Help interpreting strangely high O2 sensor fuel trims

[RMac80]

2005 Wrangler X, 4.0, auto transmission

So I have been chasing down intermittent misfire (p0301) for about a year and I have eliminated a few issues. After a month with my new injectors from K-suspension, I got misfiring a lopey idle again over the weekend.

I have been driving around with my live data scanner on, showing upstream O2 voltage, short term, and long term trims (for both banks). So here is what I observed when it was running poorly on Sunday...

-B2S1 has been replaced with NTK sensor last summer, it was showing very low and very high voltages at that time. Right now, voltages from the sensor are fluctuating between expected low and high values as normal, but the fuel trims for B2S1 are very strange...
-Short term fuel trim for B2S1 was hitting very high numbers (12,777,77 %). I have seen this before only briefly, which I read on this forum and other forums that an occasional blip reading like that was just a communication issue between the PCM and the scanner. Butt this was pinned for 5-20 seconds at a time then reset to zero. It actually began ramping up the long term trim the longer it was pinned.
-When parked and idle, B2S1 fuel trim stay normal, but moving the vehicle from park to any gear would jump the trim up to high numbers instantly.
-The only codes I am getting are p0301

Questions:
-How could an issue with bank 2 cause a bank 1 misfire (p0301)?
-What would cause the B2S1 fuel trims to jump up or down depending on whether or not the transmission is in park?

Theories:
-My main theory right now is a short in the B2S1 sensor wiring, all the data I have is from the live data scanner so I probably need to get my multimeter out and check this.
-Might be MAP sensor, since transmission gear selection will change vacuum pressure (I think?)

 

[Chris]

2005 Wrangler X, 4.0, auto transmission

So I have been chasing down intermittent misfire (p0301) for about a year and I have eliminated a few issues. After a month with my new injectors from K-suspension, I got misfiring a lopey idle again over the weekend.

I have been driving around with my live data scanner on, showing upstream O2 voltage, short term, and long term trims (for both banks). So here is what I observed when it was running poorly on Sunday...

-B2S1 has been replaced with NTK sensor last summer, it was showing very low and very high voltages at that time. Right now, voltages from the sensor are fluctuating between expected low and high values as normal, but the fuel trims for B2S1 are very strange...
-Short term fuel trim for B2S1 was hitting very high numbers (12,777,77 %). I have seen this before only briefly, which I read on this forum and other forums that an occasional blip reading like that was just a communication issue between the PCM and the scanner. Butt this was pinned for 5-20 seconds at a time then reset to zero. It actually began ramping up the long term trim the longer it was pinned.
-When parked and idle, B2S1 fuel trim stay normal, but moving the vehicle from park to any gear would jump the trim up to high numbers instantly.
-The only codes I am getting are p0301

Questions:
-How could an issue with bank 2 cause a bank 1 misfire (p0301)?
-What would cause the B2S1 fuel trims to jump up or down depending on whether or not the transmission is in park?

Theories:
-My main theory right now is a short in the B2S1 sensor wiring, all the data I have is from the live data scanner so I probably need to get my multimeter out and check this.
-Might be MAP sensor, since transmission gear selection will change vacuum pressure (I think?)

Questions

How could an issue with bank 2 cause a bank 1 misfire (P0301)?
It's possible that a problem on bank 2 could affect the overall air-fuel mixture or the combustion process enough to manifest as a misfire in bank 1. For example, if there are fuel delivery issues, vacuum leaks, or incorrect sensor readings, the engine management system might be compensating by adjusting fuel trims across both banks, leading to an imbalance. Another possibility is that a significant issue in bank 2 could cause a lowered engine power or increased load, causing additional strain on bank 1 during operation.

What would cause the B2S1 fuel trims to jump up or down depending on whether or not the transmission is in park?
The change in fuel trims with transmission gear selection can suggest an issue with the load on the engine. When the vehicle is in gear, the engine has to work harder due to increased load, leading to a richer mixture requirement. If the B2S1 readings jump drastically during this transition and affect trim adjustments, it could indicate a problem with the O2 sensor, wiring issues, or even an issue with the engine's vacuum, which is changed by the load.


