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Thread: 99 L67 breaking up over 3000 rpms

  1. #1
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    99 L67 breaking up over 3000 rpms

    Hey guys,
    I have a motor from a 99 GTP swapped into my 85 Fiero. I'm at my wits end with an acceleration issue, and I was hoping someone on here who has more experience can take a look at my data logs and tune and help me figure out what is going on. I have verified that the fuel pressure is good, the entire ignition system has been replaced and plugs were gapped properly. Every single time I try to rev past 3000 rpms the engine sputters and won't accelerate. I can see in the logs that there is occasionally knock involved, but timing gets pulled out every single time it happens.
    thanks in advance. Fiero see notes(rear o2 code delete).hptfiero2.hpl

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    Tuning Addict 5FDP's Avatar
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    The 15 degrees of knock is surely not good. Is anything rubbing on the frame or exhaust?

    Are the knock sensors in good condition?

    The log is missing the o2 sensor so it's hard to see if that is staying richer under heavy load or if it's dropping out lean. A wideband would obviously be better for this.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  3. #3
    Advanced Tuner ZeroBoostBuick's Avatar
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    Nothing stands out in the tune. Looks fine to me.

    Did you try and load the tune again just to see if it persists ?

    I've had a hiccup with fueling once. But when I re-loaded the same tune, the problem never returned.

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    Quote Originally Posted by 5FDP View Post
    The 15 degrees of knock is surely not good. Is anything rubbing on the frame or exhaust?

    Are the knock sensors in good condition?

    The log is missing the o2 sensor so it's hard to see if that is staying richer under heavy load or if it's dropping out lean. A wideband would obviously be better for this.
    Just to reiterate, the break up happens consistently even when there is no knock detected. Obviously knock is not good, but I do not believe it is the reason this is happening.

    I don't believe anything is hitting, I'll have to double-check though. As for the knock sensors, I know the motor was rebuilt before I bought the car, I doubt they replaced the knock sensors though. It is possible they are faulty.

    I do have plans of installing a wideband at some point, but the o2 info is in there. Obviously it is not exact, but both long term and short term fuel trims seem to be consistently negative under heavy load, so I would guess it is rich if anything.

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    Quote Originally Posted by ZeroBoostBuick View Post
    Nothing stands out in the tune. Looks fine to me.

    Did you try and load the tune again just to see if it persists ?

    I've had a hiccup with fueling once. But when I re-loaded the same tune, the problem never returned.
    I have flashed a few different tunes, but this has been a consistent problem even before I touched the tune.

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    Mods? What are your plugs gapped to? I owned a L67 car for 22 years...
    Daily Driver= 2003 BMW 330xi
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat)
    Project Car= 1991 Chevrolet Camaro RS (LQ4 w/ Gen 4 Rods, LS3 heads, turbo...)
    Truck= 2007 Chevy Silverado 1500 LT LY5 4x4

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    Quote Originally Posted by jaydubb71 View Post
    Mods? What are your plugs gapped to? I owned a L67 car for 22 years...
    Bone stock as of right now
    I'm running delco 41-606s at 0.057 gap.

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    Quote Originally Posted by 853800 View Post
    Bone stock as of right now
    I'm running delco 41-606s at 0.057 gap.
    Tighten the gap to 0.040 and see what happens. No trouble codes?

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    Quote Originally Posted by jaydubb71 View Post
    Tighten the gap to 0.040 and see what happens. No trouble codes?
    Correct, no codes. The recorded rpms are doing weird stuff pretty consistently now, a lot of fluttering and random drops and surges despite consistent throttle and load. I think I'm going to change out the crank position sensor, if the problem persists I'll close up the spark plug gaps.

  10. #10
    Advanced Tuner ZeroBoostBuick's Avatar
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    In the scan it shows right at 3000 RPM is exactly when you're going into Power Enrichment. Maybe a coincidence. I also don't see O2 readings in the scan.

    Try and scan as many parameters as logically possible.

    Only time I had sputter under throttle is when one of the spark wires was jumping ground when giving it some throttle. It idled normal, but as soon as I gave it more than 10 % throttle it sputtered.

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    Quote Originally Posted by ZeroBoostBuick View Post
    In the scan it shows right at 3000 RPM is exactly when you're going into Power Enrichment. Maybe a coincidence. I also don't see O2 readings in the scan.

    Try and scan as many parameters as logically possible.

    Only time I had sputter under throttle is when one of the spark wires was jumping ground when giving it some throttle. It idled normal, but as soon as I gave it more than 10 % throttle it sputtered.
    I don't see any arching from the wires.
    Am I wrong in thinking that LTFT bank 1 and STFT bank 1 are showing the o2 readings?

