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  1. #1
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    Negative Long Term Fuel Trim Diagnostic

    Hey All,

    Car - 1998 GTS
    Exhaust - M&M long tubes, 3 inch catback with high flow cats from belanger. All with less than 1000 miles.

    So I am getting pretty close on the car after chasing down what seems issue after issue. I thought I was there after having the heads rebuilt by nth moto. Since everything seemed great thought it would be time for the final tune. Working with Unleashed (Terrie), but we unconverted that the long term fuel trim on bank 2 is excessively negative. To date I have replaced pretty much everything due to not having a ton of history on the car. Short terms on bank 2 were also really bad, but that seemed to be cause by the passenger throttle body being adjusted wrong, it was taking in quite a bit more air than the driver side, after adjusting short terms lined up.

    I have been trying to track down if it could be a vacuum issue, the intake manifold gaskets were replaced twice, so those should be good. Once for good measure, once when I had the heads rebuilt.

    I'm at a loss. At idle, Short term is fine generally averaging in the -1 to +1 range on both banks. Long term bank 1 average -2 to -1. But bank two is -13. I've reset the pcm and it will read zero, car gets warm, and bank two long term starts climbing.

    I'm data logging using SCT live.

    Ideas?

  2. #2
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    Also, at the expense of sounding like an idiot, bank two is passenger, correct?

  3. #3
    Enthusiast Old School's Avatar
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    I'd swap the upstream O2 sensors left/right, and see if it follows the sensor.

    And yes, passenger(right) is bank 2.

  4. #4
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    Quote Originally Posted by Old School View Post
    I'd swap the upstream O2 sensors left/right, and see if it follows the sensor.

    And yes, passenger(right) is bank 2.
    I actually just replaced all four. Same issue. I just read more about the fuel trims. As I was trying to get it working I was more focused on the short term trims, sounds like I didn't fully understand how it works. Seems like with time the short term will find therir way to zero while the long term still report the problem. I thought I could adjust by reading the short term live, which doesn't seem to be the case.

    Maybe those TB adjustments didn't fix anything. I will have to take a look again tomorrow. I don't think it could be a cat. Bank 2 was the side I was having a problem with the cylinder 10 plug going brown after a 100 miles,but that resolved when I had the heads rebuilt, found two cracked guides.

  5. #5
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    Just came across this. https://www.justanswer.com/dodge/1dz...re-timing.html

    "You may be aware of the manner fuel is supplied to the injectors on your Viper. It's fed in through the upper intake plenum casting and can develop leaks, leading to fuel system variances (side to side) or even single cylinder misfire. Testing isn't easy, requiring a fuel system pressure gauge at the least."

    We that require a new intake manifold?

    Now that I better understand the LTFT I will do a couple more scans tomorrow.
    Last edited by RetroJonez; 09-09-2021 at 10:25 PM.

  6. #6
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    You should not even be monitoring your long term trims for tuning. You should be looking at your short term trims only. Matter of fact the best way to tune is with the long term trims turned OFF. Then once you have your base maps established with the long terms off you can turn them on.

    STFTs are the instantaneous sensor reading as processed by the PCM. If you have long term trims that almost immediately go whacko after a PCM reset you may need a new PCM. Long term trims should not begin to develop offsets that quickly. If you can locate a PCM to swap out I would consider that.

    Torrie should know all of this. Find another tuner.
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  7. #7
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    Torrie has been great, it's just been a matter of taking a look at everything, this was just one thing that stood out. Trying to get it in top shape. It's does start climbing after the pcm reset. I can get a video. Are the PCMs year specific?

  8. #8
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    Are there any pcm repair services that are viable?

  9. #9
    Enthusiast GTS Dean's Avatar
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    If your primary O2 bungs are near the stock locations, then you are really only getting short term trims from two primary pipes - (likely cylinders 5 & 6). The OE manifolds have all 5 pipes joined by that time so the primary O2s are reading a much more representative average of each bank.

    For months, I chased an engine miss that I just couldn't fix - with sizeable LTA imbalance. I tried injectors, wires and plugs; pulled the exhaust and tested each component for flow restrictions, swapped cats L/R and nothing solved the problem. Although I had replaced both primary O2 sensors with brand new OE units at rebuild, I decided to swap left and right bank units. It took only 1 short drive for me to get a MIL indicator on the cluster and the car started running really bad. I plugged in the DRB and it showed the Left Primary O2 (formerly the Right unit) had failed. I screwed in another new Mopar unit and it ran great at the Viper Days Reunion - but the majority of that driving was all Open Loop. I still have to get my car back from the transport unloading location this weekend so I can clean the car up and check the Adaptives.

