Civic VX getting crappy gas mileage
 

I've got a '92 Civic VX (2nd one I've owned!) that I bought used about 3 years ago.

Before that, I owned a '92 VX that I bought new, and that one consistently got between 45 and 55mpg. Unfortunately, it got totally crunched in an auto accident with 185K miles and was unrepairable.

The problem is that my current VX has never gotten anywhere near the 40-50 mpg that the first one got.

Here's what I've done so far

- Car had a tune-up right before I got it (now 20,000 miles ago) I was told it probably didn't need another one.
- O2 sensor was bad when I bought it, so I replaced it. It's a 5 wire sensor.
- Cylinder head had a bent valve, so got that replaced

Just recently, the idle air control valve has crapped out and apparently didn't respond to cleaning (engine fluctuates between stalling out, normal idling, and idling at 2500 rpm) but I haven't replaced it yet.

Engine has about 100K miles on it, near as I can tell the car was well cared before before I got it, but low mileage (82,000 miles on a 12 year old car) so perhaps it sat around a lot. Given the 5 wire sensor and (I seem to remember) results of a Carfax check when I bought it, I believe the car is not originally a California car, though that's where I now live.

Any ideas? I doubt the IACV alone is the problem, since gas mileage has been crappy but stable (27-33 mpg) since I bought the car 3 years ago.

"I believe the car is not originally a California car"

Good. And chances are Carfax gave you the right answer. But here are some other things you could check. What does the sticker under the hood say? And what's the number on the ECU?

Speaking of numbers, what's the number on the engine block? There's a tiny chance that for some weird reason the engine isn't a D15Z1 anymore. And speaking of long shots, maybe you have the wrong transmission.

Do you have the proper wheels? What brand/size are the tires? What's the tire pressure?

Maybe you should do a compression check.

Has the timing belt been done? I wonder if it's off by a notch.

I wonder if the ignition timing is correct.

I wonder if you have a vacuum leak.

I'm really curious about how the car ended up with a bent valve at 82,000 miles. I also wonder why the O2 sensor went bad that early. Likewise, I wonder why your IACV had a short life.

I wonder if your brakes are dragging. Does the car coast well? Is it possible your handbrake does not release properly?

Ever drain the tranny? I wonder if someone put the wrong fluid in there.

Does the car have A/C? Is the belt too tight? Is the alternator belt too tight?

Ever check the thermostat? Are there signs that the engine does not warm up properly?

Is the exhaust restricted? Did someone drive over a rock and crush the pipe somewhere?

Has 100% of your driving in this car been uphill (jk)?

Just a few wacky ideas you might look into.

Obviously your VX is running very rich. Too much fuel being consumed or fuel leak. If a fuel leak then you can tell by not using the car for five days or so to see if the gauge level changes. A fuel injector might be bad - i.e. spraying too much fuel. A Helm shop manual is a good starting point if you want to repair your VX by yourself. I just found out there are OBDI scanners available that read out short term and long term fuel trim values. Go to pgmfi.org and search for obdI D15Z1 and you will find threads about these diagnostic tools. You can buy them for as little as 65 euros or what $100 US now? I might get one myself after more searching/learning about them. These scanners connect to the Data Link Connector or DLC by the glove box. Keep us posted with your progress.

Speaking of fuel injectors....I have a question: My VX has NEVER been over 40MPG, regardless of how I baby it, coasting, etc. Is it possible the previous owner changed the fuel injectors to get better performance? Which may be a major part of my problem. How do you tell if you have VX fuel injectors?

"My VX has NEVER been over 40MPG"

The only time I've ever been under 40 mpg is when I do a steady 85-90 mph for a long distance. Then I've had tanks as low as 36-38 mpg.

"How do you tell if you have VX fuel injectors?"

As far as I know, there's nothing special about the VX injectors. But there are lots of other things the previous owner could have messed with. Wheels? Tires? Are you sure the whole engine wasn't swapped? Or the tranny?

Are you sure your O2 sensor is good?

In my previous comment I mentioned lots of other things that could affect mpg. And I'm sure I missed a bunch of things.

I have a new(er) engine w/5sp trans that was transplanted about 5 months ago, when we determined the old one was losing compression and needed an overhaul. It's a D15B VTEC-E engine and an S20 trans. This was an "import" from Japan I bought from https://www.jdmsource.ca/ in Canada, supposed to have 40-50,000 mi on it.

I sold the aftermarket wheels/tires and installed OEM VX rims and LRR tires. Not sure about the O2 sensor, had to move it over from the old engine, since some fool cut all the wires on the new(er) engine, instead of merely unplugging them.

I usually drive 60-68mph, use coasting and power off coasting whenever possible.

"It's a D15B VTEC-E"

I think that's the issue. I don't know much about that engine, and I can't find much info about it, but I think it's something fairly different from the D15Z1 you're supposed to have.

See here for someone who seemed to have an experience very similar to yours.

S20 sounded familiar (maybe because I have one) and I looked it up on google. People seem to say that it's a d16z6 transmission (ie. performance oriented) so maybe that could contribute to some of the problems.

Thanks to grasshopper in another thread, I just read some more about your engine. Maybe you should take a look at that article. Your engine is supposed to provide the best of both worlds (economy and power), but maybe in practice there are some compromises. Or maybe it has to do with the way your engine is matched to your car (via the ECU and the trans).

I see now that grasshopper has posted here, making that same point (about your trans).

Yeah, the trans on the D15Z1 engine was an S20, too...but prev owner was a wannabe tuner, so he may have swapped it.

I did find what looks like an excellent link on the D15B:

https://www.d-series.org/forums//showthread.php?t=32597

They claim it's a VX block with an EX/Si head. Should still get better FE than what I'm getting. May have to change the ECU....they claim the stock ECU won't enable VTEC....would a P28 ECU work with the VX wiring harness?