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Old 03-03-2011, 04:37 AM   #1
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Default poor gas mileage

i actually posted this somewhere else first but didn't get a response so maybe someone here can help me out:

1999 4 cyl. 4 dr honda accord ULEV automatic trans

right now i am getting horrible gas mileage - about 17 mpg highway and maybe 10 city. i have no CEL light, engine starts, runs fine. idle rpm are normal (about 650) - everything seems fine except for the mpg. prior to this problem mpg was good about 27 highway. just one week all of the sudden i had to fill up about 60 miles before i usually do (300 miles vs the usual 360 miles).

8 months ago i had a CEL code for the upstream 02 sensor, along with some stalling. changed that with honda oem part for ULEV and the code went away and so did any problems.

i just changed the downstream 02 sensor to see if that would help with the problem but no change.

can someone give me a list of things that i could try and diagnose myself?

i have a scanner and have monitored the 02 voltage but i am not sure how to interpret it. the upstream voltage is about 0.77 v and pretty much stays there from cold start to warm up to hot throughout driving. it might fluctuate a little, but mostly stays at 0.75 to 0.80.

the downstream sensor goes from 0.11 to 0.80. i have read that an 02 sensor that doesn't change much may be bad so could it be that the upstream one went bad in 8 months? also i thought it would give a voltage of 0.45 v when bad. is there any way to check besides spending $150 for a new one?

maybe i am way off thinking that its the 02 sensors. everything seems to be fine with the car otherwise. pcv valve is working but that is all i have checked. i do not see any fuel leaks on the ground, tranny shifts great, wheels move freely...

thanks
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Old 03-03-2011, 05:36 AM   #2
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Default Re: poor gas mileage

Always a bunch of stuff -

Tires (both type and air pressure)
Alignment (haven't had one in awhile?)
Fluid changes (coolant, oil, tranny, etc)
Air filters (clogged/dirty air filters = )
O2 sensors (you already changed these, did you change the downstream with OEM?)
Spark plugs and wires (relatively cheap to replace both of these)
Injectors (just pull them and look for any gunk or whatever)
EGR ports (on 5th gens these are a bigger deal, I believe the same applies for F23 motors)
etc
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Old 03-03-2011, 07:43 AM   #3
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Default Re: poor gas mileage

tires, fluid, alignment, plugs are all good/newer. next i will check injectors and egr ports.

can anyone comment on the readings that i posted from the 02 sensor? my thinking is that there is something going on with that upstream sensor. it doesn't fluctuate all that much (= bad sensor?) and the reading is high (because the sensor is bad or because its running that rich?).

i'm inclined to just buy another o2 sensor and try it but thats a lot of money. i can also get more info from my scan tool if that would help any diagnosis.

i am having such a sharp decline in mpg (city driving got 66 miles to the half tank last fill up).

i did replace the downstream with OEM also.

thanks again
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Old 03-03-2011, 01:31 PM   #4
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Default Re: poor gas mileage

Do you travel often with weight in your car? It's possible that there is added weight on the vehicle to reduce your mpg. It could also be your fuel pump? Possible miscommunication with computer? Good luck.
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Old 03-04-2011, 12:00 AM   #5
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Default Re: poor gas mileage

The oxygen sensor voltage is supposed to fluctuate like crazy. If it doesn't then it's either a vacuum leak that has to be fixed and retested or the sensor is bad and can't do its job. I would atleast pull it and look at it. You will see it either black or white on the part that sits in the stream. Or you can just buy one if you've never bought one for it before. I would go oem, usually denso, from rockauto.com and call it a day.

Normally you will smell unburned gas from the oxygen sensor overfilling the chamber when the sensor starts going back though.
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Old 03-04-2011, 05:08 AM   #6
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Default Re: poor gas mileage

no extra weight in the car...

so you think the fact that the upstream sensor doesn't fluctuate could mean that its bad? what i am asking is, is if it were a vacuum leak, that would make the engine run rich and constantly rich?

i have already replaced both the upstream and downstream sensors (upstream 8 months ago, downstream just recently) with oem parts directly from honda.
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Old 03-04-2011, 12:42 PM   #7
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Default Re: poor gas mileage

Seafoam the gas tank.
New fuel filter.
Vacuum leak.

