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Old 07-12-2008, 11:44 AM   #1
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Another new VX owner

I just acquired a '92 VX (teal). Man, are they hard to come by, especially a stock VX! I'm not a mechanic, so I've learned a lot already from this site.

Here's an issue I have: It's very difficult, if not impossible, to maintain a steady speed (at 55 or 60 mph for example). While trying to maintain a speed while highway driving (though I've noticed some of this behavior at other speeds), the engine hesitates or stutters, and then starts dropping speed. When I apply slight pressure to the pedal to maintain the speed and break through the hesitation, it then slowly keeps accelerating. So, I back off and the process starts all over again.

I've read a lot of threads on this site about O2 sensors, but I have no idea if that's the issue.

Only once has the CEL come one. About 10 minutes into highway trip, the light came on, but then actually started running much smoother and I was able to maintain a cruising speed. But that was only once, it's back to the behavior mentioned above. BTW, the shift indicator light rarely comes on.

I've only put about 600 miles on it so far, but I've put gas in it twice and recorded 45 mpg and 50 mpg, even running like I've described.

We added new plugs and filters, as well as a timing belt and rear muffler. Just the other day I bought some stock size 165/70 13 Sumitomo HTR T4s. I don't mind putting some money into this car to get it up to speed, so to speak. Price a hybrid lately, even a used one?! I'll still be ahead by $thousands before it's all said and done.

Thanks,
Rick
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Old 07-12-2008, 12:03 PM   #2
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try changing the spark plugs, but since you had a CEL, maybe your O2 sensor is the problem, but the spark plugs will be much cheaper to try first.

Make sure you get the good ones. "The VX uses NGK V-Power Spark Plugs No. ZFR4F-11".

And here is a good thread about the Civic VX: http://www.gassavers.org/showthread.php?p=49664
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Old 07-12-2008, 02:28 PM   #3
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I had the exact same symptoms you described with my first VX. I could not maintain a steady speed by holding the gas pedal in one position. I had to keep "goosing" the pedal to avoid decelerating. I also got a code 48 - LAF sensor indication when driving at highway speed every once in a while that would disappear upon a restart.

I never did find out why my first VX had the throttle issue because I sold it in 2002. I have read that the throttle position sensors of Civics occasionally fail to operate properly through their full operating range. You should check the output of the TPS sensor - somehow.

Changing the LAF sensor (aka the O2 sensor) might help you as well. I never changed it on my first VX and have not changed the LAF on my current VX either since I have been getting 50 mpg which indicates to me that the sensor is still good.

Keep us posted on your progress. I've always wondered what the cause of the throttle problem was.
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Old 07-12-2008, 02:41 PM   #4
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If the TPS is anything like one I've tested before, it's easy to test with a multimeter set to measure resistance. Just hook it up and watch the meter as you move the TPS through its range.
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Old 07-12-2008, 03:44 PM   #5
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OK. I checked my plugs and had NGK ZFR5J-11, and gapped a bit too small. Got excited thinking that must may be it. I have a parts store just a few blocks away, so I got the ZFR4F-11, gapped them correctly, and went on a test drive. Results:

Same symptoms, but then at around 5 miles out, the CEL came on and it immediately ran smooth and I could hold the speed with no problem, just like the only other time the light came on. Went 5 more miles, stopped, shut it off. Then drove 10 miles back and CEL did not come on and ran rough again.

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Old 07-12-2008, 04:14 PM   #6
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I read up on how to check the CEL code from the instructions found on this site. It's giving me a code #48. LAF sensor issue, and is that the same thing as an O2 sensor?
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Old 07-12-2008, 04:38 PM   #7
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Edit: I'm a dumbass. If you remember what I deleted here, ignore it. The rest should still apply.

When the CEL comes on it's likely that the computer is ignoring the bad LAF data and using pre-fabricated lookup tables to guess what value the sensor might provide if it were good. That results in it running smoother but not as efficiently and clean as possible.
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Old 07-12-2008, 05:07 PM   #8
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HOLY COW!!!!!

The LAF stands for Linear Air Fuel and it is a wideband O2 sensor that measures the amount of oxygen in the exhaust. It is screwed into the exhaust manifold. It is capable of measuring air fuel mixtures from 12:1 to 25:1. Its response is generally linear hence the acronym.
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Old 07-12-2008, 07:13 PM   #9
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Perhaps this is just semantics, but I don't feel comfortable with the way theholycow is putting it. Yes, when the CEL comes on, you're going into limp mode and ignoring the O2 sensor which is clearly bad and should be replaced in your case. However; the O2 sensor is just a feedback device. It's not the basis of the fuel injection system.

The fuel map aka "prefabricated lookup table" is table of fuel injector time values. How much fuel is sprayed into your engine is controlled by how long the injectors stay open. One axis is load, or how much air is drawn into the engine, and the other is RPM. In a perfect world your car would run forever on this table resulting in exactly the air fuel/ratios that the OEM intended. However, there have to be corrections applied to these values to account for changes in air temperature, sudden changes in throttle, coolant temperature, low battery etc. All these things affect the actual air/fuel ratio that your engine burns. Your O2 sensor is a sort of a catch-all for anything else that might affect the air/fuel ratio. The engine wears down, sensors wander from their original calibrations, etc, etc. Sensors really do wander away from their original calibrations. Gunk builds up on them quite rapidly.

So in other words, your engine runs on this fuel table all the time. Not just when you're in limp mode, but always. It is the basis of your fuel injection times, not the O2 sensor. The O2 sensor, along with the TPS, IAT, CLT, and battery voltage are all taken into account to make small corrections from this table that affect the actual output to the injectors. When you're in limp mode, all sensors, including your faulty O2; are ignored and no corrections are applied to the fuel map.

Most OEM's run very rich mixtures on their fuel maps and rely on the sensor trims (especially the O2 sensor) to pull the mixture to stoich. They do this to avoid getting murdered on warranty claims and to avoid getting a reputation as an unreliable brand (your engine could blow up if it runs lean). But ideally, the fuel map would be tuned in such a way that you could disable your O2 and run near optimal air/fuel ratios without it until many years down the line as your sensors and your engine wear down and you'll have to re-tune the fuel map.

I have a wideband O2 sensor on an aftermarket programmable ECU that was professionally dyno tuned. I run virtually the same air/fuel ratios whether I have O2 sensor feedback enabled or not.

BTW: The ECU will often store a code so that even if the CEL is not on permanently, you can retrieve it and see what it was trying to tell you.
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Old 07-12-2008, 07:39 PM   #10
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I had the exact same symptoms too. Changing the spark plugs to the right ones helped, but the real fixer of the problem is a new o2 sensor. The reason the car went smooth when the CEL came on is because the overworked, tired o2 sensor was no longer doing its job (the car was probably in open loop and operating very inefficiently)
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