Theories

Wiring Issue with B2S1:
This is a valid suspicion. Corroded or damaged wiring could exhibit erratic readings under varying loads, especially during transitioning between gears. Testing with a multimeter will help identify any inconsistent voltage or grounding issues.

MAP Sensor Issues:
The MAP sensor is another key component to consider. If it’s malfunctioning, it can cause erratic fuel trims and impact how the engine calculates the required fuel mixture based on load, which is influenced by gear selection.


Additional Suggestions

Vacuum Leaks: Double-check for any vacuum leaks, as they can drastically alter sensor readings and affect fuel trims. This can be exacerbated during load changes.

Throttle Position Sensor (TPS): If not already checked, ensure that the TPS is functioning properly, as it plays a crucial role in calculating fuel injection needs under different loads.

Consider a Fuel Pressure Test: Poor fuel delivery could exacerbate issues across both banks. Checking fuel pressure when the misfire occurs might provide additional clues.

Further Live Data Review: Keep an eye on other parameters, like the MAF readings (if equipped), as they can also provide insights into what's happening when the engine experiences misfires.

 

[macleanflood]

I think I'd summon @Wranglerfix for a quick PCM opinion.

-Mac

 

[Strizzo]

if a bad sensor reading is causing the stft to peg out at max adjustment, the ecu could very well be dumping enough fuel to cause a misfire. at that rate i would think the injector could be running at close to max duty cycle.

Can you try logging injector duty cycle or pulsewidth to see what it is doing when the faulty data starts?

 

[RMac80]

Thanks for the suggestions, folks!

Vacuum Leaks
I have used a smoke machine to eliminate a few vacuum leaks. The only thing I did not do was a hot engine smoke test. I checked the manifold bolts for tightness but that doesn't rule out a crack or warped area that only shows up when warm.

B2S1
This is a new NTK unit, the voltages are showing normal fluctuations but the fuel trim is what gets pegged high. I will still check the wiring here but this may point to another issue (PCM or another sensor that has input into the f/a mix)

PCM
I have an email trail going with WranglerFix but at this time they don't think any of my symptoms indicate a PCM issue, I am not seeing any hard shifting or the other classic symptoms.

TPS
This sensor seems good. Nice smooth ramp up and down curve with throttle. No jumps, no idle hunting. And I replaced the IAC with a mopar unit.

MAP
The live data from it looks good (matches atmospheric pressure at idle, ramps up with throttle) but I still suspect this sensor since one of the vacuum leaks I replaced was the little elbow under the MAP sensor right after I bought this jeep. Who knows how long it was open. Plus I had the throttle body off to rebuild the seals and did not replace it...

Fuel pressure
I swapped out my fuel rail for an older model with the schrader valve at the same time I installed the DEI heat soak shilding kit, and then I did a fuel pressure test. Steady pressure 32-35lbs from idle all through RPM range, no leak down in one hour.

Injector duty cycle monitoring
In my live data scanner I have only seen PWM/duty cycle for injector 1, I will keep looking for more PIDs

Compression test was good 80-85 lbs on all cylinders, did this test while I was replacing the valve stem seals. (did not perform leak down test officially but the needle on the compression gage did not fall any within a minute or so, this was a dry compression test so I feel pretty good about it)

So it seems like my to-do list is:
-check for vacuum leaks when hot, especially around intake/exhaust manifold
-B2S1 wiring check/inspection
-monitor MAP sensor readings while driving, see if anything strange correlates to B2S1 fuel trim pin high (possibly replace thiss sensor)
-monitor injector PWM readings if possible, see if anything strange correlates to B2S1 fuel trim pin high.
-I might need to check my injector wiring while I am at it. I recently installed a set of refurbished injectors, but if one of them on bank2 is not operating, there may be a legitimate lean condition causing the PCM to dump fuel into bank 2 (but why is my o2 sensor not showing lean?)

I'll keep this thread updated with progress! Thanks again for the suggestions!

