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Thread: 95 LT-1 Idle Cell Comparison - Humidity?

  1. #16
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    At startup the IAC are set at 144, I think that setting can be increased somewhere in the bin. The intake passage will affect only idle quality and you should eliminate every other possible reason before doing it.
    Too much and you will never have a good idle again, The individual cylinder trims will get all around and will need a fresh tune from zero. Even GM made different size holes to compensate for the unequal air distribution.
    Once I made the mistake to make the holes the same size and the idle was crap with constant surging. Only fine tuning ind cyl trims make it run stable.

    Humidity hits the ignition system to the limit, so it will be good to take a look at that first.

  2. #17
    Fuel Injected! spfautsch's Avatar
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    Quote Originally Posted by kur4o View Post
    That 02s and BLM reading looks weird enough. Did you mess with the closed loop settings. Does the AIR system present and functioning. 02s looks frozen at stratup and really slow moving after that. They should be pig rich at startup and I noticed blms were 120 at open loop meaning they are pulling fuel.
    AIR system has been deleted. Closed loop settings aren't stock b/c of headers and cam. But I suspect the injector constant is proabably too high causing the weirdness.

    I wasn't aware that long-term trims were used before the engine has run in CL.

  3. #18
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    the maf isn't really good with air density, im sure it uses barometric pressure to tweak fueling in some way in MAF mode (although i haven't looked at the code). either way ~1 KPA of baro difference shouldnt' do this to you. something else is going on.

  4. #19
    Fuel Injected! spfautsch's Avatar
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    Quote Originally Posted by steveo View Post
    the maf isn't really good with air density
    I'll have to dig up where I read this, but the word "mass" is used in the name because it's designed to measure air mass through the sensor. This implies it compensates for temperature and pressure by design. I'm going to have to dig but I believe this was the same source that mentioned humidity being the only thing a maf can't compensate for, only the O2s could do that. It might have been a Jeff Hartmann book or it might have been something I found online.

    I'm going to work with the throttle blades to give it more air at startup and try to tune around that b/c it responded very well there. I suspect most of my other problems stem from moving the injector constant too far.

    I'm also going to bump up the dwell in my controller to see if anything improves because I'm not sure where I came up with the dwell figure I used for these coils. I'm 99% confident there are no other issues with the ignition system. When the air was cooler and drier it was firing immediately every time and idling better than it ever did with the mechanical distributor. When I first saw 42-43kpa at idle I had to double-take on it because the last I recall seeing that good of idle vacuum was when it was stock.

    Thanks for all the ideas guys.

    I'd really like to know about the BLMs question two posts back. If no-one is sure I'll have to dig into the disassembly.

  5. #20
    Fuel Injected! Terminal_Crazy's Avatar
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    I’ve lowered my IAC values at startup around 40 i think.
    Thereseems to be startup values in tables but it usualy starts as 160 ish.

    I also reduced the air bleed size in the throttle body after reducing the idle timing as the iac valve couldn’t go lower.
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

  6. #21
    Fuel Injected! spfautsch's Avatar
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    Mitch am I mis-remembering, or did I read somewhere that you'd also drilled out your port orifices (the holes pointing at the injector tips)?

    It sounds like there might be some misunderstanding and confusion on the subject - the how-to I've seen about setting a base bypass hole size and adjusting the throttle blades fails to mention that it should be done at maximum air density for the climate you drive in. For example if you're in a cold weather season and consequently high air density, you want lower IAC counts - say in the 20s at hot idle. This is the point where the engine needs the least amount of idle airflow.

    I'm at the other extreme of the spectrum. In minimum air density situations you want enough bypass air to reach your idle target speed during a cold start without maxing out the IAC motor (160). This is when the engine needs the maximum amount of idle airflow.

    Quote Originally Posted by Terminal_Crazy View Post
    Thereseems to be startup values in tables but it usualy starts as 160 ish.
    If you're logging 160 IAC counts during cold starts it's an indication that the engine speed is below your idle target and the ecu has opened the IAC as far as it can trying to correct the situation with more airflow.

    All this of course assumes your IAC position has been learned by the ecu. There's no feedback mechanism for the pintle position. The ecu has a relearn procedure it uses to extrapolate when the pintle is fully closed and then stores that position (not sure where or how) and tracks it whenever it commands a change in IAC position. But if you replace your IAC motor or the position is otherwise lost by the ecu, the counts aren't going to be very meaningful until the relearn routine runs. I other words, if your ecu thinks the IAC motor is at 50 and it's actually fully open, there's no way for it to know and it will continue to command it open in steps until the ecu's tracking variable is also at 160.

  7. #22
    Fuel Injected! Terminal_Crazy's Avatar
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    Hiya Scott,
    Yes I drilled the manifold bleeds, no 2 i think is smaller of the top of my head. I drilled them as the drilling was crap, flash left in so needed deburring and they come out all over the sides of port rather than consistantly at the bottom like the chevy manifold.
    Throttle body was drilled originally to get iac counts down. As timing was increased vac would improve and counts went down. As the tune and idle has improved (timing lowered) counts started dropping very low and ive fitted a restrictor back in. That’s why i suggested not drilling until your idle settles.

    I’ll check logs tomorrow but i’m sure this always starts with counts around 160 and drops. Idle starts about 1100 rpm when cold. I’ve replaced iac and done relearns etc, iac vs temp tables all lowered and remembered values don’t seem to work.

    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

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  9. #24
    Fuel Injected! spfautsch's Avatar
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    Had to play used car salesman tonight so no progress on any front of interest. Thought I'd share this tidbit I found - not the source I recall finding years ago, but makes me feel better knowing I'm not hallucinating memories / facts / whatever.

