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Thread: Stumbling After Cold Start

  1. #1
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    Question Stumbling After Cold Start

    Recently, my car has started running really rough when it first starts in the morning. The throttle is very unresponsive, and under light load below about 1400 RPM (i.e. going up a hill, lightly accelerating, etc.) it stumbles pretty badly. It's weird, at around 1400 RPM it instantly picks up and feels fine. For testing purposes, I've pulled timing way back below a certain RPM, and that felt very similar to this - bog, bog, bog, then instant smooth acceleration. Also, it feels like the engine kind of jolts immediately after I add throttle (possible related to the acceleration enrichment function?). After the engine runs a few minutes (haven't been able to watch the datalog, but it seems like it's when the engine goes to closed loop), it seems to go away entirely.

    I initially thought it was running lean, so I gradually increased startup enrichment to about 15 percentage points higher than stock, but that didn't seem to help or hurt. It doesn't smell abnormally rich, so I'm hesitant to blindly decrease enrichment.

    I've made a few changes recently that could have caused this. I put long-tubes on it just over a month ago, but I don't recall seeing this behavior immediately after. A few days after adding the headers, I drove out from CA to school in VA, and I'm wondering if the humidity or cooler morning temps could have something to do with it. Lastly, a few weeks ago I discovered my PCV system was set up backward (the check valve was on the wrong side, causing the air to flow the wrong way and a strong vacuum in the crankcase), so I switched it to the proper orientation.

    Looking back at my BIN dates, it looks like the stumbling popped up shortly after I fixed my PCV system.

    Do you guys have any idea where I could start with diagnosis?

  2. #2
    Fuel Injected! sturgillbd's Avatar
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    Can you record a datalog and post up?

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    Well, I took a couple datalogs where it was stumbling really badly, but ALDLDroid didn't save them for some reason (looks like there's a hidden 256MB limit for all saved datalogs?).

    I took another this morning, but it didn't seem to be stumbling at all, which could have been for one of several reasons: it took a few minutes to get the datalog started, so it warmed up a little before I started driving; I left the house a bit later, so it was warmer; and it felt quite a bit less humid than previous mornings. It still sounded like it was idling rough though, so I'm attaching the datalog I took. (I attached a ZIP that has both the CSV and the XDL.)

    I'll try to go for a drive early tomorrow morning to see if I can get a better log.
    Attached Files Attached Files

  4. #4
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    Here's a better datalog. Still wasn't doing it real bad for some reason, wonder if it's weather-related.
    Attached Files Attached Files

  5. #5
    Fuel Injected! sturgillbd's Avatar
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    I viewed the first datalog this morn and didnt see anything that stood out. I need to find the more complete adx for 8D and will review both logs in more detail. I'll try to do it this eve but it may be late before I get a chance.

    Brian

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    Thanks Brian, really appreciate it.
    What do you mean by the "more complete" adx?

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    I have checked the log, you are running a little rich with BLMs around 120, and the moment you tap the throttle it gets momentarily lean from what I can see. Check your fuel pressure and have a look at the spark plugs.
    If you still have stock injectors it's a good idea to check them or replace them anyway to D3 type injectors. Stock ones tend to finish their life after those 30 years.

    EDIT: I read on other topics that your car is far from stock... I'd say to give it more AE on low temperatures in: "AE Delta TPS Pulse Width Factor Vs. Temp. ".
    And work on the tune, log shows rich everywhere.
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    Last edited by dzidaV8; 09-29-2018 at 01:31 PM.

  8. #8
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    Thanks for looking at my logs. I tried checking the fuel pressure before I drove out to VA, but the gauge I got from O'Reilly was leaking like crazy. I don't have time (or tools) to pull a spark plug right now, but the last few times I've had them out they've looked pretty sooty.

    I've got new 33.3 lb/hr injectors, so that's not a problem.

    Tuning AE was on my to-do list, but man does it look complicated. Here's my current table; about how would you change it?
    AE Table.PNG

    I have adjusted and updated my VE tables based on several long datalogs, but isn't it normal for it to shift rich and lean as temperature and humidity change?

  9. #9
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    Right, the fueling can change due to humidity and temp, but I suspect you are hitting a weird transition on the VE table when this stumble occurs. Are your VE tables smooth? Try increasing the TPS AE factor at the temperatures at which the stumble occurs.
    "AE Delta TPS Pulse Width Factor Vs. Temp. "

    Try using the XDF I attached, I tweaked it some to make it more intuitive, make sure tu use it in category view.
    Attached Files Attached Files

  10. #10
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    I've smoothed my VE tables pretty extensively, although there are a few little areas that are a little jagged. Here's the graph of my lower VE table:
    VE graph.jpg

    Whoops, found the right table. Here it is:
    AE Table.PNG
    It's stumbling around the 20 C cell, so I guess increase it there from 1.00 to 1.25 to match the above cells?

