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Old April 16th, 2011, 08:18 PM   #1
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bad thermostat = bad gas mileage?

so ive been getting horrible gas mileage on my 07 accord i4 ~24k miles. about 15mpg mostly city. one of my friends suggested that the thermostat may be bad. what do you guys think? is it easy/cheap to replace?
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Old April 16th, 2011, 09:05 PM   #2
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bad o2 sensor = bad gas mileage. not thermostat.
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Old April 16th, 2011, 09:43 PM   #3
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but i only have 24k miles on it. do the o2 sensors go bad that easy? are they covered under warranty? i was going to swing by the dealer the other day to check on it but they wanted $90 just to "diagnose" it. so i said forget it.

wow i used alot of "its" in that sentence....
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Old April 16th, 2011, 10:41 PM   #4
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if the thermostat is stuck open it could cause bad mileage, but its more likely an o2 issue.
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Old April 17th, 2011, 01:11 PM   #5
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i have never truely seen a honda thermostat fail.unless its got 150k plus.i never replaced one....02 sensor if theres any issue with an o2 your gonna get a engine light.possible bad gas, do you start ur car and let it warm up in the morning?or do you drive the hell out of it.lol.
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Old April 17th, 2011, 02:35 PM   #6
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i have never truely seen a honda thermostat fail.unless its got 150k plus.i never replaced one....02 sensor if theres any issue with an o2 your gonna get a engine light.possible bad gas, do you start ur car and let it warm up in the morning?or do you drive the hell out of it.lol.
Often times the o2 sensor won't throw a code. The ecu reads it based on range and unless it passes a range or unplugged or bad noCEL will occur
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Old April 17th, 2011, 08:40 PM   #7
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^^ +1 I had a bad o2 sensor but it didnt throw a code. Only way to really know is take it out and look at it. That still is not a perfect way but it cant help to do a physical inspection.
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Old April 17th, 2011, 09:24 PM   #8
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yea i always thought i would get some kind of light/code if an o2 sensor is bad, so i never thought that would be the problem. are those sensors covered under warranty? cause theyre pretty pricey.. especially like 4 of them or something? not sure how many there are in the car haha. and i usually dont let it warm up cause ive seen a few threads saying warming up the car isnt really needed for newer cars? maybe ill let it warm up for a few minutes before moving from now on..
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Old April 18th, 2011, 07:19 AM   #9
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yea i always thought i would get some kind of light/code if an o2 sensor is bad, so i never thought that would be the problem. are those sensors covered under warranty? cause theyre pretty pricey.. especially like 4 of them or something? not sure how many there are in the car haha. and i usually dont let it warm up cause ive seen a few threads saying warming up the car isnt really needed for newer cars? maybe ill let it warm up for a few minutes before moving from now on..
Normally o2 sensors are not covered under warranty unless throwing a code during your warranty period. However since you have such low mileage I would take your car to your dealer and see what they say. Please keep us posted on your outcome.
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Old April 18th, 2011, 07:26 AM   #10
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Hmmm......this might be my problem too. I have a CEL that has came on 3x every ~1K miles for the past 3K. I've also been getting bad gas mileage, around 275 to a full tank. I drive spiritedly but not all the time. I used to get at least 330-350. I'll have to check mine!
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Old April 18th, 2011, 08:14 AM   #11
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make sure you only use Denso or NTK sensors.
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Old April 18th, 2011, 09:10 AM   #12
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Hmmm......this might be my problem too. I have a CEL that has came on 3x every ~1K miles for the past 3K. I've also been getting bad gas mileage, around 275 to a full tank. I drive spiritedly but not all the time. I used to get at least 330-350. I'll have to check mine!
i have this exact same problem and i have an AT av6 coupe 2003. is there a procedure in determining if the o2 is going bad? can i find out by going to autozone and getting the code read? im getting 270 to a full tank and i dont think i drive very aggressively especially since i got the drop. i got an alignment just in case to see if thats causing the bad gas mileage but im still getting 270 to a tank.
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Old April 18th, 2011, 11:12 AM   #13
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Check owners manual or call Honda, I'm pretty sure the emmisions parts have a factory extended warrenty way past the 36/36 warrenty.
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Old April 18th, 2011, 01:23 PM   #14
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How are you calculating your gas mileage?
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Old April 18th, 2011, 02:15 PM   #15
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i know this isnt the proper way but i just wait until fuel light comes on, fill her up and then clear the mileage just to get a general idea of how much im getting. when i first got the car she was ~310 to a full tank.
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Old April 18th, 2011, 02:31 PM   #16
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Fill my tank all the way up, reset odo, and drive until my next fill up, divide miles driven by the amount of the 2nd fill up. So I usually get around 210-220 miles out of one fill up (I try not to drive to the light, probably around 2 "bars") and fill up around 13 gallons. So around 16mpg.
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Old April 18th, 2011, 02:45 PM   #17
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i get better gas mileage on my tuned j35a8... :\
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Old April 18th, 2011, 03:38 PM   #18
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how can a bad o2 sensor give bad gas mileage ?
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Old April 18th, 2011, 03:51 PM   #19
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how can a bad o2 sensor give bad gas mileage ?
because an 02 sensor gives your ECU readings and based on the readings your ECU tells you injectors how to work.
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Old April 18th, 2011, 03:58 PM   #20
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called dealer, wanted $90 to "check it", went to mechanic i usually go to and asked about the o2 sensors, said thats probably not the reason why since theres no light. told me to try using 93 gas. and my TL-S rims may be one of the causes as well.. regular gas + TL-S + city driving = 16 mpg?
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Old April 18th, 2011, 03:59 PM   #21
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called dealer, wanted $90 to "check it", went to mechanic i usually go to and asked about the o2 sensors, said thats probably not the reason why since theres no light. told me to try using 93 gas. and my TL-S rims may be one of the causes as well.. regular gas + TL-S + city driving = 16 mpg?
mechanic is an idiot. dealership charge is understandable.