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    Tuning Addict 5FDP's Avatar
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    Yeah, you are wrong.

    You need to log the actual bank 1 o2 sensor mv channel.

    Fuel trims stop working once the computer enters open loop. There may be a carry over LTFT into open loop (PE enable) but that tells you nothing about what is really happening. If the o2 sensor goes from 880-900mv (richer) at the start of OL/PE to something like 50-100mv under load, then it's leaning out big time. This is why we always suggest a wideband because o2 sensors only tell you so much and don't tell you how rich or how lean the fueling really is.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

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    Quote Originally Posted by 5FDP View Post
    Yeah, you are wrong.

    You need to log the actual bank 1 o2 sensor mv channel.

    Fuel trims stop working once the computer enters open loop. There may be a carry over LTFT into open loop (PE enable) but that tells you nothing about what is really happening. If the o2 sensor goes from 880-900mv (richer) at the start of OL/PE to something like 50-100mv under load, then it's leaning out big time. This is why we always suggest a wideband because o2 sensors only tell you so much and don't tell you how rich or how lean the fueling really is.
    Okay, I'll get it scanned with that channel selected.
    I was also considering forcing it to stay in open loop and seeing if the problem persisted. If I'm correct I believe that would keep it from entering PE mode as well as the info from the o2 sensor not being used. If the problem does not happen in open loop, I would assume I have an issue with the o2.

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    Tuning Addict 5FDP's Avatar
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    Power enrichment still happens in open loop. You must have PE working when you get into boost, otherwise you'd be hitting 10psi of boost with a commanded AFR of 14.68. These 3800's would just love to chip a piston while doing that.


    I don't think you are quiet grasping what we are after here. The problems you have aren't o2 sensor related. We want you to log the o2 sensor so we can see what the fueling is doing with that sensor. A wideband o2 sensor would be better because it's going to tell you the real time air fuel ratio in the exhaust, the factory narrow band o2 sensor can not do this.

    If the car was going lean, you have a fuel supply issue. Be it tune related or fuel pump related. It should be maintaining steady fuel pressure at whatever the GM spec for that car was stock. I think a 3800 runs 43psi.

    The only thing else I can think of is some crazy exhaust restriction.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  15. #15
    Advanced Tuner ZeroBoostBuick's Avatar
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    With the Factory Narrowband O2's we can still see if the fuel/air mix is Rich or Lean. And we can guesstimate by the O2 millivolts during Power Enrichment if it's Rich, Very Rich, Lean or Very lean.

    Besides O2 millivolts, also set up in your scanner to read MAF LB/min, Injector Duty cycle, Injector Pulse Width.

    Play around with your scanner to add new channels in the main table (White table on the left).
    Last edited by ZeroBoostBuick; 08-22-2020 at 01:26 PM.

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    Quote Originally Posted by ZeroBoostBuick View Post
    With the Factory Narrowband O2's we can still see if the fuel/air mix is Rich or Lean. And we can guesstimate by the O2 millivolts during Power Enrichment if it's Rich, Very Rich, Lean or Very lean.

    Besides O2 millivolts, also set up in your scanner to read MAF LB/min, Injector Duty cycle, Injector Pulse Width.

    Play around with your scanner to add new channels in the main table (White table on the left).
    Cruise Wot 2.hpl
    I believe I captured all the data you guys asked for, if not let me know.
    Thanks again for the help

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    Weird that the MAP sensor isn't working in that log, it just stops are 86kpa.

    The fueling does take a nose dive per the o2 sensor, it drops way out. This is a weird one.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  18. #18
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    Quote Originally Posted by 5FDP View Post
    Weird that the MAP sensor isn't working in that log, it just stops are 86kpa.

    The fueling does take a nose dive per the o2 sensor, it drops way out. This is a weird one.
    According to my fuel pressure gauge, the pump is supplying enough pressure. At least it was as of last fall.

    I went ahead and replaced the crank position sensor because I saw some bizarre rpm readings here and there, but now the car won't start. It cranked for a few seconds very unhealthy sounding then backfired incredibly loud through the exhaust. I can't get the vcm scanner to connect to the pcm for some reason, but my obd scanner isn't showing any codes.

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    The balancer was 180 degrees off, i fixed it and the break up is gone. Still getting the same hp tuners error, I emailed customer service

  20. #20
    Advanced Tuner ZeroBoostBuick's Avatar
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    What HPT Error are you referring to ???

    Re-Install HPT's latest version and see if the error persists.

    I usually shut town all programs on the laptop, then restart the laptop, then proceed to download and re-install the latest vcm BETA version. Probably overkill, but gives me a piece of mind.