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Discussion Starter · #1 ·
I have observed that when idling for exceptionally long times such as in the drive thru and such, that my xterra's engine starts to stumble and shake the entire car. All of the fuel trims are not what they should be and a few O2 sensor voltages aren't fluctuating much. The short term fuel trims are between 18% and 25%. I have replaced the worn out and split intake boots and that did not solve the problem. What could this be? Other than idling for long periods of time, the truck seems to run great and it gets decent fuel mileage. What could be the issue here? I have also looked everywhere for vacuum leaks and I have not found any more than the intake boots that I replaced. Starting to get somewhat confused here. Any ideas? Pictures of the conditions below.
 

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I have observed that when idling for exceptionally long times such as in the drive thru and such, that my xterra's engine starts to stumble and shake the entire car. All of the fuel trims are not what they should be and a few O2 sensor voltages aren't fluctuating much. The short term fuel trims are between 18% and 25%. I have replaced the worn out and split intake boots and that did not solve the problem. What could this be? Other than idling for long periods of time, the truck seems to run great and it gets decent fuel mileage. What could be the issue here? I have also looked everywhere for vacuum leaks and I have not found any more than the intake boots that I replaced. Starting to get somewhat confused here. Any ideas? Pictures of the conditions below.
 

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How does your exhaust pressure feel out of the tailpipe? If you put your hand right behind your tailpipe and have a helper rev it up you should feel some significant pressure. If it seems weak or even if it seems fine, I would take the upstream 02 sensors out and see how it idles with both of the sensor ports open. You can get to them pretty easy if you take the front wheels and splash guards off. If it idles better after that then you might have bad sensors or blocked cats.
 

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Discussion Starter · #4 ·
I have observed that when idling for exceptionally long times such as in the drive thru and such, that my xterra's engine starts to stumble and shake the entire car. All of the fuel trims are not what they should be and a few O2 sensor voltages aren't fluctuating much. The short term fuel trims are between 18% and 25%. I have replaced the worn out and split intake boots and that did not solve the problem. What could this be? Other than idling for long periods of time, the truck seems to run great and it gets decent fuel mileage. What could be the issue here? I have also looked everywhere for vacuum leaks and I have not found any more than the intake boots that I replaced. Starting to get somewhat confused here. Any ideas? Pictures of the conditions below.
How does your exhaust pressure feel out of the tailpipe? If you put your hand right behind your tailpipe and have a helper rev it up you should feel some significant pressure. If it seems weak or even if it seems fine, I would take the upstream 02 sensors out and see how it idles with both of the sensor ports open. You can get to them pretty easy if you take the front wheels and splash guards off. If it idles better after that then you might have bad sensors or blocked cats.
Well I cleaned the MAF and at idle and all around the fuel trims are now reading what they should be at. Not sure if that fixed it though
 

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Your long term fuel trims are pretty high as well - which is a better indicator. Since its both sides it must be something that affects both - like the MAF sensor may have been it - if the fuel trims come down your likely good now. If it comes back it could be something like a dirty throttle body, or possibly a weak fuel pump or clogged fuel filter - which unfortunately is part of the fuel pump.
 

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Discussion Starter · #6 ·
Your long term fuel trims are pretty high as well - which is a better indicator. Since its both sides it must be something that affects both - like the MAF sensor may have been it - if the fuel trims come down your likely good now. If it comes back it could be something like a dirty throttle body, or possibly a weak fuel pump or clogged fuel filter - which unfortunately is part of the fuel pump.
I just went and idled it again and most of the time the short term fuel trims are reading at 14% at idle. They rarely go normal as most of the time theyre at that stated percentage. I have cleaned the throttle body recently as well.
 

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Sounds like you need to do the throttle position relearn the idle re learn and the air fuel ratio reset
 

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I just went and idled it again and most of the time the short term fuel trims are reading at 14% at idle. They rarely go normal as most of the time theyre at that stated percentage. I have cleaned the throttle body recently as well.
Short term fuel trims are sort of meaningless as a diagnostic. Long term fuel trims are what you need to pay attention to. They should stay between +/- 5% in most cases.

MojaveDesertman gave sage advice. If you haven't relearned everything -you need to do that next.
 

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Discussion Starter · #9 ·
Short term fuel trims are sort of meaningless as a diagnostic. Long term fuel trims are what you need to pay attention to. They should stay between +/- 5% in most cases.

MojaveDesertman gave sage advice. If you haven't relearned everything -you need to do that next.
I tried that but I can try it again tomorrow. My long term fuel trims were 12.5% for bank 1 and 10.2% for bank 2. They both stayed at that despite what I fixed or tried to fix. O2 sensor maybe?
 

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From what I understand, the b1s1 is the bank one or passenger upstream o2 sensor and the b1s2 is the downstream sensor.. same thing for b2 on the drivers side. A 99% from the downstream sensors would make me want to look there.

