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Thread: Tuning help needed - '91 TBI 383 w/ Vortec heads

  1. #16
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    I tried out zeroing out the overspark and underspark vs RPM error as you said and it did hold my idle spark at a steady 27 degrees @725 RPM in Park, before that it would bounce around a couple of degrees.
    It didn't seem to work so well idling in gear at 575 RPM, the spark still moved around a bit (21-24) and where previously the RPM would stay pretty constant between 600-575 it was now occasionally dropping to 550 RPM. I guess that's part of the job of overspark/underspark vs RPM to try and help hold the RPM near it's commanded value, I'm also guessing there's more than just overspark/underspark trying to do that job as well.
    Great tip but once again when it looks like I might have finally solved something the final answer only ends up getting more complicated.

  2. #17
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    Quote Originally Posted by Kitch View Post
    I tried out zeroing out the overspark and underspark vs RPM error as you said and it did hold my idle spark at a steady 27 degrees @725 RPM in Park, before that it would bounce around a couple of degrees.
    It didn't seem to work so well idling in gear at 575 RPM, the spark still moved around a bit (21-24) and where previously the RPM would stay pretty constant between 600-575 it was now occasionally dropping to 550 RPM. I guess that's part of the job of overspark/underspark vs RPM to try and help hold the RPM near it's commanded value, I'm also guessing there's more than just overspark/underspark trying to do that job as well.
    Great tip but once again when it looks like I might have finally solved something the final answer only ends up getting more complicated.
    There is an idle speed table for both in gear and out. Even with the small cam in my van, I have the hot idle speed at 600 and it stays there without wavering at all.
    Last edited by donf; 02-27-2019 at 06:10 AM.

  3. #18
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    Quote Originally Posted by donf View Post
    I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf
    Thanks for the link, ton of valuable info in there. Saved it to my iPad and am part way through reading it. It doesn't seem to cover my specific ECM (PCM actually since I have 4L80e) but still pretty applicable on the engine side.

  4. #19
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    Quote Originally Posted by donf View Post
    I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf
    THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.

  5. #20
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    Quote Originally Posted by Redtires019 View Post
    THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.
    Glad to share, even with the 95 computer it's helpful. I actually tried to track the author down. I wanted to see if I could get all the pages. From the index its missing the tuned examples, I think that would be educational. The website mentioned in the pamphlet is still registered to someone since early 2000's but it's got the info blocked for privacy.
    Edit: I did some searching and actually found the complete pamphlet here zipped in docs, under CustomEfis Setup. The tuning examples were never in the info, probably why the others cut it off. It's just a link to a long gone website. It is a cleaner copy though.
    Last edited by donf; 03-08-2019 at 04:22 PM.

  6. #21
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    So I've made some more progress on my tune. I'm down to two problems, one has been persistent since the beginning.

    1. A very quick bog or stumble right off idle, light accel only. It's worse when it's cold, when warm it is still there, but very slight. I've hunted around in my log files looking for the problem, but have not had much luck. Definitely spending more time on #2.....

    2. Annoying, cyclic surge at deceleration. Most persistent at 1600-1400 rpm, but will happen higher or lower than that as well. very consistent - virtually always occurs when there's a negative load (overrun?) on the engine. RPM will drop 200-400 and then immediately come back up. Entire cycle takes about a second or even less. It will just keep repeating that until it gets to an idle state or close or the negative load is gone. Idles fine and steady. I've always thought it was fuel or spark, but I've started wondering if this could be a 4L80e problem? Overrun clutch? I live on a hill and have put it in 1st and accelerate up to 2500 or 3000 and then let it decel on it's own. Very pronounced when I do this - feels like something is letting go for split second and then reengaging. It does happen whether I select a lower range manually, in D, and in OD so that kind of leads me away from it being a tranny problem. Any advice appreciated!

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    Quote Originally Posted by Daveo91 View Post
    So I've made some more progress on my tune. I'm down to two problems, one has been persistent since the beginning.

    1. A very quick bog or stumble right off idle, light accel only. It's worse when it's cold, when warm it is still there, but very slight. I've hunted around in my log files looking for the problem, but have not had much luck. Definitely spending more time on #2.....
    I am not sure how similar the early computers are to the 94-95 but it sounds like an Acceleration Enrichment problem since its worse when cold. Do you have a wideband o2 logging data as you drive? If the wideband data looks OK, then try a slight timing increase in the same cells you are having the problem in.

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    Thanks, donf. No wideband yet. In the master plan, but I'm trying to get all the other issues ironed out first. For a while I had both exhaust leaks and an intake leak that were causing all sorts of problems - those are all fixed now so on to the other stuff.

    My #2 issue has me concerned. Any thoughts from anyone whether the cyclic surge on decel could be caused with fuel/timing parameters on the engine side, or is consensus that this is a tranny issue?

  9. #24
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    Quote Originally Posted by Daveo91 View Post
    Thanks, donf. No wideband yet. In the master plan, but I'm trying to get all the other issues ironed out first. For a while I had both exhaust leaks and an intake leak that were causing all sorts of problems - those are all fixed now so on to the other stuff.