As far as O2 sensors go, I'm pretty sure that a car dropping from 27 mpg to 17 highway is a BIG stretch, esp if you have already replaced one of them.

And judging from you post if you were getting 27 mpg on the highway then you are a normal driver, I had a 1997 Honda Accord Se Coupe - Auto and I drive hard... anyways I got 25 mpg highway with it.
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Old 03-04-2011, 03:13 PM   #8
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Default Re: poor gas mileage

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no extra weight in the car...

so you think the fact that the upstream sensor doesn't fluctuate could mean that its bad? what i am asking is, is if it were a vacuum leak, that would make the engine run rich and constantly rich?

i have already replaced both the upstream and downstream sensors (upstream 8 months ago, downstream just recently) with oem parts directly from honda.
Yes, it's supposed to fluctuate as the sensor constantly senses the oxygen ratio. Yes because it would be getting too much oxygen. But if you replaced the sensor then you might not be reading the readings correctly.

You could pull the plugs and see how they look. If they're black then you got too much fuel going in.
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Old 03-04-2011, 04:58 PM   #9
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Default Re: poor gas mileage

High is rich and low is lean.

I do not think the problem is with the oxygen sensor.

If the sensor were to be biased high ( rich ), the fuel command would be to lean the mixture, not richen it.
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Old 03-07-2011, 04:24 AM   #10
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Default Re: poor gas mileage

yeah i think you're right. i think the sensor is fine.

i pulled the upstream sensor and there was black soot kinda stuff on the base of it. i guess i'll move on to the egr.

i guess the reason why i am not getting a code is because the richness is within specs for normal?
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Old 03-11-2011, 12:41 PM   #11
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Default Re: poor gas mileage

.45v is what it should fluctuate around, if you read down to .2 adn up to .7, than you o2 sensor should be fine. it is reading rich to me. explaining the horrible gas mileage. now to find the problem. read the o2 again and try pulling a vacuum line (pcv valve). does the reading drop? is your timing correct? sometimes all it takes is bringing Everything back to factory spec. kijunglee1234@gmail.com

fuel pressure (not something you can just look at), injectors may be stuck open, intake filter, throttle body may be clogged, list goes on.
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Old 03-21-2011, 01:23 PM   #12
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Default Re: poor gas mileage

Im having the same issue. Replaced the upsteam o2 and it didnt help.
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Old 03-21-2011, 01:55 PM   #13
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Default Re: poor gas mileage

High is rich. Low is lean.
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Old 04-04-2011, 08:23 PM   #14
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Default Re: poor gas mileage

I have a friend with a Fit with the same problem. Went from 35 to 25. Last person I saw have a similar issue had filled up their oil too high and there was no response after that... I haven't actually been able to find a SOLUTION. I guess people figure it out and never bother posting the actual answer. So I'm posting mostly to follow this, since I'm the 'mechanic' friend.

Also my xB dropped from 33 to about 29 a year ago, and I have yet to isolate the problem. It's not significant enough for me to dump money into new sensors, I'm hoping I'll just get a CEL eventually before it gets any worse. ...I'm pretty sure DENSO makes Toyota, Subaru and Honda(??) ECUs, so the cause/effect should be the same.
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Old 04-05-2011, 05:25 AM   #15
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Default Re: poor gas mileage

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I have a friend with a Fit with the same problem. Went from 35 to 25. Last person I saw have a similar issue had filled up their oil too high and there was no response after that... I haven't actually been able to find a SOLUTION. I guess people figure it out and never bother posting the actual answer. So I'm posting mostly to follow this, since I'm the 'mechanic' friend.

