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Old 11-02-2008, 02:56 PM   #1
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'98 Civic HX poor fuel economy

Hello all,

The fuel economy on my wife's '98 federal HX with 124,000 miles on it is worse by 5% - 11% versus the previous two tanks. I'm sure some of it is the result of colder weather, but not all of it. Her driving hasn't changed at all (daily 85 miles roundtrip, 98% highway with the possibility of 4 traffic signals along the way, some rush hour traffic periodically, 70 mph avg speed). Last year at this time she was getting 45 - 47 mpg. This year she's getting 39 - 44 mpg. The only thing that's changed since last year is a set of new tires (Nokian i3 LRR). Perhaps some of the difference in FE versus last year is due to a larger tire diameter now registering fewer miles. But the tires do coast well, so I'm certain they are LRR.

In the past 1.5 years, the CEL has come on periodically, and the code is for the primary 02 sensor. Each time I disconnect the negative battery cable and reconnect it to reset the ECU. There has never been a change in FE during these periodic CEL episodes. During this latest drop in FE, the CEL has not come on. I did a tuneup at 109,000 miles (plugs, cap, rotor, air filter, valve adjustment). The engine appears to be running well as always...no symptoms of any problems.

Does anyone have any suggestions about the cause of the poor FE? If it's the O2 sensor, is there a test for it? And if it's bad, are there any recommended aftermarket O2 sensors that are cheaper than the $300+ Honda O2 sensor?

Many thanks for any suggestions.

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Old 11-02-2008, 03:44 PM   #2
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On a 1996+ vehicle you should never unplug the ECU or reset it w/out diagnosing and then fixing the problem, then scanning the ECU to confirm that it's fixed.

Your car is OBD2. OBD2 stands for On Board Diagnostics. What that means is that your ECU runs tests on all of it's systems related to emissions which is just about everything. It takes certain conditions and hence time for the ECU to complete these tests.

For instance, an EVAP test requires the gas tank pressure to rise above a certain point and then fall below a certain point. That way it can measure whether there is a leak in the system. This means that it has to pressure up and then cool down, in other words, you have to drive a long time on a hot day and then let the car sit overnight. In the winter in a cold climate, you may never complete this test.

When the test has not been completed, the ECU registers "not ready". The problem arises when it's time to take your smog test. Different states have different criteria. In Utah, 2001 and newer vehicles are allowed 1 not ready, and 2000 and older are allowed 2. Every day as a smog tech I have to reject vehicles because their battery is bad so the ECU doesn't store the results of it's self-diagnosis. Some cars take months to complete these tests, particularly 1996 and 1997 Nissans and Mitsubishis.

Your ECU is telling you that your O2 sensor is failing the test. Then you're resetting the ECU. It takes time for it to complete the test again, at which point it fails, lights the CEL, you disconnect the battery and then start the process over again. Resetting that light doesn't fix the problem and your poor FE is a symptom of that.

Your O2 sensor has failed to the point that it's dragging down your FE enough for you to notice, or worse, more systems have failed as a result of you continuing to drive on a faulty O2 sensor, and THAT is what's dragging your FE economy down enough for you to notice.

Stop fighting the system and get your car fixed, man.


Civic VX, D15Z7, 5 Speed LSD, AEM EMS, AEM UEGO, AEM Twin Fire, Distributor-less, Waste Spark
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Old 11-02-2008, 04:02 PM   #3
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Old 11-03-2008, 07:31 AM   #4
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if the code has come back repeatedly after resetting the ecu. the the sensor is probably bad and you should replace it. a bad o2 sensor can cause the car to run rich meaning more fuel, meaning bad fuel economy. suspended hatch already covered this, but the best test is to switch out a known good o2 sensor.

also your sensor is 5wire and so the added cost comes with the territory of the hx.
don't waste your time or time will waste you
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Old 11-03-2008, 08:09 AM   #5
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Thanks for the information, and a couple of points of clarification:

The reason I kept resetting the ECU after the CEL periodically threw the primary O2 sensor code is because someone on this forum had recommended trying that awhile back when the car had about 85,000 miles and the light came on for the first time. My experience with my former '95 VX was that the O2 sensor lasted about 120k - 130k miles, so 85,000 miles seemed a little premature for O2 sensor failure. And absent any other symptoms from that first CEL until the most recent one about 2 months ago (i.e. no decrease in FE, no drivability problems), I continued resetting the ECU. It has only been recently that the fuel economy has become noticably worse....and without any CEL.

It sounds like the consensus is that the sensor needs replacing, so I will do so unless there are any other possibilities for the poor FE.
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