Hi folks,
As many others have posted, i am currently having a somewhat decreased MPG experience. Purformance-wise everything is great. I am not going to ask what i need to do to improve MPG - i searched and there is plenty of info here. And even before i searched i knew i need to replace my spark plugs - the new ones are in my garage for a few months now and waiting to be put in...

However, i have been wondering how can the mileage deteriorate and the CEL not throw a code? if i drive the car the way i did 1 year ago and got 300-350 miles a tank and now the same driving gets me 220-250 on the same routes then that means i am using more gas for the same amount of air and at some point the computer should be able to figure that out and tell me that i am running too rich, or not burning all the gas in the mixture or whatever (after all that is the O2 sensor bunch's job). All of the above leads me to believe that the only explanation is mechanical issue that is not tracked by the ECU. But my car appears very solid. No, i do not have a stuck caliper and yes, my tire pressures are good, around 33psi.

Car is 2002 max MT with no mods whatsoever ... i have a weekend toy that gets all the goodies

 

Well, the O2 sensors can easily deteriorate beyond the point of even somewhat-nominal functioning without throwing any codes. Read all about them here.

Basically, as the sensor ages, two things happen:
1. Crap builds up on the sensor itself where it sticks into the exhaust stream. This crap reduces the sensor's ability to quickly detect changes in oxygen content, resulting in the engine computer being unable to change the fuel trim from rich to lean, or vice versa, at the proper speed needed to run it at maximum efficiency--several times per second at even low RPM.
2. The sensor slowly puts out fewer volts as it ages. The ridiculously brutal environment the little guys hang out in for the duration of their working life results in the eventual death of the electrical bits. As they die, they put out fewer volts. When the engine sees low voltage coming from the sensor, it reads that as "the exhaust has too much oxygen, enrich the fuel mixture to burn more of it." This rich condition kills fuel economy, hurts power, and will dramatically shorten the life of your catalytic converters.

My O2 sensors are dying, but haven't thrown any codes. I'm not going to wait for the important two, the ones not there to simply monitor the cats, to die completely before changing them.



From the FSM:
"The mixture ratio feedback system provides the best air-fuel mixture ratio for driveability and emission control. The warm-up three way catalyst can then better reduce CO, HC and NOx emissions. This system uses a heated oxygen sensor 1 in the exhaust manifold to monitor if the engine operation is rich or lean. The ECM adjusts the injection pulse width according to the sensor voltage signal. For more information about the heated oxygen sensor 1, refer to EC-240.

This maintains the mixture ratio within the range of stoichiometric (ideal air-fuel mixture). This stage is referred to as the closed loop control condition. Heated oxygen sensor 2 is located downstream of the warm-up three way catalyst. Even if the switching characteristics of the heated oxygen sensor 1 shift, the air-fuel ratio is controlled to stoichiometric by the signal from the heated oxygen sensor 2.

(pg 249)Under normal conditions the heated oxygen sensor 2 is not used for engine control operation."

 

My car (2000, AT, stock, 93K) has done the same thing, gets 3 to 4 fewer mpg than it used to get over a year ago.

So a question I have for Erik regarding your point # 1 - do you think that the O2 sensor(s) could be cleaned and therefore improve/correct this situation?

 

thanks Eirik - very nice explanation that makes a lot of sense. I did replace one sensor at 90k(car now has 120k) but it was a while ago and i can not remember if it was 1 or 2. I will check to see if i can find out the code it was throwing...

I guess the question really will be do i want to spring a couple of hundred bucks to replace the the two oxigen sensors if the MPG still does not improve once i do the usual culprits on the maintenace list.

 

at my worse gas mileage i've never gotten below 330 miles to a tank...i've gotten upwards of 400 miles to a tank on mine...its crazy you lose gas mileage THAT bad. sorry i cant answer the original question, just wanted to put that out there

 

That's something you'll have to answer on your own, I suppose. Obviously, I don't know your financial situation, nor can I provide exact figures on what will change if you put in new O2 sensors. If we knew, exactly, what it would do to your fuel economy, we could calculate how long it will take before they pay for themselves.

Research the sensors and see what luck people have had with aftermarket sensors. I know Denso makes them and people have had good luck, if I recall, with them. Bosch and the usual domestic subjects make them, too. I do know one 4th gen owner who tried several different Bosch electrical items and found they were far inferior to their OEM/NGK counterparts. I don't remember if he bought a Bosch O2 sensor or not. You'd think that, since Bosch invented them, theirs would be the best.

Courtesy Parts is listing the Sensor 1s for $132 each, so figure DaveB will give them to us for ~$120. RockAuto has the Densos for a bit less than $80 each. I plan on buying two new Densos from them.

For me, I am making a conservative wager on an improvement of 2 miles per gallon and, with gas at $3 a gallon, even that modest an improvement will pay for the sensors after roughly one year.


Stores sell Oxygen Sensor Cleaners, I know, but will it actually improve anything? I think the science suggests it won't do much. Since the electrical failures from the aging wires and deteriorating elements in the sensor can't be fixed by cleaning off soot... I've read that, in some ways, the O2 sensor gets "used up" like the catalyst inside the cats as it ages.

If you think the labor involved in pulling them off, as well as the ~$12 cleaning investment, outweighs the benefit of just buying new plugs, then I'd suggest giving a cleaning a go. Personally, I don't think spending $250 on O2 sensors every ten years is asking too much.

 

I completely agree.

But first, OP, if you really want have a clue how much gas you are using with any accuracy, do not quote "miles per tank" as it tells you about as much as a guesstimate. Make the effort and calculate your actual fuel economy over 1-3 tanks. It's not that hard, seriously. If you think of it, monitor where/how you drive, avg trip distance, etc. This can all cause a change. btw how many miles does the car have?

