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Old 11-20-2013, 03:22 PM
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P0174 code, what does lean and rich actually mean?

'97 F150, 4.6L V8
This is going to be a bit of a read, so thanks to anyone who looks.


My truck has been showing this code for a bit now (P0174), which my scanner says "System too lean, bank 2. Okay, so I have also noticed worse mileage (normally a tank of gas takes me about 500KM, now I am only getting to around 400KM before I fill), and there is also a slight gas smell in the cab when I start the truck. It goes away after a while.

So since it is lean, that means that the O2 sensors are reading too much oxygen correct? So when I am getting a lean code, is my engine trying to add more fuel to compensate, to get the right ratio's? But it can never actually get the right ratio because there is too much air coming from somewhere? Is that why I get the gas smell, because the gas is being put in, to try and correct the issue, but not all getting used?

I am just kinda confused how these lean/rich codes work, because calling it lean just makes me automatically think less gas, but it is more gas?

Either way, I think this is beyond me, because I have checked all the simple solutions (MAF, PCV elbow etc.) and they all seem good. Also when attached to my scanner, my O2 sensors are providing normal voltages ranges, so they should be good? Even when I look at the fuel trim it seems to not go to double digits (negative or positive).

I did notice one thing on my scanner though, on both bank 1 and 2, fuel trim is shown as sensor 1 and sensor 2, but on my reader it constantly says waiting for data from bank 1, sensor 2, and bank 2, sensor 2. What sensors are reading fuel trim and is there are reason why only one shows up on each bank? Or is there only one sensor on each bank?

chances are I will take it to my local mechanic, but I would like to hear some input from you guys.

Thanks for reading!
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Old 11-20-2013, 07:30 PM
pdqford pdqford is offline
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Each bank has an upstream O2 sensor for fuel control.

If the O2 sensor determins the fuel mixture is too lean the PCM will increase the short term fuel trim and possible the long term fuel trim to try and compensate for the lean condiction.

If the sum of the short term fuel trim + the long term fuel trim exceeds ~30% and the bank1 O2 sensor indicates too much oxygen the PCM will set DTC P0171 and if the bank2 O2 sensor indicates too much oxygen the PCM will set DTC P0174.

The down stream (after the cat) O2 sensors on each bank are there to monitor the cat efficency to see if the cat is doing its job - they do not control the fuel.

Since you only have one bank indicating "too lean", the issue is more than likely not related to fuel pressurevolumn or MAF problems. I'd first look for something that would ONLY effect bank2. An exhausr leak could be dumping some outside oxygen into the bank2 before the fuel control O2 sensor on that bank, or maybe a leaking intake manifold gasket or a fuel injector o-ring on that bank.

Hopefully the extra fuel hasn't destroyed your cats yet.
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Old 11-20-2013, 07:41 PM
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Thanks for the reply, and yeah hopefully no damage has been done. I only drive it to work on Monday and home on Friday (I use a company vehicle for everything else) so it hasn't had much driving in the past year. I'm booked into see my mechanic on next week so hopefully they can figure it out for a reasonable price. I wish I had a garage and could do more work myself, doesn't feel too good working on your truck outside when its -25 C haha.
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Old 11-21-2013, 01:23 AM
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Can your scanner do freeze frames?

When a code is set, the system takes a freeze frame snapshot of engine conditions (rpm, temps, etc). Some scanners can display them. More common now than years ago.

Might give a hint as to when this occurs -- it might not be constant.

Good Luck,

hj
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Old 11-21-2013, 02:23 AM
Bobbycolt Bobbycolt is offline
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My truck had the same problem the tube that ran from the PVC valve to the back of the throttle body broke off at the back of the throttle body had to replace the whole pice on a 99 on the 97 like yours you can replace just the insert in the throttle body if I'm not mistaking. Hope this works for ya
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Old 11-21-2013, 12:01 PM
Bluegrass 7 Bluegrass 7 is offline
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...To try to help clear up your confusion or create more, remember the computer is running on a program.
...The codes are a result of the program's 'reaction' to input signals from hardware sensors 'about' what is happening to the outside hardware and conditions.
...It's a 'closed loop' operation for the fuel system based on an input device called an Ox sensor. This device is calibrated to offer an output voltage based on Oxygen content in the exhaust within a limit set by it's design and to match up with requirements of the program.
...This sensor has analogue output who's voltage value at any given time is converted to digital counts by a circuit device converter in the PCM.
...The digital results constantly adds to or subtracts from the averages in the fuel tables.
...The value of the short term tables at any given time is transferred to a fuel injector driver for each cylinder that can vary the injector 'on' time for fuel amount control for every combustion event x 8 for all rpm ranges.
...You can see how deeply the electronics gets to control all the functions in real time and how fast the processor clock speed has to be.
...This is only one part of the total system control.
...Controlling spark and timing is an equally in-depth function all it's own.
...Bottom line for this is the codes are triggered when the fuel table are driven out of limits set in program trying to account for changes in combustion.
...This becomes an 'inferred' function that too much 'unaccounted' for air is entering the engine causing a 'lean' condition.
...The only way this can be detected is by the OX sensors as a remote detection, if you will, after the fact.
.
...The second sensors down stream are the same type and calibration as the fronts but are performing a (different function).
...They are not controlling fuel in any way but still are detecting Ox in the outputs of the cats as their function.
...The detection of OX at this point become a measure of cat operation and health.
...This function has a set of codes all it's own to only address the cat system operation again by inference. It's called cat oxygen storage.
...When the detected value goes outside a fixed table limit for each side a code is set. Look up codes 420, 421, 430, 431.
.
...Overall the system is a very clever piece of both software and hardware engineering working together that hardly ever fails to be accurate.
...Within this framework of operation of moving tables during real time operation, the system was given the ability to self adjust to varying conditions such as fuel quality, temperature changes and barometric pressure changes and...and can account of average wear and aging of major parts such as ring seal and valve seal average being the keyword.
...It is an incredible total design that the average person is not a where of until the system tells you there is a fault and interpretation is needed to understand it.
Good luck.
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Old 11-21-2013, 12:01 PM
 
 
 
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