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Stock Grand Cherokee Tech. All ZJ/WJ/WK Non-modified/stock questions go here!ZJ (93-98), WJ (99-04), WK (05+)
All ZJ/WJ/WK specific tech questions asked here!
The number never changed and i rev at 1.5 rpms and 2.5 rpms
i test this numbers at parking never moved
but the short term and the long was the same -38%
Why would the values for both, the STFT and the LTFT be elevated that much? The PCM is having to adjust, or trim the fueling by shortening the pre-programmed injector pulse width. There is an allowable normal parameter to obtaine the proper pre-programmed stoichiometric air-fuel ratio, but -38% is too much.
STFT and LTFT can be difficult to grasp. I think because it is a dynamic evolving value. It keeps changing all of the time. It is aimed to balance the fueling to the air intake, maintaining everything at a pre-determined set equivalence.
The operator controls the airflow, or load with the accelerator pedal. The powertrain control module (PCM) can only control the fuel. It uses sensors to measure or calculate airflow, consults an air/fuel ratio map in its permanent memory, then chooses the correct injector pulse width to match the airflow. That pre-programmed injector pulse will provide exactly one gram of fuel for each 14.64 grams of air (a stoichiometric air/fuel ratio). Although, the PCM almost always adjusts injector pulse width to provide more or less fuel than what is actually being specified by the map. That adjustment is “fuel trim.”
When everything is stable and working correctly, fuel trim numbers should be no greater than 10%.
The 'total fuel trim' should be no more than 10% when the numbers are added together. For example, if LTFT is 4% and STFT is 4%, the total is 8%: that’s acceptable.
If LTFT is 12% and STFT is negative 6%, total fuel trim is 6%. This shows that the PCM has enough control to keep things working correctly, but the high LTFT number shows it's compensating for something. On an older engine, LTFT is typically a bit higher as the PCM compensates for normal wear.
As the engine ages and the cylinders don’t seal evenly anymore, the short-term fuel trim may trend high or low most of the time. The PCM has the ability to learn this trend and store it in memory, and it will use that number in the fuel trim calculation to compensate for the changes causing the trend. That's the long-term fuel trim value.
If fuel trim is significantly greater than 10% positive or negative, the PCM is compensating for more than just normal wear-and-tear.
You said you are getting code P0172 (bank 1).. This is saying and it means the system is too rich. The PCM is shortening the fuel cycle at the injectors to compensate. It is doing this by decreasing the length of time of their injector pulse width. Reducing the fuel being delivered to compensate for a rich running condition on cylinder bank 1.
Some things it could be.. is not getting enough air resulting in an overly rich air-fuel ratio. Maybe it has a restriction of exiting air at the exhaust somewhere in the muffler, piping, catalytic converter, exhaust manifold etc? It sounds like it could also possibly be due to having too much fuel? Maybe injectors are leaking is one possibility.
The LTFT value is a value of multiplication. For example, the effects of having a leaking, or of a partially clogged injector increases as speed and load increases.
The STFT value is a value of addition. It only changes to increase or decrease slightly, but it does this constantly.
I don't think the PCM ever needs to re-learn or re-set the trim values, since it is based on a constant..
Probably one of the easiest and simplest things I would think to look at would be the oxygen sensors. I would look at the oxygen sensors first. If one of them is bad somehow? The oxygen sensor could be faulty or shorted to ground?
The most obvious excess fuel rich condition is probably leaking fuel injectors.
If LTFT or total fuel trim is more than 10% negative (like yours is at -38%) the PCM thinks the air/fuel ratio is too rich, so it is leaning out the mixture calculation to return the STFT control back in the correct range. This presents three possibilities: 1. Not enough air in the combustion chambers. 2. More than the commanded amount of fuel is reaching the combustion chambers. 3. One or more of the sensors is reporting incorrectly.
Based on the fuel trims, I do not really think it could be a restriction of exhaust gasses. If there were a restriction in the exhaust, the trim values would not swing to the left like they are.. They would swing opposite each other, as a positive STFT with a negative LTFT.
