Ok, i was just reading some of the tips for improving gas mileage as my 2000 F150 Xl with the 4.2 5 speed is getting about 13.5 mixed mileage. ( i have an airraid intake system also) It said to replace the oxygen sensors around 100000. I have about 115xxx on mine. Recently towing a car trailer with but 1000 pounds on it, i got 10 mpg. My question is, i have no codes thrown, but will this help my MPG by the 2-3 that they claim. If so, i believe mine has 2 upstream and 2 downstream sensors, should i do all 4 or just 2? And what tools would be needed (special tool or wrenches/deepwell sockets).

 

If you do not have a CEL on that is throwing a O2 sensor failure code, then I would not waste the time or money on replacing them. I would however look into cleaning the MAF and the air filter. Good luck!

 

They are a maintenance item and are definitely do but I wouldn't expect 2-3mpg gain from changing them. All 4 should be done and the upstream are different from the downstream just as a heads up. They should run around $50ea. anywhere except Ford who will want about twice that for the same sensor. You can get to two sensors with a simple open ended wrench the rest are a PITA to get to and will most likely require an O2 sensor socket that has a slit in its side for the wire to stick out. I've also had the best luck pulling them out with the exhaust being a little warm. The hardest thing is getting the wire clips undone though, usually you can get a hand on them and then do nothing to unclip them

 

it would definitely help, they might not be completely gone, but i bet they are nice and black

 

An O2 sesnor that is worn enough to throw a code is way bad. y the time they get that bad, you might as well kiss your expensive catalytic converters goodbye, and you will have already spent hundreds of dollars on wasted fuel. An O2 sensor by itself is not terribly expensive, but the components that can be destroyed when they go out are very expensive.

Go to your local parts store, and buy Bosch brand sensors, those are original equipment on most Fords. Replace both upstream sensors. The downstream sensors do not need to be replaced. They do not have any bearing on fuel management, so if the downstream sensors fail, the worst thing that can happen is that you have to replace them. I do however suggest that you remove them, put some anti-seize on the threads and put them back in. This helps ensure that if they ever do fail, that they won't be hopelessly stuck.

 

An O2 sensor will give different codes for a problem. You can get a lean, rich, heater, and most of all a lack of responce code if not working correctly! Replacing an O2 for no code and just for trying to gain fuel mileage is a waste of money! I wont argue they do recommend replacing at approx. 100,000 mi. but I see many run far longer than that. As far as ruining your cat, only if it's running way too rich and it will flag a code before it does! This is mostly told at parts stores trying to get your buisiness and money! Changing the fuel filter, air filter and plugs will accomplish much more if not done so yet. But after all it's your money and your decision! Hope this helps.

 

Thanks guys for the responses. I dont normally pay too much attention to these articles, but seeing it on this site made me wonder a little more. I may just go ahead and replace the upstream sensors and measure my mileage. Just to throw it out there, i used electric connection cleaner to clean my maf because i had heard of that before, would spraying that on it be sufficient or should i just get the maf cleaner and spray it on there?

 

That will work fine, I use brake cleaner.

 

This is one persons opinion, but my experience harshly clashes with this opinion. My business is just catalytic converters, and as such, it is my business to know why they failed, and what went wrong. So perhaps I should go into greater detail as to what happens when an O2 sensor starts to fail.

Lets start with the basic concept of what an O2 sensor does. Quite simply, it measures the amount of oxygen in the exhaust, or more specifically, it measures the difference between ambient oxygen levels and exhaust oxygen levels, and generates a voltage bases upon this difference. If the oxygen in the exhaust is too high, indicating a lean mixture, the signal strength will be low. In a low oxygen environment, the signal will be stronger, indicating a rich mixture.

The O2 sensor has two primary functions. The first is to ensure that the air to fuel ratio maintains a stichiometric average. The second function is to ensure that the oxygen levels in the exhaust are always changing. This second part has to do with catalyst operation, and is too complex to go into detail here. However, this means the O2 sensor has to meet two different criteria to function properly. Firstly, it must produce a signal that is accurate to within 2%, and secondly, it must respond to changes in the oxygen level fairly quickly.