TL-S rims and regular gas should not make that much of a difference.
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Old April 18th, 2011, 04:19 PM   #22
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i get better gas mileage on my tuned j35a8... :\
With that stroke, I'll bet, lol.
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Old April 18th, 2011, 04:26 PM   #23
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i always think the mechanic i go to dont know ****. lol. but my family goes to him so i was like might as well ask him about it. how many o2 sensors does the 4 cyl have? (im sayin 4 cyl cause im not sure if the v6 has a diff amount of sensors). im guessing two cause i went to the denso site and it has a upstream and downstream. is it easy to replace yourself?

i might try ONE tank of 93, and try to switch back to my stock hubcaps and see if that fixes anything.. if not.. then i dunno, might look more into the o2's.

i also see some are like "california equipped" what does that mean? i have a SULEV engine and i heard cali is like the only place with them. weird cause i bought mine in NY. can anyone give me the exact part # for the right sensors? so i have an idea how much i have to dish out if it comes to that.
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Old April 19th, 2011, 11:32 AM   #24
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because an 02 sensor gives your ECU readings and based on the readings your ECU tells you injectors how to work.
Yeah, Stock cars are tuned to a slightly richer A/F ratio. Its the o2 sensor's job to read how much fuel is being burnt and to tell the ecu to adjust the fuel. If your o2 sensor is shot or "responds slowly" it cant make the proper adjustment and you burn more fuel.

If you have only 24k you could try to reconnect the connectors in the ecu and o2 sensor. I was having problems with my o2 and it was due to a loose connection. I have k-pro and most of the time the sensor went offline without throwing a code. (I could see it in my datalogs)
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Old April 20th, 2011, 10:43 AM   #25
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To fully understand Programmed Fuel Injection Systems it helps to have a college education in Automotive Technology. Correct, oxygen sensors do operate in a certain range. Between 0.1 and 0.9 volts. The Powertrain Control Module constantly monitors O2 sensor parameters (value). When the O2 sensor reading drops below 0.1 volts the oxygen sensor is biasing lean. When the O2 sensor reads above 0.9 volts the oxygen sensor is biasing rich. So looking at a O2 sensor on a oscilloscope (graph), the reading constantly jumps from 0.1 volts to 0.9 volts. This reading changes due from the amount of oxygen in the exhaust stream. By changing the amount of oxygen levels in the exhaust stream this makes the efficiency of the catalytic converter higher. The catalytic converter needs to trap oxygen atoms to convert hydrocarbons into H20 and CO (carbon monoxide). So, now you understand what the oxygen sensor does, the engine is constantly changing the injector pulse width. This creates the up and down change in the amount of oxygen in the exhaust. This is done to get the fuel ratio at "stoich" or stoichiometric efficiency. Stoichiometric Efficiency is when is engine is running to its greatest efficiency possible and without excess hydrocarbons coming from the exhaust. When stoichiometric efficiciency is achieved, you are getting the best possible fuel mileage the vehicle can get. Thermostats only cause fuel mileage concerns when the engine is overheating or when the engine is not able to reach normal operating temperature. Hence when the engine is cold the fuel injector pulse width is increased (Dumping fuel in the cylinders to warm the engine faster). The engine must be at normal operating temp to go into "closed loop" (Using O2 Sensors to adjust fuel to reach "stoich") Suggest stop reading forums with a bunch of uneducated jumble in it to help you fix your car and start listening to your educated technicians. A new thermostat won't fix your fuel mileage problem.
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