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Old 02-03-2013, 01:12 PM   #1
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Unhappy VX Low MPG Conundrum

Hello everyone, my name is Craig, and I've been a proud Civic VX owner for 5 years. Recently I've decided I want to move up to the awesome factory mpg king that is the Honda Insight. (I know I can get similar mpg with the VX by doing a bunch of mods, but it's a bit of work and still won't have some of the nice creature comforts that the Insight has standard). But before I make that purchase, I need to sell the VX, and I've been fixing up the little problems that have accumulated over the years of my ownership that I've just lived with. One of the main problems I'm trying to solve is that as of about 2 years ago, the mpg dropped down quite a bit. Currently it's averaging around 38-40 mpg which is pretty pathetic for this model. I've investigated a little bit here and there over that time but not put a whole hog effort into it since I was still happy with 40mpg. So I'm coming here to ask for help in a last ditch effort to fix it myself before I have to give up and take it to a mechanic. Any help would be appreciated!

So here's the details on the car and what I've done so far. It's a 1992 Civic VX with 250k miles on it. The motor is still in great shape, doesn't burn much if any oil at all. I replaced the valve stem seals and adjusted the valves at 230k. Compression check shows that the cylinders are only 5-10% below factory spec. The oxygen sensor was supposedly replaced by the previous owner around 200k (I've checked that it is in fact the factory L1H1 sensor. The rest of the car is also in great shape mechanically. Being a 1992, it does have the 5 wire sensor and harness and I've swapped in a federal ECU. There is currently no check engine light, but there is a code 22 (VVT and control pressure switch) and a code 48 (Heated Oxygen sensor) stored in the memory. The only times I've seen the CEL come on was occasionally when I would drive over the local Santa Cruz mountains, and the light won't come on again the next time I start the car. My first suspicion was the oxygen sensor of course, so a buddy of mine let me borrow his SnapOn scanner that showed the codes, and the recommended test procedures to test the reason for the codes. I didn't follow through the test because there were quite a few and they were pretty involved and lengthy. So instead I drove around and watched the scanner monitor to see what the ECU was doing while I was driving. The car almost never entered closed loop mode, so I think that means it's running only on the fuel map tables stored in the ECU and not taking readings from the O2 sensor. The vtec solenoid was however being activated and deactivated as needed based on load. So it still seems that the O2 sensor is the culprit.

I found this posting on the forums (http://www.gassavers.org/f7/civic-vx...itor-2427.html) when looking at ways to test the O2 sensor. Using my DMM and connecting the positive lead to D16 and negative lead to chassis ground yielded a reference voltage of 2.7v, which according to my searches on the forums here seems to be the right reading. Connecting the positive lead to D14 and negative lead to D16, and driving around gives me the following results

idles at around +0.3v
Under normal acceleration around -0.5v
Under hard acceleration it goes to -1.5v
Maintaining speed in 2-5 gear it sits around +1.2v
Coasting with the clutch out it sits around +0.7v

These results seem to match the ones thewizard182 reported from his car with a federal ECU in this post http://www.gassavers.org/f5/vx-o2-se...ings-6306.html

Connect the positive lead to D14 and negative lead to chassis ground yielded these results if they are helpful

idles at around +3.0v
Under normal acceleration around +2.2v
Under hard acceleration it goes to +1.5v
Maintaining speed in 2-5 gear it sits around +3.9v
Coasting with the clutch out it sits around +3.3v

I've also performed the three following tests outlined in the Haynes repair manual on the oxygen sensor circuits

1. Tested the oxygen sensor heater circuit on the sensor with an ohm meter, getting 3 ohms (manual said 2-13 ohms is acceptable)
2. Tested oxygen heater sensor pin against all other pins on the sensor with an ohm meter, and got no continuity (manual says there shouldn't be any)
3. Checked voltage on oxygen sensor plug on harness for the heater circuit with the ignition on and got about +7.0 volts (manual didn't give a specific voltage, just said there should be some)

I'm not sure what all this means for the status of the O2 sensor, or what could be causing the ECU to stay in open loop mode. I read that it will stay in open loop if there's a CEL on, but it is rarely on (the CEL works, it's how I got the stored codes).

