO, knowing it's effect on lean burn and 02 temps.
Tires are also a crucial part of the VX package. I could push the car on flat smooth concrete with one thumb.

regards
Mech

You should check your tire pressure a few times a year so if you haven't yet then it's most likely low, combine that with the wide, non LRR tires you have and you'll see a few mpg drop right there.
On the two VX's I've owned I've noticed that exhaust issues and non stock exhaust seems to lead to the o2 sensor failing, so you might not be able to revive a failing o2 sensor but unless you want to replace it twice, like I ended up doing on my first vx, I would fix the exhaust with stock parts first.
Also, does your VX have any dealer options? passenger side mirror, mud flaps, or A/C? each of those will cut your mileage back a little, not as low as you are seeing but they all have an affect.

I'd check the part of the HT-leads that go down to the plugs for cracks and damage.

It looks like only the axleback has been modified with a simple bolt-on.
I'll add those to my list, and it looks like there aren't any other mods under the hood except for an aftermarket fuel filter.

I will add these to my list. Thanks!

The tire pressure was a *little* low. 33PSI on a 35PSI max tire. I put them all to 37 for now. I will also try to exchange my muffler for a stock one. Is the VX muffler different to the regular EG muffler?
The car has no AC, no mud flaps, but it does have a passenger side mirror.

Unfortunately my commute is currently mostly city driving. It's about 4 miles at about 30mph (city driving, but not bumper to bumper). I do drive plenty outside of my commute too, but rarely on a highway.

I will try to time the warm-up and check the EGR circuit (have to google to find out how). I'll also try to push my car, although it doesn't have the original tires.

Just checked, my spark plugs are the ZFR5F-11 plugs (replaced by the dealer). Google seems to indicate that those make a fairly big difference to the proper VX plugs (ZFR4F-11?)

Does anyone have a snippet of Honda documentation stating that the ZFR5F-11 plugs are the incorrect ones to use for the D15Z1? Those plugs were put in by the dealer in October by the previous owner since he gave me his invoice.

Thanks for all the tips, I'll update my original post with my new extended todo list

check the engine rpm at 100 km h it should be 1900 rpm in fifth gear
if rpm is 2400 you got a tranny from civic 96-2000 hb
if rpm is 2700 you got a dx 92-95
if rpm is 2900 you got a si tranny

poor mileage
check air filter - should be able to see the sunshine through it when you hold it up in front of the sun.
check timing. too advanced causes it to retard a lot. too retarded gives crappy mileage. other hint is exhaust gets really hot.
check fuel pressure. too low and engine runs like crap.
02 sensor is another thing to check. you can put a volt meter on it and verify it is working.
check compression. a dead hole can mess things up.
pull valve cover. make sure valves all seem to lift correctly.
check you have the right engine. LOTS of swaps and if someone stuffed a d15b7 with a Cx/Vx ecu, you will have a weird combo.