Then, if you feel it is using significantly more fuel, do a simple cost vs. benefit analysis. For example (let me spoon-feed you):

12,000 miles per year
$3.50 per gallon (premium)
22 mpg (last year)
19 mpg (this year)

Total cost per year @ 19 mpg: $2210
Total cost per year @ 22 mpg: $1909
Difference: $301

Cost to replace O2 sensors: $250

Worst case scenario: no improvement, required maintenance performed a little bit early, eventual failure/crappier mileage avoided
Best case scenario: fuel economy improves, "save" money on gas, car runs better, you feel good because you solved the problem

I say go for it.
fyi "3 to 4 fewer mpg" is not that helpful without a reference value. For example, a decrease of 4 mpg corresponds to a decrease of 17% at 24 mpg, but a whopping 25% at 16 mpg, or just 12.5% at 32 mpg.

And, yes, I am an engineer.

 

One of the best posts I've seen on the org since I joined. I have the same problem with deteriorating gas mileage but I had somewhat dismissed the o2 sensors as being the problem because I don't have any CELs.

Great information here, thanks a ton. This is an example of why car forums are great.

 

Eirik - thanks! When i replaced mine i got it from DaveB and while i do not remember the exact price i remember i felt it was very reasonable for a oem part.

tcb 02 - please try to add value to the original discussion when you post - there is really no need to teach people basic math. Do not make recommendations for something that is not being asked please.

thanks,

 

Actually, he provided more in depth information which is what you asked for in your original post. Do you recall?

And he is right, you need to calculate actual MPG, not the miles per tank to do a proper calculation. Furthermore, he provided some great insight on a cost benefit analysis if replacing the O2 sensors is even worth it.

If you don't like posts people provide, then don't make a thread. Be prepared to deal with any and all feedback if you start a thread. You can take your panties out of a bunch now.

 

OK, here's your reference point - I used to get 20 to 21 mpg and now I am getting 17 to 18 mpg. Now you can calculate a percentage.

And once you have calculated said percentage, how will that improve my gas milage?

 

My point was simply the following: in order to have a quantifiable measurement of how much more fuel is being consumed, you have to calculate it as a percentage. For example, under 5% change is probably insignificant, as it can be due to things like temperature, calculation error (in the measurements of miles, gallons, etc. - not your math), or driving style/traffic. Greater decreases would indicate a significant change, and would point you in the direction of sensor failure, etc. I am just encouraging you to be more scientific about how you approach the problem in order to take some of the guess work out of it.

And you can calculate that yourself.

 

Guys, i surely do not want to get into a spit-spat here, my only point is there is a topic in the first post and i would really like to keep the discussion on it. Do not need anyone telling me you need exct mpg rather than per tank - i have that data but is not pertinent to the questions i am sking. If someone wants to teach people basic math they can start a separate thread and do that.

Anyways, enough time and energy on this side issue. Thanks all for posting and pitching in.

 

You are absolutely correct on all your points.

I saw your post, and felt I had a similiar situation. When I read Erik's response, he provided some information that I did not know and I asked him a question to which he responded with a to-the-point answer.

And I thank Erik for that.

 

I usually get right around 300 average I drive about 50/50 city & highway. You have 5 spd?

 

I'm thinking this thread may be the answer to my mileage issues

I bought an 02 SE about 5 months back and have never been happy with the mileage she gets (that and the clunky shifter are my only issues). She got a quick spruce up early on with new oil, plugs, air filter, etc and there was minimal improvement... the MAF swap to a 2k1 hasn't helped mileage much either (that said, I am happy with the switch).

So today I checked the voltage of the O2 sensor and got basically a 0.1v to about 0.7 readings... Since my understanding is I should see 0.9 about as much as 0.1 I'm thinking maybe the O2 is on it's way out...

Of course this requires the tenuous assumption that I know what I am doing and measured off the right wire on the right sensor...
(no seriously... I am a newbie on this)

Great forum... for a great car!

Incidentally, stats: 160k km, mileage city best 17mpg (80 city), hwy best 24mpg (90% hwy) ... and that's light footing it 90% of the time. (edit: 95% light footing to be honest)

 

Answer in original post:

 

+1

While I usually can't stand OT replies in a thread, I have to say that it is pertinent to the question. The math is a huge part of the assessment. If you've been around this forum long enough, you'll know that people do not accurately track their gas mileage. Which is fine, unless that person is trying to use that data to diagnose a problem. I believe tcb_02 may have been addressing an important part of the puzzle for others besides the OP that may not be aware of the correct method and are reading this thread. Even more so, threads like this are often found during a search by someone with a similar problem. We might as well clarify the correct procedure for diagnosing. Again, judging from the hundreds of posts that pop up week after week about 'I get blah-blah miles per half tank' there really is a need for repetition of this point as often as possible.


OP: OAN, great thread. Inquisitively thought provoking.

 

The gaps in my plugs were about double what they were supposed to be (doubtful the original owner ever changed the plugs) and it wasn't until there were over 150k miles on the car the plug gaps got bad enough there was a misfire and an SES light. With new plugs I am getting about 3 mpg better on the freeway than I was with the old plugs.

 

Just to provide a contrary data point, when I changed my plus at 140+ K miles, the gap was more than doubled. After the plugs and coils change, the car ran as good as before with no changes either in mileage or performance. I get 30 in summer and 25 in winter depending upon how easy I am on the gas pedal. This is 2K 5-spd 3.0 and roughly 80/20 hwy/town split.

By the way, always compare your seasonal mileage. At least in New England, the difference between winter and summer mileage is quite big on all vehicles that I own.

- Vikas

 

There are differences here in Texas due to the winter gas blends and the unusual cold we've had the last two years. Usually lose 2 mpg in the city and 1-2 on the freeway.