If it were because of a vacuum leak then the LTFT value decreases, or comes down as current and long-term engine speeds continue rising and going up.. So, it does not sound like a vacuum leak to be the reason either.
MAF sensors make the LTFT value a more positive value.. But since there is no MAF in JGC, it is not the reason.
MAP sensors do not have any affect on fuel trims. Fuel trim values would not be related to anything to do with a MAP sensor.
Im gonna change fuel injector in case of a leak or there bad it has something to do just with bank 1 so just the driver side gonna start changing injector and let u know what the results thanks
Forgot to say when i first got the car the catalyst converter was clogged i changed already gonna try to take of the exhaust muffler after the fuel injector and see what happens
Another simple easy thing to do right now, is to disconnect your 'up-stream' oxygen sensor(s). See what it does? How it behaves when you have the O2 sensor(s) removed out of the equation? Depending, this could provide more clues..
Fuel trim is an ongoing calculation based on data reported by the oxygen sensor(s). Extra oxygen in the exhaust picked up by these sensors indicates a lean air/fuel mixture, so the PCM increases injector pulse width to add more fuel (positive fuel trim). Too little oxygen in the exhaust picked up by these sensor(s) indicates a rich mixture, causing the PCM to reduce injector pulse width (negative fuel trim).
The fuel control fuel trims occur while in closed-loop operation. Closed-loop happens when the engine is warmed up to its normal operating temperature. This happens quickly ussually within 5-10 minutes following a cold start. This is important to know, so you do not rely on readings that were obtained from a scan that was done too early, before it was in a closed-loop.
Change the 4 fuel injector from the driver side that it’s supposed to be bank 1 and didn’t notice any difference.
i also take the exhaust muffler out and no difference
i change the bank 1 sensor 1 of the o2 sensor and no difference here’s a pic .
i think my fuel trim should be working i see the bank 2 moves up and down while the car its running and the bank 1 nothing
Bank #1 (-32.8% + -32.8%) = -65.6%
Bank #2 (7.8% + -9.4%) = -1.6%
Bank #1 is bad, it is very far out!
The -9.4% LTFT on your bank #2 is kind of high. I think it looks like a normal-high value. Ideally, this number here the value would be as close to 1% as possible. Anything 10% or greater is considered outside of normal acceptable operating parameters. Your Jeeps bank #2 LTFT is only 0.1% away from being outside of the acceptable 10% allowance. In the near future, I think you may want to pay close attention to what may happen on your Jeeps bank #2, re-checking the trims. I think by seeing a high-normal value like this at your bank #2 LTFT.. you now need to find out, and see how bank #2 trends. The total trim value for at bank #2 is only at 1.6%. This total trim value is well within the acceptable 10% variance range. Currently, the PCM is able to fully compensate by changing its STFT levels on bank #2, in order to reach the desirable stoichiometric air-fuel ratio. I also think having a value of +7.8% here for the STFT is getting too close. This value +7.4%, it is only 1.7% away from being greater than the acceptable variance percentage range as well.
It seams strange having both the STFT and LTFT being that far out in the same negative direction, having the exact same extreme number values like that too...
Could it be the computer?
when i just got the car the lady told me that it was changed for a new one .
or it could be oil it looks kinda bad but i never tough that could it be what u think
This is a tough one. I would not want to pretend to know exactly what I am talking about either. I'm learning it better now for the first time.
I would not want to give out incorrrct information, or accidentally divert you somehow looking in the wrong directions..
I am not sure the relationship banks 1 & 2 have on one another? I imagine you have not driven your Jeep as often lately.. You probably have not been able to accelorate or run through the RPMs normally.. There may now be more moments of extended idling time.. I'm not sure what kind of effect these things ultimately have on the current trim value readings you are getting? All of this probably has to be taken into consideration.
Bank #1... There may be a simple explanation for why this banks trim values are what they are?
My schedule right now is jam packed full, but I am interested to know this stuff, and I want to try to continue and hopefully help you when I am able.. if I can
Good thought dont know i guees she try to find the same problem a solution cuz she said she change the computer and a mechanic told her that it could be the heads but its not overheating so didn’t think on that
09-29-2019, 05:02 PM