    My #2 issue has me concerned. Any thoughts from anyone whether the cyclic surge on decel could be caused with fuel/timing parameters on the engine side, or is consensus that this is a tranny issue?
    The wideband info can help on both. We are on this side of the computer screen, so its very hard to diagnose a problem. You have to start with what you know and your flying blind on a lot of this without logging wideband readings. If it starts acting weird you can log that and know for sure its not or it is fuel related. With a narrow band you may know its rich or lean, but they are basically Stoic sensors and nothing more accurate than that. If you are looking at the block learn averages to change to spot something like this, it may if it happens for long enough, but its not the best way. If you positively cant afford a wide band I would look at the logged data in spreadsheet form anyway without it. See what it changing frame by frame over that particular problem. Whether its IAC counts, or timing, or the very limited fuel mixture info you already have access to. Post the info in spreadsheet form for the few seconds when the problem is happening and maybe others can take a look too.

  10. #25
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    Quote Originally Posted by donf View Post
    The wideband info can help on both. We are on this side of the computer screen, so its very hard to diagnose a problem. You have to start with what you know and your flying blind on a lot of this without logging wideband readings. If it starts acting weird you can log that and know for sure its not or it is fuel related. With a narrow band you may know its rich or lean, but they are basically Stoic sensors and nothing more accurate than that. If you are looking at the block learn averages to change to spot something like this, it may if it happens for long enough, but its not the best way. If you positively cant afford a wide band I would look at the logged data in spreadsheet form anyway without it. See what it changing frame by frame over that particular problem. Whether its IAC counts, or timing, or the very limited fuel mixture info you already have access to. Post the info in spreadsheet form for the few seconds when the problem is happening and maybe others can take a look too.
    thanks again Don, I’ve started shopping for WB. I’m convinced. In the meantime I’ll see if I can do what you suggest with the log files frame by frame in excel. I haven’t done that yet but I think is should be able to figure it out.

  11. #26
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    If you have an android phone and the blue tooth ALDL, Johns free app "ALDL Scan Tool" works well for this too. The wide band requires a totally different input setup, but for ease it saves the log file as a csv and seems to be just as accurate as all the others. Sometimes I just hit the record button and look at the data when I get home.

  12. #27
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    You could always do what I did before cheap, good commercial widebands were available. Disablw the BLM and tune off the integrator alone. Integrator reponds much more quickly.

  13. #28
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    Quote Originally Posted by donf View Post
    If you have an android phone and the blue tooth ALDL, Johns free app "ALDL Scan Tool" works well for this too. The wide band requires a totally different input setup, but for ease it saves the log file as a csv and seems to be just as accurate as all the others. Sometimes I just hit the record button and look at the data when I get home.
    No android phone, I use an iphone. I'll just stick with windows laptop/TunerPro/ALDL cable. But thanks for the tip.

    Quote Originally Posted by Fast355 View Post
    You could always do what I did before cheap, good commercial widebands were available. Disablw the BLM and tune off the integrator alone. Integrator reponds much more quickly.
    Hmm, thanks for the tip. Might try that.

    Guys, I've thought about disabling closed loop just for diagnostic purposes. I'm curious to know if these decel surge issues are still present in open loop - seems like if they're not that would help me zero in on the problem at least a little more. Can I just unplug the O2 sensor or should I do the disabling in the BIN? If BIN do I just increase the timer a big amount?

    And one other thing I'm trying: have a new MAP sensor on order. I'm not big on throwing parts at problems, but I found a NOS GM on ebay for $15. MAP is one of the few things I reused after my rebuild and I just want to eliminate that being the problem.

  14. #29
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    Quote Originally Posted by Daveo91 View Post
    No android phone, I use an iphone. I'll just stick with windows laptop/TunerPro/ALDL cable. But thanks for the tip.



    Hmm, thanks for the tip. Might try that.

    Guys, I've thought about disabling closed loop just for diagnostic purposes. I'm curious to know if these decel surge issues are still present in open loop - seems like if they're not that would help me zero in on the problem at least a little more. Can I just unplug the O2 sensor or should I do the disabling in the BIN? If BIN do I just increase the timer a big amount?

    And one other thing I'm trying: have a new MAP sensor on order. I'm not big on throwing parts at problems, but I found a NOS GM on ebay for $15. MAP is one of the few things I reused after my rebuild and I just want to eliminate that being the problem.
    There are different ways to end up with different results. Look at the voltage shape of the typical narrow band o2 sensor. You may be able to get you block learn dialed in and its fine for that, but what about your WOT mixture that's not suppose to be Stoic?? After all you have put into you project do you really want to skimp with $160 and a few wires to log the data? If money is that tight, sell it on craiglist when you are done. No one will ever say a narrow band is superior for tuning, you may be able to "get by" but really for the money involved, why? I did not want to bother with mounting a gauge or cutting holes so the new AEM line has one that just data logs to a computer. The accuracy is very close to the big $$ Haltech setup on my chassis dyno.

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  15. #30
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    Have you blended your main and idle spark tables in the areas where they crossover (I'm assuming you have tracing turned on)?

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