Also my xB dropped from 33 to about 29 a year ago, and I have yet to isolate the problem. It's not significant enough for me to dump money into new sensors, I'm hoping I'll just get a CEL eventually before it gets any worse. ...I'm pretty sure DENSO makes Toyota, Subaru and Honda(??) ECUs, so the cause/effect should be the same.
i never found a solution, but my gas mileage has improved. its not back to normal, but i get around 24 mpg highway. and thats driving nice and easy. i guess its not that far off normal mpg anymore. if i use it for city driving it seems to get worse mileage than it should though.

i only changed the downstream sensor (upstream is 8 months old) but never had a CEL. maybe it was a few bad tanks of gas? i dont have much time to diagnose the problem, and i guess i'm not going to give my mechanic $100 an hour to try.

good luck finding out what it is.
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Old 04-05-2011, 07:01 AM   #16
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Default Re: poor gas mileage

You might try the fuel filter. When I changed mine I got a definite boost in efficiency. It's supposed to be changed every 40-50k.
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Old 08-14-2011, 11:11 AM   #17
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Default Re: poor gas mileage

I am experiencing pretty much the same problem with my '99 Accord EX 4 cylinder with auto transmission. The car has 134k miles on it and has been serviced "by the book".

I noticed the problem shortly after having the starter replaced (by a Honda dealer) on the road. I think that is unrelated, however.

Since then I have had the plugs pulled and I went into the shop to look at them. They were all the same and looked to be reasonably good for having 13k miles on them (conventional NGKs replaced at 120k mile service) although the gap looked rather large. (I did not attempt to measure it.) I had spoken with NGK tech support and had new NGK Iridium plugs installed. I did not replace the plug wires. The tech at the shop said they looked OK and should be good for 200k miles (current mileage is 134k).

The distributor shaft seal was leaking some oil and the rotor and cap contacts showed oxidation and fouling. Replaced the distributor body, rotor and cap.

Had a BG Induction system flush/cleaning performed although the throttle body was supposedly cleaned at the 120k service. I ran a can of BG44 through it after this service and have just put half a can of Seafoam in the tank of gas now in the car.

The A/C condenser fan motor was just replaced and A/C serviced. A 4 wheel alignment done while it was in the shop. Oil was changed. (I have used M1 5w30 since I got the car.)

The engine appears to idle and run normally. I have not experienced any check engine lights. Front and rear brakes were done (dealership) in the last year. I have 4 OEM spec Michelin tires within the last year. The car has always run the same spec Michelin tires.

I am in Central Texas and it has been hot (what's new?) so I run the A/C all the time this time of year. That costs a bit of mileage compared to the cooler months, but I used to get high 20s MPG cruising at 65-70 MPH. Around town in mixed surface and Interstate/Loop driving I used to get 22-24 MPG or a bit more in cooler weather without running the A/C. Recently I have been getting 22-23 MPG highway and 18-20 or a bit more around town.

I also replaced the battery somewhere in the sequence after the starter replacement when I noticed that it was not spinning the engine as quickly as it should. (It turned out one cell was going bad.)

I am not sure what to look at next. I would have expected either a check engine light or smoky exhaust with poor throttle response if an O2 sensor was involved. I am not sure whether an EGR problem would trigger a check engine light or not, but I would have expected an idle and/or drivability problem if it were acting up. Although the fuel filter was replaced about 14k miles ago, I suppose I should replace it "just in case".

All the "gas" in this area is that 10% ethanol variety. I run the 87 octane gas for which the car is rated. I normally use Exxon, Shell or occasionally Valero gas from stations that have a lot of traffic so I think the gas is fresh. Whenever possible I try to fill up at the same station and at least facing the same direction (because of the slope) if I can not get on the same pump. I fill to the first click and then to the nearest 5 cents (for no particular reason).

I would very greatly appreciate any thoughts or suggestions as I am about to run out of ideas.
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Old 08-14-2011, 02:39 PM   #18
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.45v is what it should fluctuate around, if you read down to .2 adn up to .7, than you o2 sensor should be fine. it is reading rich to me. explaining the horrible gas mileage. now to find the problem. read the o2 again and try pulling a vacuum line (pcv valve). does the reading drop? is your timing correct? sometimes all it takes is bringing Everything back to factory spec. kijunglee1234@gmail.com

fuel pressure (not something you can just look at), injectors may be stuck open, intake filter, throttle body may be clogged, list goes on.
Absolutely right!I would check the coolant sensor to start with.You won't have any problems science you got scanner.Check it first thing in the morning or at least 8 hours of sitting down.Check the corelation between coolant temperature and air(IAT) sensors .It should be almost identical.If the ECT sensor reads always cold the ECU will enrichen the mixture to the max.Another thing I would check if the fuel pressure regulator.By the way,how are the fuel trims?
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