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  #1  
Old November 8th, 2018, 18:04
Henrique - RS Henrique - RS is offline
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OBD2 - Fuel trims issue

1998 XJ 4.0 Stock Engine
5 Speed
Hollowed Cat

Hello everyone,
My 98' has been struggling while taking off after a red light for instance. Sometimes my engine would even stall. Sometimes it runs perfectly fine like today all day.... Never had an issue while on high RPMs and load so I guess that may rule out fuel pump issues, right?

Cleaned my IAC, changed TPS, didnt find any vacuum leaks by spraying carb cleaner around vac lines, injectors, brake booster, CCV, etc...

My long term fuel trim is up to 20%, while the STFT oscillates around -1+3%. My upstream o2 readings seem to be a little bit lazy, even though they are in the expected voltage range. Since downstream o2 does not control fuel AND I have no cat, I can assume we are not worried about downstream (B1S2) o2 readings.

Readings at idle:



Does anything seem out of usual? I guess I should replace my upstream O2 sensor because of it's "lazyness" in oscilating. What you guys think?

TIA
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Old November 8th, 2018, 18:40
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Re: OBD2 - Fuel trims issue

Do you have a cracked header? Are you running stock injectors?
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Old November 8th, 2018, 19:00
Henrique - RS Henrique - RS is offline
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Re: OBD2 - Fuel trims issue

Quote:
Originally Posted by sidewaysstarion View Post
Do you have a cracked header? Are you running stock injectors?
Header has a small crack which has been sealed with exhaust sealant (I know it is not a long term fix) to no change on the readings.
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Old November 9th, 2018, 01:04
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Re: OBD2 - Fuel trims issue

The LTFT is very high and the injectors are cycling at a high percentage rate.
If you were tuning a new engine, a larger injector would be chosen to get the LTFT closer to zero and to get the cycling rate to around 80%.
How old are the injectors? One or all of them could be dirty and not flowing at their original rates, leading the ECM to increasing the injector's cycling percentage.
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Old November 9th, 2018, 02:18
Henrique - RS Henrique - RS is offline
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Re: OBD2 - Fuel trims issue

Quote:
Originally Posted by lazyxj View Post
The LTFT is very high and the injectors are cycling at a high percentage rate.
If you were tuning a new engine, a larger injector would be chosen to get the LTFT closer to zero and to get the cycling rate to around 80%.
How old are the injectors? One or all of them could be dirty and not flowing at their original rates, leading the ECM to increasing the injector's cycling percentage.
The injectors are all stock from factory and are 20 years old hehe..

So I guess I'll clean them, first, right? I'll take them off and take it to a shop where they have those injector cleaning machines. Thanks for the input, I'll let you know how it turns out next week.
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Old November 9th, 2018, 07:46
Henrique - RS Henrique - RS is offline
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Re: OBD2 - Fuel trims issue

I don't know if O2 sensor appearance tell us anything like spark plugs do, but anyway, here is what my O2 sensor looked like. Removed it and cleaned it before scanning it. I guess it should look black, not white?



After letting it soak overnight in gasoline, I haven't noticed the jeep running rough nor misfiring at take offs, but as shown before, the fuel trims are very high, indicating a lean condition.
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Old November 9th, 2018, 12:16
Henrique - RS Henrique - RS is offline
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Re: OBD2 - Fuel trims issue

Ohhh...
I forgot to mention that in Brazil, our crappy gasoline has 27% Ethanol. But I don't know how that could affect the fuel trims.
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Old November 9th, 2018, 14:05
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Re: OBD2 - Fuel trims issue

27% ethanol will probably increase your fuel trims significantly. I'm not sure how much, but I do know that guys up here who use E85 as budget race fuel have to run significantly larger injectors to supply enough fuel compared to original, even at the same power level.

I don't know if it is your whole problem though.

Don't replace it just yet but have you checked if your ignition coil is bulging or cracked? How many miles has it been since you replaced your spark plugs?

Your downstream sensor is used as the goal voltage for the upstream sensor so you may have some issues from the hollowed out cat, but I would not expect it to cause this much of an issue.
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Old November 9th, 2018, 20:33
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Re: OBD2 - Fuel trims issue

And this thread epitomizes is why I love ... can I ask how you got those graphs?

I have a bluetooth OBDII unit and the torque app on my phone, but I don't know that I can log all that (or even what it all means)... but I want to.
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Old November 13th, 2018, 08:17
Henrique - RS Henrique - RS is offline
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Re: OBD2 - Fuel trims issue

Quote:
Don't replace it just yet but have you checked if your ignition coil is bulging or cracked? How many miles has it been since you replaced your spark plugs?
My coil is a little bit cracked indeed, but there is no hesitation when starting the jeep nor sparks in the dark. I purchased a new coil just in case, but it hasn't arrived yet.
I bought the jeep with the spark plugs that are in it. I mean, they are at least 1 year and 3 months old, LOL... But this is not a daily so I guess there aren't many miles on them.

I guess they look good, right?



Quote:
Originally Posted by bridgerx View Post
And this thread epitomizes is why I love ... can I ask how you got those graphs?

I have a bluetooth OBDII unit and the torque app on my phone, but I don't know that I can log all that (or even what it all means)... but I want to.
I got those graphs using ProScan software, in my notebook, not cellphone.
There are a large variety of software you can use when you have an USB OBD reader.

Those graphs mean the following:

Blue line: That is the short term fuel trim. It represents the PCM making little corrections in injection times according to the readings in the O2 sensors. If an o2 sensor reads a lean condition, the short term fuel trim will be positive, meaning the PCM is raising the injection pulse times on the injectors. The opposite happens when the o2 sensor sends a rich mixture signal to the PCM.
The best condition is when that the short term fuel trim is oscilating between -3% and 3%.

Red Line: Long therm fuel trim. The PCM has several "injection maps" pre programmed in order to keep the mixture stoichiometric (14.7:1 air to fuel rate). If a lean condition is being read and the short term fuel trims cannot correct it within the (around) -3% to +3% (approximately) rates, the PCM shall select another injection map that will make the short term fuel trims "be happy" and the engine run in the correct Air-Fuel proportions.

When you have a long term fuel trims higher than 10% for a long time, it means that something might be wrong, because the PCM is having to add more fuel than the expected.

As you can see, in my jeep the long term fuel trim is very high to make the short term "happy". Usually this means that I have unnacounted air going into the engine (big vacuum leaks - I say "Big" because our jeeps uses MAP sensors which can account for air entering from a vacuum leak since it's a pressure sensor, instead of an Airflow Sensor - MAF), clogged fuel system, or even bad spark.

I don't seem to have any of these conditions. I have replaced the o2 sensor and the readings didn't change. The jeep has been running fine, even though the fuel trims are kept high. I guess that's the 27% ethanol to blame.

I will post later to tell if my problem has been solved or not, although nothing changed except for the new o2 sensor reading the same as the old.
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Old November 13th, 2018, 16:29
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Re: OBD2 - Fuel trims issue

Bosch platinum plugs are gapped way to large for most applications. I'd switch to a copper plug at the correct gap.

I would also try an antifouler on the second o2. Shrouding it from the untreated exhaust stream may trick the pcm into dialing back the long term. (Contrary to popular belief, B2S2 readings WILL affect fuel trim) I'm going to be make the assumption that a new cat is cost prohibitive, so i wont bark up that tree.
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