92 VX only getting 33mpg
 

----Edited to update progress as i check things, so people don't have to read the whole thread - last edit 1/31/08---

Started with 33mpg
Best tank so far with california ECU 43mpg
Swapped in Federal ECU and got 49 mpg

Background info:
90% highway driving, 100 miles a day
Average speed is 55-65mph
I don't drive like a hypermiler, but i do get passed alot
No CEL on
It is a california car, has the l00 ecu but has a 5-wire o2 sensor. Apparently this happens sometimes on the 92 models.

So far i have changed/tested:
Switched to VX rims and 175/70r13 tires losing a total of 44lbs
Compression tested 170-180 all cylinders
Replaced zfr5f-11 spark plugs with zfr4f-11
Front calipers not dragging
One of the rear drums had a little resistance, will check further
Checked O2 sensor, readings posted below
Checked and adjusted timing
Swapped in federal ecu

So far on the list to do is:
Get alignment
Check EGR (my egr ports on manifold look to be welded/sealed up, is that normal?)
Check vacuum

What else should i check?

My favorite tire size calculator:
https://www.wickedbodies.net/Tire-Size-Calculator.htm

Completely aside from rim size, if the overall tire diameter is different from that of the original spec tire then your speedometer will be off. Larger tires would mean you've actually driven further than the odometer shows. So use the calculator to check your current front tire size vs. the original spec size. It will show you the % difference.

Check your engine vacuum at idle then slowly increase it to 3000 rpm.

If you don't have at least 20 inches of steady vacuum under these two conditions, then you have a compression issue in one or more cylinders. An exhaust gas analyzer would help to confirm if your VX is running super rich.

My 92 VX holds 24 inches of vacuum at idle and at 3000 rpm. I'm getting about 42 mpg but I live in Michigan and it's been cold lately. I average 44 mpg over the entire year. You should be able to achieve this easily. I'm running the 175/70R13 size now on the OEM VX wheels and have lost 1 mpg or so from what I had with the stock 165 width. I drive 70% highway at 70 mph.

That is my tire corrected mpg.

I will try and test the vacuum later and do a compression check.

Thanks.

Haven't got a chance to check anything yet, but i did pick up some VX rims. Spent all day stripping the green paint off of them, I'll either paint them white to match the car or if I'm feeling bored try and polish them.

The car seems to run just fine, except for the low mileage. The only thing is it makes a little noise with the clutch out and the car in neutral, goes away when you press the clutch in. Not sure if that is normal or not though as this is my first honda, and first front wheel drive car.

You might have a bad bearing in the transmission. It is usually one of the two ball bearings on the mainshaft that goes bad eventually - especially when the axle seals leak. Don't let it go for too long or the bearing may fail - which will trash the tranny altogether. I was lucky to catch it before the cage broke. The ball bearings were all gouged up. The bearing issue should not affect the mpg issue though.

I replaced the bearing myself but it isn't a easy one man job. The toughest job is pulling the tranny from the engine. The dowel pins hold the tranny pretty tight to the engine. I did this with the engine/tranny still in the car. You might as well put a new clutch in while you've got it apart.

It could also be something like a caliper that is dragging.

Or a wheel bearing that is going bad and dragging.

Also check your alignment and all related suspension components.

How fast are you driving on the highway? I get the best mileage at 60mph...anything over that and the mileage drastically decreases.

jadziasman...which bearing did you replace? You make internal transmission work sound like a job for a novice, which to my understanding wasn't? lol.

Can you provide a little more info? I've got 230K+ miles on my transmission and am getting ready to do a clutch job, and if it's something a mechanically inclined guy can do, please let me know as I wouldn't mind replacing any bearings that I can access.

It depends on how much noise the transmission is making, and what kind of noise. The fact of the matter is that ALL transmissions will make SOME noise when spinning in neutral. And this will cease when you disengag the clutch. If this noise is just a fairly subtle sound that kind of sounds like water flowing in a faraway mountain stream (for lack of any better way to describe it), then you are okay. But if it is a louder hum, howl, whine, or grindy sound that you just can't ignore, then you have problems. Of course, you have a choice here - tear down the transmission or replace it. Replacement is certainly MUCH easier,and likely cheaper, too. But you risk getting another bad one. Replacing the bearing is a much more sure fix. But if you are not familiar with tearingdown transmissions, then itis probably not the best idea. Since D-series trasmissions are cheap, you can always get one used, replace it, and pracice working on your bad one to gain more familiarity.

As for your mileage, this doesn't sound right. I get the same mileage with my CRX that is powered by a 200HP JDM ITR motor. And I drive that car 80+MPH on a daily basis.

I will add calipers to my list of stuff to check, thanks.

I will be getting it aligned again after i switch to the vx rims and raise up the suspension a little, its currently on eibach coilovers and at about 3" lower than stock. I know my camber is off.