    From this page talking about some newer humidity sensor equipped MAFs being used on the newest Duramax engines.

    https://gearsmagazine.com/magazine/a...rflow-sensors/

    Quote Originally Posted by gearsmagazine_pleasedontsueme
    Humidity has a major impact on the density of the air and the amount of oxygen contained in the air column traveling into the intake manifold. Humidity takes up space, leaving less room for oxygen molecules.

    Standard 5-pin MAF sensors are unable to compensate for the water vapor in the air, which leads to rich air fuel ratios when high water vapor conditions are present.

  10. #25
    Fuel Injected! Terminal_Crazy's Avatar
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    Quote Originally Posted by spfautsch View Post
    What are your ambient temps like this time of year?
    Anywhere from 5C to 35C. Generally it’s very cloudy so in 20’s. 30 ish when the sun comes out.
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

  11. #26
    Fuel Injected! spfautsch's Avatar
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    We generally don't see lows near 5C from about mid May to mid October. I'm guessing your average monthly temp (i.e. several hundred samples per day averaged over 30 days) is a solid 3-6C lower. That doesn't sound like much but in terms of air density it is.

    Whatever the case, I think a case of mistaken identity has been a major contributing factor. I won't be able to test until later tonight, possibly even tomorrow but I think the combination of all of the aforementioned factors have stacked up to cause me the severe cold start problems.

    This, boys and girls is the part of the show where Scott eats his hat in front of you all. :-) Anyone have any ketchup?

    After a bit of research it seems like this coil, which is what I have:



    Is not one of the supposed "hot" or "truck" LS coils. Seems it's electrically similar to the common LS coils and needs a target dwell of 6ms.

    Whereas this coil which looks quite similar:



    Is a "hot" or "truck" LS coil and needs a target dwell of 4.5ms.

    Anyone want to guess what target dwell I was using?

  12. #27
    Fuel Injected! Terminal_Crazy's Avatar
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    Not the one you thought! ��

    Would anyone consider opening the manifold idle air passages up any bigger?....

    Went to the track on Saturday. (1/8) Had a fantastic day but traction was diabolical.
    Spun all through first even feathering the throttle and clutch. It’s never that bad on the road.
    Best 9.18 @81. 60’ 2.399 so more mods over the winter.
    However the rear seal on the manifold needs resealing after I blew it out arsing about with pcv so I’ve just ordered manifold gaskets and need to pull it off again.

    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

  13. #28
    Fuel Injected! spfautsch's Avatar
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    Oh no, it was exactly what I'd intended it to be. I'd just labored through the whole diy-ltcc project under the assumption the coils I have were the 4ms "hot" ones. I wish there were datasheets available for these items like injectors.

    Unfortunately bumping dwell up didn't miraculously solve any problems. Cold start (albeit from 33C and ~50% RH) was pretty much the same. IAC was pegged until about 4 minutes in. O2s hovered around 460-560mv and the ecu dropped out of CL for a frame or two numerous times during the 5-1/2 minute log. So I guess I'm back to the drawing board.

    I think I'll try a SD flash and see what happens.

    When / if I ever get this figured out I intend to experiment with different sized bypass orifices to answer the question of whether enlarging the port feed holes is in any way necessary / beneficial. In fact at this point I have nothing to lose by removing my 1/4" npt orifice plug (leaving a 1/2" bypass hole) and see if it doesn't let the engine run away when started.
    Attached Files Attached Files

  14. #29
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    Well, I was going to say, "unplug the MAF, tune for speed density", but you beat me to it.

    All this talk about humidity being a major factor reminds me of when I determined the exact nature of the MAF sensor. A typical sensor heats an element 100 deg C above ambient temp to determine the specific heat of the airflow. This crudely compensates for humidity as well, but not in an exacting way. So technically, the output of the sensor is less sensitive when it is humid out, since the cooling effect of humid air is less (and also less o2 by way of water vapor displacing the usable oxygen). Crude, but effective for the emissions at the time of manufacture.
    I just hope the MAF is accurate at low RPM with a choppy idle, but I suspect not. Tinkering with the MAF calibration might be viable, only if the sensor is still good. I say clean the MAF sensor, just in case. I suspect the MAF sensor won't be used to a 383 sucking on it as opposed to a 350. Best to eliminate it for now, then come back to it.

  15. #30
    Fuel Injected! spfautsch's Avatar
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    Hey vilefly - have any plans this Saturday? Care to make a trip to central misery? I'll pay for your fuel. I could use some help measuring dwell time and led and coil igniter current draws with my circa 1984 CRT oscilloscope.

    I'm nearly tempted to ask the mods to delete this thread. There's enough inaccurate information in it regarding the nature of mass air flow sensor operation that it's quite rudely reminded me why I hate asking for help.

    All I was hoping for is someone who perhaps

    a) also has a MAF enabled 383 gen 2 LT-1

    and

    b) lives in a similarly hot and humid climate

    to chime in with any thoughts about the idle airflow numbers I'm seeing and IAC bypass size so I can assess how much effort I should exert looking for a consistent misfire / weak spark situation.

    MAFs aren't evil. I can't believe there aren't thousands of cammed up 6.0, 6.2, and 7.0L LS based cars running around still using the airflow sensor for it's intended purpose. However, those guys all have the benefit of ETC to control idle air, so there's no need for an IAC motor, or a bypass. I can however believe no-one is doing the same with a 6.3L gen 2 LT-1, because most of the tuning "experts" encourage disabling the MAF and using speed density only. Things like this are what drive me to drink.

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