    Hmm, that's a nice-looking XDF. I like how you added the patches section - maybe I'll finally try patching my BIN if it's that easy. The XDF I'm using (wish I could give credit, but I don't remember who on this forum I got it from) breaks parameters up into sections though, which I've really gotten used to. Check it out. Maybe if I have time I'll make a hybrid of the two.
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    TPS AE is just a short burst. If it still stumbles, it may need the MAP AE increased also in that area. I booted into windows this eve and replayed your logs. Its a little rich overall but not terrible. Only other thing that stood out was a slow oscillating integrator Do you feel a surge while cruising? I viewed the log using the monitors in Tunerpro and it looks like the integrator is driving the o2 crosscounts. Usually this means you do not have enough proportional gain. Green is O2 voltage, Red is TPS and Blue is the Integrator. If you don't feel a surge, I wouldn't fool with it.
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    Quote Originally Posted by 90Vette View Post
    The XDF I'm using (wish I could give credit, but I don't remember who on this forum I got it from) breaks parameters up into sections though, which I've really gotten used to. Check it out. Maybe if I have time I'll make a hybrid of the two.
    Right, I removed all the breaks as I prefer category view.

    Quote Originally Posted by 90Vette View Post
    It's stumbling around the 20 C cell, so I guess increase it there from 1.00 to 1.25 to match the above cells?
    Yes, increase that but also adjust the surrounding cells to smooth the table.
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    I increased the AE table like so:
    Modified AE table.PNG
    and hooooly crap that was bad. Whenever I touched the gas, the engine just about died. After about 30s to a minute though, it ran great. Check out the datalog I took, and let me know what you think.
    Since adding AE TPS fuel seemed to make it worse for the first little bit, I'm wondering if it's running too rich. The weird thing is, it would have been in a cell with 20% enrichment (20 C), which doesn't decrease until 56 C. I don't understand why it would run better so quickly after starting.

    Quote Originally Posted by sturgillbd View Post
    TPS AE is just a short burst. If it still stumbles, it may need the MAP AE increased also in that area. I booted into windows this eve and replayed your logs. Its a little rich overall but not terrible. Only other thing that stood out was a slow oscillating integrator Do you feel a surge while cruising? I viewed the log using the monitors in Tunerpro and it looks like the integrator is driving the o2 crosscounts. Usually this means you do not have enough proportional gain. Green is O2 voltage, Red is TPS and Blue is the Integrator. If you don't feel a surge, I wouldn't fool with it.
    How exactly does MAP AE work? Somehow I've only ever read about TPS AE.
    Also, what are you looking at to determine whether TPS AE needs to be adjusted? I'd looked at O2 volts vs TPS %, but I couldn't find a clear pattern. Do you use the integrator for that?
    Interesting about those oscillations. I haven't noticed a surge at cruise, but when I'm coasting in neutral I notice the idle speed oscillates up and down a few hundred RPM. Could that be related? Because that drives me crazy.

    Quote Originally Posted by dzidaV8 View Post
    Right, I removed all the breaks as I prefer category view.

    Yes, increase that but also adjust the surrounding cells to smooth the table.
    Oh wow, I didn't even realize category view was a thing. Don't know how I've never played around with that drop-down. That's a lot better, thanks!
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  14. #14
    Fuel Injected! sturgillbd's Avatar
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    Too much AE can cause a stumble too. Too little usually cause a pop back through intake. Too much causes the engine to bog. TPS AE is a short burst as throttle blade opens. MAP AE follows though when the throttle is held open during acceleration. If adding AE makes it worse, you may try to remove 10 % of the TPS AE from your original value at 20C then try 25%. View your log and watch for a TPS increase. If the O2 voltage goes lean, It usually needs more AE. If it goes rich, you need less AE.

  15. #15
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    I've driven it the past couple days with the tune with more AE, and it's definitely worse. I just made a new tune with less AE, and hopefully that will take me in the right direction. If that helps, I'll also try leaning out open loop at those temps.
    I was trying to look at a couple new datalogs I've taken though, and I'm really having trouble seeing any sort of pattern in O2 sensor volts when TPS % goes up. I've attached them; can either of you tell me anything from them?
    Attached Files Attached Files

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