Water flowing is a decent description of it. It doesn't sound like an obvious problem to me, i didn't notice it until driving the car for a few days. Most other people can't hear it. I'll have to have someone familiar with hondas sit in it and see, but i think it's probably ok.

sounds like the bearing to me, I didn't think anything of mine until I had the dealer mecanic take it for a drive, and he pointed it out to me, 20,000 miles later I replaced the tranny, it was cheaper to get a used tranny then to replace the bearing.

If you have a bad bearing in the tranny, it will be obvious. The noise is a kind of growling sound or like rocks in a blender. It is most noticeable at idle with the clutch disengaged. When pressing the clutch pedal to the floor, the noise goes away. Why? because the mainshaft in the tranny stops spinning since the engine and tranny are no longer connected. I wish I had recorded the sound before I repaired it.

The repair is not easy but a patient, semi-skilled amateur can do it. I had help - the Helm pdf manual and honda-tech. If you're lucky to get a used tranny that has good bearings then you don't need to open the tranny case. It's not really too hard to replace the bearings. The hardest part is getting the tranny out of the car!!!

Regardless of how you decide to do it, a transmission or clutch repair on a front wheel drive car is a lot of labor. Well worth it though. My 16 year old VX has a one year old clutch and rebuilt tranny and that only cost me $100 in parts. The bearing itself was about $18.

Spark plugs were zfr5f-11, I changed them to zfr4f-11

I changed out my rims and tires and lost 11lbs per corner for a total of 44lbs. Better acceleration and much easier to turn. Previous combo was 16" tenzo dc-6 and 205/40 falken azenis tires and weighed 35lbs. Vx rims and tires weighs 24lbs.

Compression check was 170-180 on all cylinders.

Should know if any of this helped in a few days as I drive around 100 miles a day.

What was your average freeway speed? 33mpg is way off the mark. There is something more than just heavy tires or a wheel alignment. Unless the big tires is just completely throwing off the odometer, which was already suggested.

Keep in mind the oxygen sensor can go away and cause a loss in MPG without throwing the CEL light.

I also have a California '92 VX, 5 wire sensor and L00 ECU. I'm going to swap in the FED ECU in a week when it gets to my door. I can't think of anything I would need to change but the ECU.

I doubt it's just the rims. My friend had a wheel/tire combo that was around 35-40 lbs on each corner, and after switching to VX wheels, his mpg went up maybe 2.

I think the tranny issue sounds normal. All Hondas I've owned that had from 140-247k on them made more noise in neutral than I thought they should, but they were all fine and I never had one of the trannies go bad on me.

Hopefully your MPG goes up this tank.

Average speed is 55-65 mph.

After putting on VX rims, changing cap and rotor, and switching the spark plugs to the proper zfr4f-11 I am up to 38mpg.

Still need to check the timing, whenever it stops raining.

I am also wondering if my aftermarket rear hatch spoiler is causing some drag. I've attached a pic. It's not a huge one but it looks like its less aerodynamic than the oem one, looks like it would make a decent amount of downforce.

I bet it does add some downforce. While ruining the clean aerodynamic line.
Looks like a wind catcher to me.
I'd sell it and pocket the money. Ebay, probably.

Can you take it off without wrecking stuff and run a tank or two without it? If you do, see if you can find some screws or plugs to fill any holes. Consider rubber "well nuts" from a hardware store's fasteners section.

A Kamm back treatment would continue the roofline rearwards and downwards; that would likely reduce overall drag. This one sticks up; I really think it adds drag rather than reducing it.

However I'm curious what other folks who've taken a course or two in aerodynamics will say.

Checked the o2 sensor voltage from d14 to d16.
As far as i can tell it is the correct l1h1 sensor.

Until it warms up it's completely random, anywhere from -1.9 to +1.9
After it's warm it idles at +0.2 to +0.4
Under normal acceleration it goes from -0.4 to +0.7
When i accelerate and then let off the gas it goes to +1.9
Under hard acceleration it goes to -1.9
At 2k rpm and 60mph it goes between 0 and -0.4
If i let off the gas to the point where i'm losing speed it gets into the 0 to +.02 range

It's never very smooth of a reading, the only thing i can predict is the idle voltage, everything else jumps around alot.

It will go from 0.2 to -0.4 to 0.4 to -0.6 with each refresh of my meter, with as constant of a throttle as i can give it.

Does this seem right? I should be in the positive range at highway speed right?

Here's the spark plugs that were in it, not sure how long they were in.

Nice looking VX. I have a CA VX too. Have you checked the air filter? Could be dirty. Also the plug wires. Good luck on getting into the mid to high 40s.

My advice: Get rid of those plugs ASAP!

Those are NGK Iridium ZFR5F-11s, they are not meant for a lean burn motor and can potentially damage the D15Z1 combustion chamber.

The iridium tips are too small and the combustion temps gets pretty high when in lean burn, this can melt the tip off and cause damage to piston rings, valves, and cylinder walls.

Go get yourself some NGK V-Power Spark Plugs No. ZFR4F-11 copper plugs, no iridium, or any plugs with more than one ground terminal, lol.