    The PCMs did have some board architecture changes over time, along with different tuning flashes and software capabilities. I'm pretty sure that Dan Lesser can repair a JTEC if it isn't too far gone.
    Last edited by GTS Dean; 09-10-2021 at 09:18 AM.
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  10. #10
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    Quote Originally Posted by GTS Dean View Post
    If your primary O2 bungs are near the stock locations, then you are really only getting short term trims from two primary pipes - (likely cylinders 5 & 6). The OE manifolds have all 5 pipes joined by that time so the primary O2s are reading a much more representative average of each bank.

    For months, I chased an engine miss that I just couldn't fix - with sizeable LTA imbalance. I tried injectors, wires and plugs; pulled the exhaust and tested each component for flow restrictions, swapped cats L/R and nothing solved the problem. Although I had replaced both primary O2 sensors with brand new OE units at rebuild, I decided to swap left and right bank units. It took only 1 short drive for me to get a MIL indicator on the cluster and the car started running really bad. I plugged in the DRB and it showed the Left Primary O2 (formerly the Right unit) had failed. I screwed in another new Mopar unit and it ran great at the Viper Days Reunion - but the majority of that driving was all Open Loop. I still have to get my car back from the transport unloading location this weekend so I can clean the car up and check the Adaptives.

    The PCMs did have some board architecture changes over time, along with different tuning flashes and software capabilities. I'm pretty sure that Dan Lesser can repair a JTEC if it isn't too far gone.
    The O2s are located in the collectors on each side. Those should be good. This problem has shown with 2 O2s, I could still try switching them. I had one in and it was reading this way, so I swapped with a new one just in case. That was all in the last week. I could trying changing the locations just to see, but I am not sure two new O2's would be faulty. I'll give Dan a call. I did order a new old stock intake just in case, was able to get it for $300, if it's not needed, I'll just hang it on the wall in the bar.

  11. #11
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    Make sure your long tube header collector does not have any leaks as it exits into your 3" catbacks...if there are any leaks, it will draw a pulse vacuum back into the collector where that 02 sensor sits skewing your actual AFR readings and fuel trims.

  12. #12
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    Quote Originally Posted by ellowviper View Post
    Make sure your long tube header collector does not have any leaks as it exits into your 3" catbacks...if there are any leaks, it will draw a pulse vacuum back into the collector where that 02 sensor sits skewing your actual AFR readings and fuel trims.
    I just had them off and reapplied some copper gasket to the flange and retorqued them, same reading. Not to say it might not be leaking. Any other way to double check for a leak, it's so hot and loud under there I can't really tell. Seems like soapy water just burns off quick.

    Unless my PCM isn't fully resetting. Prior to the headers, the passenger sides stock exaust manifolds kept leaking through the vclamp. One actually broke. So it wouldn't surprise me if it was reading that way prior. But since the long tube headers everything should be tight.
    Last edited by RetroJonez; 09-10-2021 at 10:41 AM.

  13. #13
    Enthusiast Old School's Avatar
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    Is it throwing a CEL? or is this LTFT in the acceptable range.

  14. #14
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    Not throwing a code. Something is weird though. Reset the computer. Bank one long at zero initial read, bank two long immediately starts at -7.81, bank one short 0, bank two short 0. Then after it ran awhile the other banks started reading and values were fluctuating slightly, as I would expect, but bank one long just stuck at -7.81, then after a while it just jumped to -13.1. So something is weird there. This was the same with two new O2s as well. After a PCM reset those shouldn't just jump to a locked value. I did find the header bolts did loosen up a bit so I retorqued those.
    Last edited by RetroJonez; 09-10-2021 at 12:05 PM.

  15. #15
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    Well, I am going to chalk this up to a gremlin. Still a bit off on bank 2, still ironing out some stuff in the tune. But SCT starting reading it better, and it's now down into an acceptable range.

  16. #16
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    For what its worth, my left and right STFTs are never the same. THe drivers side (bank 1) is closer to zero but the passenger side (bank 2) is always in the -8-13 range...meaning its reading rich at idle. I'm nowhere near stock (GEN-IV top end), different injectors, and a big cam, so trims at idle are expected to be near impossible to dial in.

  17. #17
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    Quote Originally Posted by ellowviper View Post
    For what its worth, my left and right STFTs are never the same. THe drivers side (bank 1) is closer to zero but the passenger side (bank 2) is always in the -8-13 range...meaning its reading rich at idle. I'm nowhere near stock (GEN-IV top end), different injectors, and a big cam, so trims at idle are expected to be near impossible to dial in.
    That's goo to know. THen I am in the same range. Actually better after the tune. Everything seems good!

