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Bad 02 sensor without code?

Old 02-14-2014, 02:25 PM
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01 Trueblue
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Default Bad 02 sensor without code?

My 01 gt is pretty much stock (as far as I know) cai, o/r prochamber, flowmasters, bama tune 3.73's.

Im getting 130 miles out of 3/4 of a tank of gas. I drive it hard but not terribly. I shift the car most often around 2500 or so and I dont think I have revved past 5k ever. I live in a high traffic area and lots and lots of lights. I expect low mpg but not old big block numbers

Plugs are new, the cai is brand new, new fuel filter.

Issues that I will attend to this weekend.

clean maf, Tb, iac.

I think the filler neck gasket is forked because if I over fill it will puke gas on ground, this might be contributing. Im curious to if I have bad forward 02's I have no idea how old they are, or how to know if theyre bad or not. I looked at them all when i swapped the midpipe last week and none of them looked particularly fouled, but who knows. Is there a way to test it out? Is the stealership the only place to get motorcraft?


Because I know this isnt the original engine Im probably going to swap out for an 03 or 04 with less than 100 on it, just so I know whats going on with it and build a forged for a KB.
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Old 02-14-2014, 03:07 PM
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Oh and when I changed out the plugs they didnt look as though I had been running rich, they had a nice brown color other than #3 which was a lil wet with oil (possibly)


With this being said, just running to the autoparts store to get some maf cleaner, (granted no cats) the inside of my car smells FAT FAT FAT where I can sort of smell gas laden exhaust on my hands.
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Old 02-17-2014, 09:03 AM
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Originally Posted by 01 Trueblue View Post
My 01 gt is pretty much stock (as far as I know) cai, o/r prochamber, flowmasters, bama tune 3.73's.

Im getting 130 miles out of 3/4 of a tank of gas. I drive it hard but not terribly. I shift the car most often around 2500 or so and I dont think I have revved past 5k ever. I live in a high traffic area and lots and lots of lights. I expect low mpg but not old big block numbers

Plugs are new, the cai is brand new, new fuel filter.

Issues that I will attend to this weekend.

clean maf, Tb, iac.

I think the filler neck gasket is forked because if I over fill it will puke gas on ground, this might be contributing. Im curious to if I have bad forward 02's I have no idea how old they are, or how to know if theyre bad or not. I looked at them all when i swapped the midpipe last week and none of them looked particularly fouled, but who knows. Is there a way to test it out? Is the stealership the only place to get motorcraft?

Because I know this isnt the original engine Im probably going to swap out for an 03 or 04 with less than 100 on it, just so I know whats going on with it and build a forged for a KB.
Hmm.. It sounds like you're on the right track. If you have a new intake, new fuel filter and plugs that is definitely a good start. A Bama tune should also help your MPG if the car is driven with gas mileage in mind.

I would definitely recommend cleaning the MAF, Throttle boy and IAC. Fixing the filler neck leak is also probably a good idea. From there, I would pop the O2's off and see how they look. If you don't know the last time they were replaced, it may be a good idea to replace them. But before you throw parts at the car I would recommend inspecting them first and see if they seem to be in bad shape.

I hope this helps. Let me know how you make out!

-Dan
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Old 02-17-2014, 11:07 AM
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"Looking" at the O2 sensors will reveal very little unless they have been totally fouled by running rich for a long time--here is how you can bench test them using a propane torch and multimeter.

Getting back your original concern, it is quite possible for O2 to be bad and not (yet) throw any DTCs. This is because as the sensors age their output voltage drops, their output may still be switching in response to the PCM's continuous changing of the injector pulse width in closed loop mode¹, however as lower sensor output indicates a leaner mix the PCM will try to correct things by adding fuel via the short term fuel trim.

BUT the mix really isn't lean, it's just that the failing O2 sensor is telling the PCM it's lean--so when the PCM tries to correct the AFR it is really making the mix rich; which of course means using too much fuel and decreasing mpg.

-----------------------------------------
¹ - A narrowband O2 sensor can only detect air/fuel ratio across a narrow range (band) of 14.55 to 14.85:1--that's how they got their name:



They also react relatively slowly to changes in the air/fuel ratio of the sampled gas, 250 to 300 ms: here is what an O2 sensors output looks like with the engine idling:



And at 2000 rpm:



An old and near death sensor would have a similar waveform, however the voltage would be less, dropping from 0.2 to 0.9 V as shown above to as little as 0.0 to 0.6 V.

Because of these limitations all EFI systems using NB sensors do so only during closed-loop operation, and do so by constantly varying the amount of fuel injected so as maintain an average O2 sensor output of 0.45 V which represents the stoichiometric air/fuel ratio; 1.0 Lambda, 14.7:1 for gasoline. The O2 sensor senses these swings in AFR and reports rich or lean as appropriate.

A common misconception is that it is the sensor itself that is "switching" its output, and that when they stop switching they are bad. This is 110% incorrect. It is the PCM that is "switching" the AFR, the O2 sensor is just reporting the current AFR in response to the PCM's actions.

Also as the sensors age their response to the changing AFR slows, this is what will cause the "slow response" DTCs, P0133 and P0153...
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Old 02-17-2014, 11:59 AM
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I have a bama tune running currently. Visual inspection didnt provide much info as theyre were not fouled really just a little dirty but, its unknown if they have ever been changed (Ive only had the car a couple months now)

I did clean the maf, and everything else, and it did add a little snap, it still smells pretty dang fat.

I have some concerns with this engine. It is my daily driver (for now) but I intend to get it set up for track days and such. So, Im intending to swap out the engine because there is some various clangs in the engine at 2800-3200 that most likely is chain guides but, im not willing to bet on being that lucky when it really matters. So, I will throw new 02's in it, and fix the filler tube issue, and go from there.

I really need to find some local mustang friends so I can learn what is normal mustang noise and what is my cars issue lol
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Old 02-17-2014, 06:48 PM
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If you can monitor the O2 sensor output then do so (you can do this with a fast responding multimeter, but an oscilloscope is best), if the lower voltage is dropping below 0.1 V then they are likely all done. If the lower voltage drops below 0.1 V and the higher voltage is less than 0.7 V then they are definitely over-the-hill...
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Old 02-17-2014, 07:45 PM
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Thanks cliff, i will check em with my multimeter and see whats up.
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