So if the O2 sensor is ok, and maybe the scanner was incorrect and the car is going into close loop mode, here are the other things people have said to check in other posts I have read on these forums.

Tires- tires are always at around 44 psi (max pressure indicated on sidewall), they are the correct size tires for the VX, low rolling resistance, and I have the factory VX wheels
Brakes- Checked the brakes, they are not dragging
PCV- checked the pcv valve on the intake manifold between the #3 and #4 runners, it's a little oily, but not clogged
Spark plugs- These are the incorrect NGK ZFR5F-11s, and I will get the ZFR4F-11s, but can that reduce the mpgs so drastically?
Spark plug wires- mine look pretty good, and testing them with an ohm meter showed them to be in good shape
Cap and rotor- visually they look good, but I could replace them if you guys think they could still be bad.
Ground straps- all factory ground straps are in excellent shape. I did add a beefy one at one point from motor to chassis, but it didn't change anything
Air filter- air filter was replaced about 2 years ago

So that's about it. I've tried to be as thorough and clear as possible so we don't go chasing after the easy dead ends. If everyone is still scratching their heads, can someone recommend a good mechanic in the San Jose or San Francisco area that is familiar with VXs and has reasonable rates? Thanks for any and all replies! Have a good Super Bowl Sunday!

Cheers!

Craig
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Old 02-04-2013, 04:24 PM   #2
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2 years is way tyo long for an air filter and clear the stored codes. google how to. pull fuse for 5 min . your 02 is probably shot or the cat is plugged up. spend 10$ buy a vacuum gauge and report back.
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Old 02-05-2013, 09:35 PM   #3
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If the cat was plugged wouldn't the car be performing noticeably worse (driving characteristics wise) and not pass smog? (which it has). What am I doing with said vacuum gauge?
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Old 02-06-2013, 03:14 PM   #4
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You are going to hook it up to a positive vacuum source. The needle will tell you all kinds of things bad valves out of time plugged cat healthly motor. Google how to read a vacuum gauge. Once long ago everyone had one in their toolbox now the are all to forgotten. They can yield alot if very valuable information. And get an air cleaner. Asap. The car may or may not pass smog with a slightly plugged cat. We have no emissions in my county so.
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Old 02-06-2013, 06:26 PM   #5
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When I had a plugged cat my fuel economy did not suffer, but power suffered so severely that I eventually couldn't get the car over 40mph.
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Old 02-06-2013, 06:27 PM   #6
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I know how the vacuum gauge works, I am mechanically inclined (done all of the work on the car myself), and work at a restoration shop (as a painter), so I have access to a lot of the necessary tools. I was asking what specifically am I testing? Is there a list to run down of what to test and what the normal values are for the items I'm testing?

On the plus side, I did replace the spark plugs with the correct ones and went from 45.5 mpg on a freeway trip last week to 53.3 mpg on the same trip the other day! I'm a little skeptical of this improvement, as a data point of one does not make a trend, so I'll keep an eye on the mpgs when I fill up next week. In the meantime I replaced the air filter, distributor cap, and rotor this evening.

As far as the O2 sensor, wouldn't the voltage readings I got on it inform me as to whether it was working properly or not?

Thanks
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Old 02-06-2013, 07:26 PM   #7
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O2 sensor can give a reading in the right voltage range but it may not be measuring the oxygen in the exhaust gases correctly so you could be running richer than you think you are because it doesn't read properly.
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Old 02-07-2013, 08:09 AM   #8
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Quote:
Originally Posted by blitzkraig View Post
I was asking what specifically am I testing? Is there a list to run down of what to test and what the normal values are for the items I'm testing?
It's not just about values but fluctuations and such. Someone posted this link for me and I found it a most useful page:
How to Use and Interpret a Vacuum Gauge
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Old 02-07-2013, 05:57 PM   #9
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That's a good one cow. Like I keep stressing a vacuum gauge can tell you lots. I have a check engine light on with both 02 codes 41 47 I think. With the fault tracing with the FSM it told me o have a bad ecu. New ecu still same codes. Trust me I don't want to throw a 200$ part at my car.
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Old 02-07-2013, 07:27 PM   #10
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That is indeed a very useful link. I'll hook up a vacuum gauge this weekend and take a look at the readings. Thanks.
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