  18. #18
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    Well the mystery of the negative fuel trim is still popping up but on the short term side. My averages are fine but I'm getting some hesitation around 1,600 RPMs at Cruise. Checked the data logs and bank two is intermittently going -15 to -25. I haven't been able to track down any leaks, o2s are new, I have Casper extensions, o2s are located in the long tube collectors. I could record a log if anyone would want to give it a look. Under load everything seems fine. Averages are in the +/- 2 range but that cruise fuel pull is driving me nuts. The o2 connector on that side touched the header while I was testing it once. Melted the connector but the harness side looked fine, ordered a new extension and called it a day. Any chance the wiring could have went faulty down the harness? I have heat wrap on them now. I'm completely out of idea's. Idle is fine, load is fine, WOT is fine. Just barely on the gas at cruise is not.
    Last edited by RetroJonez; 10-31-2021 at 07:39 PM.

  19. #19
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    The barely on the gas cruise sounds like throttle body balance perhaps? Someone else may be able to chime in here a bit.

    As others have stated, STFTs are good for immediate readings, anything that needs permanent adjustment ends up in LTFTs as an overall percentage of correction across the board. LTFTs are what you wanna obviously look at here. Its odd that resetting the PCM wasn't bringing you back to 0 and holding for a while on LTFTs after startup, warmup, and a few minutes of runtime. Not holding near zero just screams leaky injector, vacuum leak, exhaust leak/issue, IAC leak or the likes to me.

    As for the harness and faulty wires, break out the wiring diagram and use a volt meter and check for resistance on the wires and continuity and see what you get from the plug to the location it runs to in the harness on the other end. That is really the only way you're going to find out for sure if that little mishap caused any issues.

    Overall, i'd check these (some i know you've already checked)

    * - Front O2 Sensors (lazily sweeping when car is warmed up, or are they cycling rapidly from 0 to 0.9ish volt?)
    * - Rear O2 sensors (should be lazy-ish and holding a lower voltage range around .60 -.80) They're checking that upstream is doing its job and cats are in place (looking for specific voltage range as to not trigger CEL)
    * - Idle air control motor stuck ? Gasket leak?
    * - Throttle bodies been adjusted or messed with recently? (in regards to your comment about light throttle cruise tip-in issues and stumbling)
    * - Throttle body gaskets all ok?
    * - IAT Sensor reading properly, temps climbing when you start the car from dead cold and let it idle and get up to operating temp?
    * - Vacuum leaks near the map sensor or to it or overall?
    * - Exhaust leaks ( Upstream of o2 sensors? or downstream just at collectors?)
    * - Intake manifold gasket leaks?
    * - Injectors (leaking?) Might be worth flowing/cleaning them and freshening up seals if nothing else is found.

    Buy yourself a cheapo smoke-machine and plumb it into the intake from the brake booster line when the car is off and cold and see if you can find anything leaking out. You MIGHT find your issue pretty fast.

    One other thing to think about is, are the sensors (O2 sensors for example) all the the factory oem location? If not, the PCM is expecting specific readings from a sensor within a certain range or distance from the outlet of the cylinder heads through the manifolds. If you move the sensor, it's going to have to adjust for this (thus the reason for LTFT). How much adjustment the PCM makes is anyone's guess however until its warmed up and throwing
    the percentage of correction into LTFT for the affected bank.

    These are where I would start. Some of these I realize you've already tried, but if you haven't tried them all, i'd run through every one of them and check them all until you find something. If nothing is found, then its possible the PCM is at fault here. Most of what i listed is controlling just about all the input/feedback to the pcm for it to make adjustments aside from things like cam/crank sensor. Of course this isnt an 100% complete list of things that affect drive-ability, but they're the biggest ones.

    Best of luck, tracing gremlins isn't fun but when found and fixed its obviously rewarding.
    Last edited by Lunchbox; 11-02-2021 at 09:27 AM.

  20. #20
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    Still tuning the short term tables with the long terms enabled? Even after recognizing that your long term tables immediately go wacko after a flash? TURN THE LONG TERMS OFF UNTIL THE SHORT TERMS ARE CORRECT.

    Another item that I learned from tuning with Dan Lesser is the primary oxygen sensor location on long tube headers. The common thought is that if you have it in the merge collector you'll read all 5 cylinders. Sounds sensible. Versus the sensor bung that would be supplied in the single down tube like Belanger does.

    We tried both locations and could never get the merge collector location to tune correctly coming off idle versus the single tube location. The reason why? Distance from the combustion chamber. At idle, port velocities are so slow that when you come off of idle the sensor is reading too late as compared to what's happening in the engine at that exact moment. Moving the sensor back to the single down tube solved all low port velocity tuning issues. Well, that and having a tuner that knows not to tune with the long terms enabled...

