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

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  1. #1
    Fuel Injected! spfautsch's Avatar
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    95 LT-1 Idle Cell Comparison - Humidity?

    Been struggling with this problem for roughly 2 months. After a bunch of experimentation I started looking for compression loss because it was struggling to start and stay running, and most of the time died when it switched to closed loop.

    While I'm not leaning toward the ignition controller being the cause, as most of you know I'm running my experimental coil per cylinder controller (link in sig). It seems to run great once up to temperature, but it is certainly possible the ignition controller is a contributing factor.

    At this point I can barely get the thing to start - it seems like it's not getting enough air. I've drilled the IAC bypass hole from 11/32 to 1/4 inch with little change. Cold starts barely stay running with the IAC pegged at 160 until the engine warms up. Once warm IAC counts are around 80 at 950 rpm.

    So I'm looking for a simple explanation because the problem only appeared as the humidity started to climb right after the 43kpa frame was captured. Note the huge difference in metered airflow. I know the barometric pressure is higher in the 43kpa frame, but so is air temp. Aside from this the biggest difference between the conditions the data was captured in is that the 43kpa frame was from late spring with relatively low humidity (and 11/32 IAC bypass hole). On the other hand the 48kpa frame was captured a week ago in 65-70% relative humidity and 17/32 bypass hole. Can humidity and air density account for this much difference? All the MAF logs I have to compare to are 2+ years old and I couldn't possibly guess what the relative humidity was.

    cl-idle-43kpa.png cl-idle-48kpa.png

    Edit: I failed to mention that this is the first summer I've run this after adding the stroker crank (384ci - 4.040" x 3.750")

  2. #2
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    I vote for compression loss due to valves not fully closed.
    I guess adding more fuel didn`t cure the problem. Iac counts seems normal.
    On the second log engine is not making good vaccuum, possibly the result of lost compression.

    Humid air is heavier and harder to move. It takes more effort to move the same amount of air. It doesn`t seem to have that much impact as you expect. If you suspect engine chocking, enlarging the 8 ports on the intake manifold is better than drilling the bypass hole.


    I hope your individual cylinder trims are not off chart. They do contribute to engine surging and stalling.

  3. #3
    Fuel Injected! spfautsch's Avatar
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    I ruled out compression loss. Engine was torn down to replace head gaskets and installed solid lifters.

    More fuel isn't generally the direction I felt it needed to go when it smells so obscenely rich as it is. This is why I was suspecting loss of compression and / or very weak spark. But I just can't imagine the spark is weak enough to cause incomplete combustion.

    I've been dabbling with the startup fueling and the injector flow constant since the initial post and have got it starting and idling relatively well from warm (approx 60c coolant temp). I really need to buy some known good / unmodified injectors because I've added an entire lb/hr to the setting I was running in the 43kpa frame and it seems to be getting better. I don't know if it's a meaningful measure of anything, but now when it switches to closed loop you can't even feel a difference in idle. A few days ago it would either die or stumble and then switch back to OL for another 90 seconds depending on how warm it was before starting. I guess I'll have to wait until it cools completely to test if I've made any progress in this regard.

    The water vapor in humid air also displaces oxygen and contributes to cooling in the cylinder, which is why I'm trying to get a grip on how much this should be impacting things. Bear in mind I'm speaking in terms of "oppressive humidity". It was 82F/28C in the garage this morning when I began working on it, and my shirt was soaked in sweat in 15 minutes.

    Individual trims were set using the wideband well before the problem began to present itself. It didn't take much - cyl 6 is at 0.98 and cyl 5 is at 0.99, all others are at 1.0. I haven't re-checked lately but don't suspect this is a major problem.

  4. #4
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    Rechecking valve lash with solid lifters might help with cold hard starting conditions. It takes a little for a valve to hang down.
    If it is a start up fueling problem, adjust the cranking VE table. It is reponsible for fueling at startup.

    At cold only hesitation, fueling will be the main suspect. The hotter the engine, the less fuel and air it needs to keep the idle.
    Bump the fuel all over to see if it cures it, and start looking for good known injectors. These knock off, no brand reworked always make more troubles than help.

  5. #5
    Fuel Injected! spfautsch's Avatar
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    I haven't checked hot lash yet, but I doubt I'm losing compression here. Primarily because it acted the same way with hyrdraulic lifters, but also because lash has probably not shrunk during break-in, rather increased.

    Startup fueling is getting my complete attention now, as is injector flow constant. I'll report back tomorrow once it cools completely.

    Trust me - I've read "the gospel" on modified injectors and couldn't agree more, but I ran these injectors enough when the engine was a 5.7L to know they aren't the problem. If anything increasing displacement should have made them less problematic for startup and idle scenarios.

    Thanks for your thoughts nonetheless.

  6. #6
    Fuel Injected! steveo's Avatar
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    just for fun lets force your IAC into operating range. crack the throttle plates open until cold start yeilds iac counts <160 and see if things improve. the air will be less even than a properly sized idle passage but that shouldn't affect startup too much.

    More fuel isn't generally the direction I felt it needed to go when it smells so obscenely rich as it is.
    it takes a lot for it to 'smell' rich. often a 'fuel' smell is a misfire due... lean will hit a misfire state more easily than rich

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