 

[macleanflood]

I thought fuel pressure is supposed to be 48-50 psi?

My Jeep did high percentage fuel trim corrections until I replaced the fuel pump and got from 30 to 48.

-Mac

 

[Wranglerfix]

You can send your Pcm to us and we can test it for you and one of our Jeeps just to be 100%.

 

[Strizzo]

I am not a jeep 4.0 expert by any means, but 80-85psi compression is very low in any engine, even in a dry test.

 

[powerstrokeunleashed]

i would try swapping the old injectors back in the jeep and see if every thing return to normal. if you don't have the old one any more up plug on injector at a time an check your trims for changes to find your malfunctioning injector(s). just went through this with my 2.5l

**2005 Jeep with the 4.0L inline-6 engine, the target compression range is typically 120-150 PSI per cylinder. A healthy engine should exhibit compression within this range, with no more than 30 PSI difference between cylinders. Readings below 120 PSI could indicate an issue, such as worn rings or valve problems. **

 

[Lou]

Thanks for the suggestions, folks!

Vacuum Leaks
I have used a smoke machine to eliminate a few vacuum leaks. The only thing I did not do was a hot engine smoke test. I checked the manifold bolts for tightness but that doesn't rule out a crack or warped area that only shows up when warm.

B2S1
This is a new NTK unit, the voltages are showing normal fluctuations but the fuel trim is what gets pegged high. I will still check the wiring here but this may point to another issue (PCM or another sensor that has input into the f/a mix)

PCM
I have an email trail going with WranglerFix but at this time they don't think any of my symptoms indicate a PCM issue, I am not seeing any hard shifting or the other classic symptoms.

TPS
This sensor seems good. Nice smooth ramp up and down curve with throttle. No jumps, no idle hunting. And I replaced the IAC with a mopar unit.

MAP
The live data from it looks good (matches atmospheric pressure at idle, ramps up with throttle) but I still suspect this sensor since one of the vacuum leaks I replaced was the little elbow under the MAP sensor right after I bought this jeep. Who knows how long it was open. Plus I had the throttle body off to rebuild the seals and did not replace it...

Fuel pressure
I swapped out my fuel rail for an older model with the schrader valve at the same time I installed the DEI heat soak shilding kit, and then I did a fuel pressure test. Steady pressure 32-35lbs from idle all through RPM range, no leak down in one hour.

Injector duty cycle monitoring
In my live data scanner I have only seen PWM/duty cycle for injector 1, I will keep looking for more PIDs

Compression test was good 80-85 lbs on all cylinders, did this test while I was replacing the valve stem seals. (did not perform leak down test officially but the needle on the compression gage did not fall any within a minute or so, this was a dry compression test so I feel pretty good about it)

So it seems like my to-do list is:
-check for vacuum leaks when hot, especially around intake/exhaust manifold
-B2S1 wiring check/inspection
-monitor MAP sensor readings while driving, see if anything strange correlates to B2S1 fuel trim pin high (possibly replace thiss sensor)
-monitor injector PWM readings if possible, see if anything strange correlates to B2S1 fuel trim pin high.
-I might need to check my injector wiring while I am at it. I recently installed a set of refurbished injectors, but if one of them on bank2 is not operating, there may be a legitimate lean condition causing the PCM to dump fuel into bank 2 (but why is my o2 sensor not showing lean?)

I'll keep this thread updated with progress! Thanks again for the suggestions!

Compression gauge hoses have a shader valve in them. If you were to see the pressure decay on your gauge that’s a tooling issue not a cylinder issue. The gauge holds pressure until you release it.

 

[Painfultruth]

Thanks for the suggestions, folks!

Vacuum Leaks
I have used a smoke machine to eliminate a few vacuum leaks. The only thing I did not do was a hot engine smoke test. I checked the manifold bolts for tightness but that doesn't rule out a crack or warped area that only shows up when warm.