So now, let me briefly explain what happens as an O2 sensor gets older. As the O2 sensor does its job, it is exposed to exhaust gases, which contain a variety of different compounds. Various hydrocarbons, some raw carbon, sulphur compounds, various metallic ions, etc. Over time, these compounds can coat the sensor element, weakening its signal strength. The computer has no ability to compensate for this loss of signal strength, and can only interpret a weak signal as a lean mixture. This causes the computer to add more fuel to the engine than a sticheometric ratio requires. This leads to incomplete combustion, wasted fuel, loss of power, This also results in the formation of additional carbon deposits. These additional deposits can then further impair the sensors active surface area, lessening its signal strength further.

When all of this reaches a certain breaking point, the effect becomes accelerated and magnified. Once you reach a mixture that is consistently rich, the catalyst can no longer efficiently handle the excessive hydrocarbons, and the carbon begins to coat the catalyst, impairing its effective surface area. Since todays catalysts are so much more complex than they used to be, they cannot take too much of this abuse before they reach the breaking point and basically stop working altogether.

In the mean time, the carbon deposits have an additional side effect. They create a haven, a quench zone if you will. It acts as a sponge, which traps fuel and air and shields it from the effects of combustion. This results in small quantities of unburned fuel and air. The O2 sensors detect this unburned air, and interpret it as a lean mixture. The computer will add more fuel to compensate, further exaggerating the problem.

As you can see, these problems can compound themselves, and for the most part, they stem from O2 sensor problems to begin with. Even a small inaccuracy can cascade into significant problems down the road. Now let me give you an idea of the window I am talking about here.

An O2 sensor at a 3% inaccuracy is all that is required to start forming lasting carbon deposits. Once these deposits start forming, the carbon starts impairing proper combustion, and coating the catalyst. At 5%, the the problem begins to aggravate itself and also reaches a mixture where the incomplete combustion leaves enough unburned fuel to deceive the sensor further. The computers on most OBD-II vehicle will not trigger an O2 sensor code or a rich code until the mixture gets somewhere in the 8% - 15% range, far beyond the point where the early damage starts to occur. Most users will not notice the power loss until the mixture gets somewhere in the 10% range.

These percentages do directly correspond to fuel economy. A mixture that is 10% rich will decrease your economy by approximately ten percent. However, this also results in a loss of power as well. If this power loss causes the driver to use more throttle to get the same performance, the economy loss can be even greater.

In my experience, most of the converters I sell are due to the dreaded P0420 (or P0430) code which indicates the catalyst efficiency has dropped too low. In most of these cases, the P0420 code is the only code in the computer, leaving no obvious clue as to what caused the catalyst failure. Many mechanics simply replace the converter since no other codes are present, only to have the same problem return after several weeks. Most mechanics have since found that even though no O2 sensor codes were present, that one or more O2 sensors had failed. By replacing the sensors, the replacement converters stick, and do not fail repeatedly.

Now maybe most of you have little or no experience with this because Fords tend to be so reliable in this area. I deal with a wide variety of makes and models, and Fords only make up a small portion of total sales in part, because their exhaust systems and catalyst designs are better than most, especially on the Ford trucks and SUVs. Also, Ford's tolerances for catalyst performance are greater, so even if the catalyst is damaged, it takes a lot more damage and time for a catalyst related code to be set. This is not true of other makes and models, make what I am saying even more relevent to them. But all makes and models rely on O2 sensors. All O2 sensors behave the same way, and are damaged by the same things. There are a lot of variables that play into this in differing degrees.

One method I like to use to identify a possible O2 sensor problem, is to look at the long term fuel trim. If it is not within 5%, there is a problem, that if let uncorrected, will eventually damage one of the more expensive components. Looking at fuel trim data requires a more than a simple code reader, it requires at least a basic scanner.

 

5% of what?

 

Zero.

Although, this would not fix the usual vacuum leaks that are common.


A far better way is to watch the switching waveforms on a graphing scan tool and look at how cleanly (or not) the voltage switches.

Steve

 

Thanks guys, learned something today.....probably forget by the time I need to remember.

 

5% of what? I should have been more specific. 5% of the fuel trim (long term). I have watched waveforms. An O2 sensor still gives a clean waveform till it gets really bad.

 

Wasn't going to post, but Bear River absolutely gave a perfect description of an O2 sensor function! My profession is O2 related. Everything he said is valid. BTW, NGK is the largest OEM supplier of O2 sensors. They are for a reason....

 

5% is set for a rich code, but you forgot about the lack of responce code which will trip first usually when an 02 sensor gets weak! I too deal with them everyday. I'm not arguing your evaluation or anything else, I just usually see a lack of responce codes for lazy 02 sensors when I check with a scan tool and check them too see how well they are working.