Bring 'em home, let the head cool off if you drove your VX to pick up the plugs, pull out the iridium ones, clean them off and sell them back to the ricer that owned the car before you and thought that they do anything good for the car. That way you can probably pay off the cost of the NGK V-Power plugs. Install the V-Power plugs and you should be good. Double check the condition of the spark plug wires. If they aren't the stock ones (i.e if they are aftermarket or "race" plugs, other than the NGK Race plugs) save up and get some OEM ones from the Honda dealer. Also check the condition of the cap and rotor. Replace if worn.

Doing that should help along with making sure you are using the proper driving technique for the VX.

And also check out the links in my signature and CivicVX.com for more info than you can shake a stick at for the Civic VX.

Good call. I just changed the ECU to the Fed one on my Cali VX and I didn't check the spark plug range. I just ran outside, everything is OK!

TomO,

I already got rid of those plugs and replaced them with the right ones from the info on your vx thread. Already did the cap and rotor, and it has new NGK wires. Atleast the previous owner got the wires right.

I would love to know what you think of my O2 readings, and what anyone else gets as their readings.

I am currently up to 38mpg.

Background info:
90% highway driving, 100 miles a day
Average speed is 55-65mph
I don't drive like a hypermiler, but i do get passed alot
No CEL on
It is a california car, has the l00 ecu but has a 5-wire o2 sensor. Apparently this happens sometimes on the 92 models.

We do the exact same driving, with the exact same car ('92 California VX). I've owned the car about a month. My first tank netted me 51.7mpg, my 2nd tank was 53.2. I try to stay at 60mph on the 2nd tank. I don't do any key off coasting, hit traffic occasionaly, with some hills. I don't know how many miles are on your car, but the compression numbers look good. Timing can greatly affect mileage, so I would look at it. Only thing I can think is how are you figuring out your mileage? Is there anything on the car that is not stock? Like headers, or maybe a transmission swap?

Also, did you check vacuum?

I wish i had those mpg numbers.

It has around 200k miles on it.
As far as i can tell it's the right vx transmission.
It is lowered and has an aftermarket rear spoiler, picture posted above somewhere.
I will check vacuum.

You can tell from rpm if the trans is right. At 53-55mph the tach is right at 2,000rpm in my car. I'm really curious as to why your car gets lower mpg. Someday my mileage might fall off and I don't want to be scratching my head for a month.

At 2000 rpm I'm around 55-60, which from what I've found is normal.
Hopefully this weekend I can get some more figured out. I leave for work in the morning when it's dark, and i get home when it's dark so working on it is a little hard.

Timing was retarded a bit, set it back to normal, no change in mileage. I'm still at 35-38 mpg.

What was the vacuum reading?

The last few tanks have been about 41-43 mpg, getting closer...
I'm going to take off that aftermarket spoiler and see if it helps, i know it can't be doing me any good.

I've seen the write up for cleaing the egr ports on the intake manifold, but mine look to be soldered up. Is that normal?

If youre compression is only 180, it is a little bit lower than stock. mine was hitting 195 in all cyls. If I were you I would probably just replace the o2 sensor. chances are it has over 60k on it. You could maybe ask the previous owner? It also takes some getting used to to learn how to drive the vx efficiently. Did you know that you can put around in town in 5th at 32 mph? Does your shift light work? You should gain at least 5 mpg with the lean burn installed.
Im at 56-57 MPH at 2,000 RPM, but I saw a picture of someone's vx that had even taller gearing than that, that was supposedly stock. Mine is a 94, I wonder if 92's had a different ratio?

I have a question. Maybe I missed something, but I am wondering if maybe the person who had this car, before you, may have dropped a different transmission into it. If you have the right transmission, you should be at around 2000 RPM, a 60 MPH. If you are running much more than that, then your transmission was probably swapped out and your going to need to change that back, to get to the mileage your expecting.

Right now, you seem to be getting more in the neighborhood of what I get with my 89 Wagon, which has essentially a DX drivetrain in it.

I saw no one responded to this. So I thought I would. I'm not familiar with the pins on the car and why you have negative voltage, but normally an O2 sensor is something like 0-5V and 2.5V is normal. The lower the voltage the richer and the higher the leaner, so I bet your pins are just subtracting a reference voltage or something.

I bet negative is right and + if lean. So I'd say running a bit lean is normal for a lean burn engine and going heavy negative while accelerating is normal since I'm sure even the VX richens the mixture for acceleration. Going hard positive after letting off the gas is just a lean spike after acceleration. That all seems normal to me.

Now you can't really correlate voltage to specific A/F ratios, but I'd expect to see a bit higher than .2-.4 at cruise/idle in a good lean burn situation, maybe...

You should try reading these measurements again now that you have the correct plugs in.

Transmission seems to be correct.

This is after the correct plugs are already in. Yes, it does use a reference voltage, 0 being stoic, positive lean, negative rich. I am negative at cruising speed, I'm not sure if that is right as i have a california car. I started another thread with this specific question but it's not getting many replies either.

I swapped in the federal ecu and on my first tank got 49 mpg. My best tank before was 43 mpg.