B2S1
This is a new NTK unit, the voltages are showing normal fluctuations but the fuel trim is what gets pegged high. I will still check the wiring here but this may point to another issue (PCM or another sensor that has input into the f/a mix)

PCM
I have an email trail going with WranglerFix but at this time they don't think any of my symptoms indicate a PCM issue, I am not seeing any hard shifting or the other classic symptoms.

TPS
This sensor seems good. Nice smooth ramp up and down curve with throttle. No jumps, no idle hunting. And I replaced the IAC with a mopar unit.

MAP
The live data from it looks good (matches atmospheric pressure at idle, ramps up with throttle) but I still suspect this sensor since one of the vacuum leaks I replaced was the little elbow under the MAP sensor right after I bought this jeep. Who knows how long it was open. Plus I had the throttle body off to rebuild the seals and did not replace it...

Fuel pressure
I swapped out my fuel rail for an older model with the schrader valve at the same time I installed the DEI heat soak shilding kit, and then I did a fuel pressure test. Steady pressure 32-35lbs from idle all through RPM range, no leak down in one hour.

Injector duty cycle monitoring
In my live data scanner I have only seen PWM/duty cycle for injector 1, I will keep looking for more PIDs

Compression test was good 80-85 lbs on all cylinders, did this test while I was replacing the valve stem seals. (did not perform leak down test officially but the needle on the compression gage did not fall any within a minute or so, this was a dry compression test so I feel pretty good about it)

So it seems like my to-do list is:
-check for vacuum leaks when hot, especially around intake/exhaust manifold
-B2S1 wiring check/inspection
-monitor MAP sensor readings while driving, see if anything strange correlates to B2S1 fuel trim pin high (possibly replace thiss sensor)
-monitor injector PWM readings if possible, see if anything strange correlates to B2S1 fuel trim pin high.
-I might need to check my injector wiring while I am at it. I recently installed a set of refurbished injectors, but if one of them on bank2 is not operating, there may be a legitimate lean condition causing the PCM to dump fuel into bank 2 (but why is my o2 sensor not showing lean?)

I'll keep this thread updated with progress! Thanks again for the suggestions!

Fuel pressure is way low

 

[RMac80]

My recollection of fuel pressure and compression from memory were wrong, sorry for the confusion. I had to go back to my previous notes/posts:

The fuel pressure was right on 60psi for all throttle ranges and at idle. After turning off the engine the pressure dropped to 55-56psi within a few seconds but stayed at that level for 35 minutes.

All 6 cylinders showed 120-130 lbs pressure on the first full compression stroke, resetting the gauge each time and doing each cylinder 3 times.

 

[RMac80]

You can send your Pcm to us and we can test it for you and one of our Jeeps just to be 100%.

I may do that this winter if I can't get this sorted. The issue is so intermittent I will continue to drive it and do debugging work on it this season.

 

[ItchyDong]

I read everything and can’t fathom why you’re chasing O2 data for a single cylinder misfire. I also think you’re getting flooded with needless info while driving and everything looks “wrong”.

Low or high fuel pressure at the pump wouldn’t target one cylinder either.

-Injector control test with a noid light.
-Injector balance test, or swap-tronics (swapping #1 injector with any other cylinder to see if problem follows)
- Swap-tronics with your #1 spark plug

Eliminate those things.

 

[RMac80]

I read everything and can’t fathom why you’re chasing O2 data for a single cylinder misfire. I also think you’re getting flooded with needless info while driving and everything looks “wrong”.

Low or high fuel pressure at the pump wouldn’t target one cylinder either.

-Injector control test with a noid light.
-Injector balance test, or swap-tronics (swapping #1 injector with any other cylinder to see if problem follows)
- Swap-tronics with your #1 spark plug

Eliminate those things.

So a bit more history. Last fall I rented a NOID light set and got good pulse on all 6 injector leads. I cleaned all my injectors and put in new filter baskets and o-rings, and at that time I swapped injector #1 and #5. The misfire stayed with cylinder 1.

Inspection of the spark plugs (at that time only a couple months old) revealed no fuel fouling or lean burning, but plug #1 was a bit oil fouled. Bore scope on cylinder 1 revealed I had a lot of oil leaking into the cylinder around the intake valve, so I changed out all my valve stem seals, inspected all my pushrods and rockers and springs while I was at it.

