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Thread: 25% ltft

  1. #1
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    Default 25% ltft

    I'm pretty much in the dark when it comes to tuning. I'm starting up a new build and have very rough running and severe backfire....
    SBE LQ4, L92 heads, LS3 stock intake and injectors. Custom cam and bench tune by EPS.
    Upon startup, everything was good. Fired immediately, idled smooth, sounded good with open headers. This was only about a minutes worth of time. I didn't even have coolant in it yet. After a few short startups it started running rough and backfiring really bad. I thought maybe because open headers. Few days later, got loose ends tied up including the exhaust on. No change. It backfires and won't idle. I changed plugs. #2 & 4 were BLACK. Tried again. Holding rpm around 2000, and entering CL, the LTFT on bank 2 goes to 25%. Bank 1 at 12%.
    Why would they do this?
    Did I screw up by starting a few times with open headers?
    If so, why would only 2 & 4 be so black?

  2. #2
    Joe (Moderator) joecar's Avatar
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    It would do that with open headers (the low pressure in between each pulse causes fresh air to be pull in enough to hit the O2 sensors, causing trims to go positive).

    Try clearing the trims (use the DVT tab), and try again.

    If it keeps doing this, then check for air leaks between MAF and header collector.

  3. #3
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    Great. thank you for the info. I'll give it a shot.

  4. #4
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    I disabled LTFT at Fuel-Trims-Parameters thinking it would stop the 25% add till I can get this figured out. Also cleared Fuel Trims using DVT. Installed new plugs.
    But I can't even get to Closed Loop now. The engine is running really rough and severe popping out the exhaust. I shut it off only about 20-25 seconds in. I did scan that if anyone wants to look at it. I don't see it telling me anything, but again, I'm new here.

    Aside from it missing so bad, what causes an engine to pop so severely out the exhaust, AF ratio, bad gas, timing?

  5. #5
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    If you disable LTFTs, I'm pretty sure you can't run in Closed Loop. The ECM needs the O2 feedback to make adjustments to the fueling based on output (LTFT/STFTs).

    At LTFTs +25%, it means the tune is stupid lean. You'll need to figure out why, is there a MAF? Where is it located? Have you talked to EPS to see if they'll get you a tune update now that you've fired the engine up?
    ~Erik~
    2013 Sonic RS Manual - 1.4L I4T E78, tuned, turbo mods, etc.
    2008 TrailBlazer SS 3SS AWD Summit White - LS2 E67/T42, bolt ons, suspension, etc.
    2002 Chevy TrailBlazer LT 4X4 Summit White - 4.2L I6 P10, lifted, wheels, etc.

  6. #6
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    Above Joe says more than likely I caused this issue by starting the engine with open headers a few times. So I understand how it would think its lean and bumping LTFT to 25% when it gets to that point.
    Standing back and looking at this....the engine fired right up, idled steady and sounded good for the first 4 or 5 times I started it. Only 15-30 seconds at a time. 5th & 6th time is when it started missing and popping severely thru the exhaust. That's the first time I saved the tune from the PCM. Did I save bad(?) data or does the PCM not change the actual tune that I would save?
    On another suggestion, a fellow helped me get the LTFT disabled so that wouldn't be adding to the problem why I try to figure this out. Thats the only reason they were disabled.
    I re-flashed the tune I saved from the PCM with LTFT disabled and clearing fuel trims like Joe suggested. I expected the car to start and run smooth just like it did on first srart. It didn't. So theres my questions....did I save a tune that was already altered in that first minute or so?

    To answer your questions, 85mm MAF, its temp mounted to a 12" long 4" tube straight in front of the TB. A cone filter is clamped on the end of it. I have not talked to EPS. This tune and cam was ground years ago. Life got in the way and I couldn't complete this till now.

    Again to back up a sec...it won't hardly run at all now. Severe popping and runs extremely rough. Shouldn't reflashing have at least got me back to square one?

    Thanks for looking at my thread!

  7. #7
    Lifetime Member 5.7ute's Avatar
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    I would be double checking the valvetrain.
    The Tremor at AIR

  8. #8
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    Really...what are you thinking is happening?

  9. #9
    Lifetime Member 5.7ute's Avatar
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    My thoughts are that if it started fine at first without a hiccup, and now has a miss, then there is the possibility that a mechanical failure has occurred.
    The Tremor at AIR

  10. #10
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    My thoughts as well...not necessarily a mechanical failure, but maybe that, bad gas, loose grounds....just something other than the PCM causing this. That's why I'm directing my questions to anyone who can tell me that when I realized I needed to retrieve the tune from the PCM, did it already change itself due to the operating conditions of those first few minutes.

    One of my replies in this thread is missing. I emailed Joecar about it. I guess this would make more sense if I typed it out again
    Last edited by KCG; August 3rd, 2017 at 12:24 PM.

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