    Dan is one of the elite tuners in the Viper world. And when on tuning this car we had gone through 70 log files and short term fuel table updates to get them correct before turning the long terms on, I respected that he doesn't cut corners. The results spoke for themselves.
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  21. #21
    Enthusiast GTS Dean's Avatar
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    Dave is correct. The STFT have to be read close to the combustion chambers for quick response. Look at the OE manifolds - the O2 sensor is probably within 12" of the port, and it is reading a blend of all 5 cylinders for a representative average. If your O2s are way downstream - like by the side sill turnouts, there are too many milliseconds of delay between the combustion event and the measurement downstream. Port scavenging with headers is also going to be affected at different engine speeds. The stock setup stays bottled up and is less sensitive relative to a better system.
    Last edited by GTS Dean; 11-02-2021 at 01:13 PM.
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  22. #22
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    To add to the above, collector locations generally just SUCK on long tubes, both for delay, but also sensor temp and contamination. It doesnt take much O2 to screw up a reading, and the heaters generally are far from great, resulting in sensor cooling below thresholds that far from chamber.

    General rule is keep sensor within 12-18" of the head, unless you have a full standalone capable of both overdriving the heater and controlling response times to compensate.
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  23. #23
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    My options for the O2s I have now are in the collector or in one pipe. I think cylinder 6 and 5 in the long tubes. Before tuning I was asked to move them into the collector. But those are way down by the sill before the downturn. I could move them back up and test. I don't know if the long terms were disabled during the tune. I didn't have cats and the car and couldn't find someone to tune with the cats on, so I threw the high flows back on assuming they needed to be there for the downstream. Thanks everyone for the support, the help has been invaluable. Just joined the VOA as well, guessing it might take few days for my profile to update. Some of this data stuff is a bit out of my wheelhouse.

    Sounds like I should move the O2 closer to the heads and log again before I get too crazy. The are M&M headers so pretty much the same as the belanger. I also have some of those 02 things that back the downstream sensor out of the exhaust to prevent codes, but I am not getting a code. Should I throw those in?

    Log from when the ST goes really negative.
    Screenshot 2021-11-05 141723.png
    Last edited by RetroJonez; 11-05-2021 at 03:42 PM.

  24. #24
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    * - Front O2 Sensors (lazily sweeping when car is warmed up, or are they cycling rapidly from 0 to 0.9ish volt?) They actually go negative sometimes. See screenshot, grabbed from one of my pulls where the short term takes a crap.
    * - Rear O2 sensors (should be lazy-ish and holding a lower voltage range around .60 -.80) They're checking that upstream is doing its job and cats are in place (looking for specific voltage range as to not trigger CEL)
    * - Idle air control motor stuck ? Gasket leak? Replaced
    * - Throttle bodies been adjusted or messed with recently? (in regards to your comment about light throttle cruise tip-in issues and stumbling) They were. I adjusted per some of the threads that said to have the drive open a hair sooner, this actually solved my low rpm surging through parking lots.
    * - Throttle body gaskets all ok? Haven't checked those. I do have gaskets I could put in though.
    * - IAT Sensor reading properly, temps climbing when you start the car from dead cold and let it idle and get up to operating temp? I will have to check that.
    * - Vacuum leaks near the map sensor or to it or overall? Tried to smoke test using a home made but it was not very good.
    * - Exhaust leaks ( Upstream of o2 sensors? or downstream just at collectors?) Haven't been able to find any actually pulled the exhaust and resealed everything and used new band clamps.
    * - Intake manifold gasket leaks? Replaced when I did the heads. Although the manifold bolts were a pain to line and get in, always felt like the manifold was slightly warped. I do have a new intake and extra gahskets I could try. But it should be good.
    * - Injectors (leaking?) Might be worth flowing/cleaning them and freshening up seals if nothing else is found. Injectors were cleaned.

    Buy yourself a cheapo smoke-machine and plumb it into the intake from the brake booster line when the car is off and cold and see if you can find anything leaking out. You MIGHT find your issue pretty fast.

    I'll try getting a cheap smoke machine. The home made one I tried I attempted to seal through the TB using a rubber glove. Hilariously it didn't work very well.

  25. #25
    Instead of a smoke machine, you can just pressurize the intake and listen for the leak.

    I built my boost leak tester with a 3" silicone coupler, a PVC end cap, and a valve stem. 5-10 psi is all you need.
    Last edited by usmcfieldmp; 11-08-2021 at 04:26 PM.
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