In my X I was recently getting +25 b1s1 and -20% b2s1. No readings either side from the downstream sensors. Very rough idle, random misfires, and no power. I got a new maf sensor, tried cleaning the throttle, relearning the throttle and the idle, smoking the system to look for vacuum leaks with no luck. This is all after new head gaskets, rebuilding the front end, all new oem cam and crank sensors yadda yadda. I finally got down to pulling the upstream sensors and all of the problems got way, waayyy better. Smooth rpms all the way up no misfires or hesitation. Idle rock steady at 700. And that was just from being able to exhale out the sensor ports.. no o2 sensors affecting the trim, and way incorrect back pressure. Took off the cats and b1 was hollow in the primary and blocked like a brick of carbon in the secondary. B2 was blocked solid in the primary. All the sensors looked like toast. I'm installing new cats and sensors this weekend and will update my own post with details when I'm done.

If everyone else's advice about the intake system doesn't work out, pull the upstream sensors and start it up. It's a cheap and easy thing to test.

edit Forgot to add that at no time was I ever getting a code for the cats.. idk. 🤷‍♂️
 

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Short term fuel trims are sort of meaningless as a diagnostic. Long term fuel trims are what you need to pay attention to. They should stay between +/- 5% in most cases.

.....
I have read elsewhere that LTFTs within +/- 10% is acceptable. FWIW.

Personally, I have no idea what an acceptable range is and would expect a lower ranger to be better.
 

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Short-term fuel trim acts as an immediate response to reach stoic. So yes it is not terribly useful for diagnostic.

Since long term is pretty healthy at 9% (one snapshot we don't know if it changes under load, etc) I too would be more suspect of the upstream sensor as mentioned.
However lean codes are most commonly caused by unmetered air in the intake, inspect all vacuum lines for cracks or perform a smoke test. Do you have lean codes for one or both banks?

You may also try clearing the codes/fuel map and seeing if your previous repair was most successful than originally thought.
 

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We know very little about the OP's X (age, mileage, driving habits, fuel source, maintenance). So, in the spirit of guessing, and in addition to the excellent suggestions above, I would:

+ clean the intake butterfly valve.

Reported symptoms match a dirty intake butterfly valve to a T. Air throttle body intake cleaner is widely available, cheap and relatively easy to apply with a soft cotton cloth, and a clean toothbrush. Be careful not to leave anything behind in the intake.

+ if not already done, replace the PCV valve and hose. This will unlikely resolve the current problem but will work to avoid/minimize future problems.

Once the rough idle is fixed, I would run a bunch of concentrated PEA fluid through the gas tank. That should help clean up injectors and rings. Chevron Techron is widely known and liked. There are many other similar, often less expensive fuel treatments out there.

Then I would change the motor oil and filter.

Personally, I run the most expensive long-life synthetic oil available coupled with long-life oil filters (all purchased on sale). And then I typically change the oil once a year because I put relatively few kilometers on the X and they are almost all highway/off-road klicks.

Serious trail riders and towers as well as stop-n-go commuters may wish to change the oil more often.
 
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Discussion Starter · #15 ·
I have cleaned the MAF sensor, replaced the intake hoses that were torn, I even did the idle relearn procedure multiple times and the fuel trims are still reading at 14% at idle. I’ve checked all of the vacuum lines as well and they’re all intact.
 

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How does the engine idle immediately after a cold start up? The upstream oxygen sensors (sensor 1 on each bank), which Nissan calls Air/Fuel (A/F) ratio sensors, aren't used as inputs by the ECM until the vehicle has warmed up a bit. "Open loop" control as it's called. If the engine idles fine during a cold start up, which is when the upstream A/F ratio sensors aren't be used, then consider testing the upstream oxygen sensors.

The low Sensor 1 voltage readings in the images provided are a conundrum. Both banks are reading low. Was the engine at fully warmed up, meaning at operating temperature, when those screen shots were taken? The Service Manuals call out of 2.2V for sensor 1 values.

I'd dig into this since the ECM is adding additional fuel, which if unburned during combustion, can end up downstream and burn in the cats, shortening their life.

Check the STFTs & LTFTs at both idle and a higher RPM. It was stated that at warmed up idle is when the symptoms occur and not at higher rpm. It can't hurt to spray some starting fluid at idle to confirm no vacuum leaks.
 

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Discussion Starter · #17 ·
How does the engine idle immediately after a cold start up? The upstream oxygen sensors (sensor 1 on each bank), which Nissan calls Air/Fuel (A/F) ratio sensors, aren't used as inputs by the ECM until the vehicle has warmed up a bit. "Open loop" control as it's called. If the engine idles fine during a cold start up, which is when the upstream A/F ratio sensors aren't be used, then consider testing the upstream oxygen sensors.

The low Sensor 1 voltage readings in the images provided are a conundrum. Both banks are reading low. Was the engine at fully warmed up, meaning at operating temperature, when those screen shots were taken? The Service Manuals call out of 2.2V for sensor 1 values.

I'd dig into this since the ECM is adding additional fuel, which if unburned during combustion, can end up downstream and burn in the cats, shortening their life.

Check the STFTs & LTFTs at both idle and a higher RPM. It was stated that at warmed up idle is when the symptoms occur and not at higher rpm. It can't hurt to spray some starting fluid at idle to confirm no vacuum leaks.
It was at operating temperature and running when those pictures were taken. I have checked absolutely everywhere for vacuum leaks and have even sprayed and I have found nothing.
 
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