Still getting P0301, so I checked the resistance on all my injectors. ALl were within spec, but #3 was lower resistance. At that time I was seeing negative fuel trims on bank1. My hypothesis at the time was fuel was dumping in cylinder 3 due to faulty injector so the PCM was cutting fuel like crazy on bank 1, then cylinder 1 was the one misfiring due to heavy oil build up in there. SO I got a full refurbished set of K-suspension flow matched injectors. I saw no misfires or negative fuel trims for a couple months, thought it was fixed.

Now I am seeing cylinder 1 misfires again, but no negative fuel trims on bank 1. I am seeing very high fuel trims on bank2. I want to trust the refurb set I just bought but maybe one of them in bank 2 is stuck shut?

I agree, it's not adding up, but I am running out of things that "make sense"...sadness

 

[ItchyDong]

So a bit more history. Last fall I rented a NOID light set and got good pulse on all 6 injector leads. I cleaned all my injectors and put in new filter baskets and o-rings, and at that time I swapped injector #1 and #5. The misfire stayed with cylinder 1.

Inspection of the spark plugs (at that time only a couple months old) revealed no fuel fouling or lean burning, but plug #1 was a bit oil fouled. Bore scope on cylinder 1 revealed I had a lot of oil leaking into the cylinder around the intake valve, so I changed out all my valve stem seals, inspected all my pushrods and rockers and springs while I was at it.

Still getting P0301, so I checked the resistance on all my injectors. ALl were within spec, but #3 was lower resistance. At that time I was seeing negative fuel trims on bank1. My hypothesis at the time was fuel was dumping in cylinder 3 due to faulty injector so the PCM was cutting fuel like crazy on bank 1, then cylinder 1 was the one misfiring due to heavy oil build up in there. SO I got a full refurbished set of K-suspension flow matched injectors. I saw no misfires or negative fuel trims for a couple months, thought it was fixed.

Now I am seeing cylinder 1 misfires again, but no negative fuel trims on bank 1. I am seeing very high fuel trims on bank2. I want to trust the refurb set I just bought but maybe one of them in bank 2 is stuck shut?

I agree, it's not adding up, but I am running out of things that "make sense"...sadness

Rebuilt injectors have a long history of causing problems. An injector balance test would 100% confirm or deny any suspicion.

 

[RMac80]

Weekend report. Drove the thing all weekend about 300 miles. Mixed city/byway driving. Some long (~2hr) and short across town drive cycles. There was only one time when the strange O2 sensor readings came up as described above.

During that drive cycle, the Jeep/OEM code reader was showing the 12,777.7 fuel trims consistently. But when reading generic OBDII code reader was not showing those values, but a lot of zeroes which is a good thing.

SO FOR THOSE OF YOU WHO CAME TO THIS THREAD SEEING SIMILAR STRANGE FUEL TRIMS...
I would say it is an issue with an aftermarket scan tool trying to read a Jeep/OEM data stream, as others have suggested elsewhere.

As for my phantom P0301 code, it did not happen this weekend. In fact my CEL went off. If it comes back, I will chase issues that impact individual cylinder (injector swap, injector and coil wiring, etc) as @ItchyDog suggested. Possibly get the PCM checked /flashed this winter.

It seems the O2 trims were not a real issue, but an issue with my diagnostic meter. This is a good thing because having an O2 issue (or MAP, vacuum leak, manifold leak, etc) would impact more than one cylinder. Not matching my very consistent P0301 issue.

Thanks again Jeep Gurus! You guys make owning and working on this thing fun instead of frustrating!

 

[ItchyDong]

You need to look up “substituted values” when using scan tools. There are times when viewing OEM data using aftermarket scan tools. OBD2 is not allowed to substitute values. It’s a known thing to always try to look at OBD2 when an erroneous reading pops up on the OEM side. However OEM has a lot more data PIDS than OBD2 so sometimes that’s not a possibility and has to be verified with a scope or a meter.