Bringing TBI and Multi Port Fuel Injection to a New Level.     EFI Conversions and Tuning! Seattle to Portland! E-mail Tuning Consultant!
Page 1 of 2 12 LastLast
Results 1 to 15 of 20

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

  1. #1
    Fuel Injected!
    Join Date
    Apr 2017
    Posts
    27

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

    Hi all, could use some tuning advice.

    I've built and installed a vortec-head 383 in my '91 squarebody suburban. 3/4 ton, 4wd, 4L80e tranny. 16147060 PCM, using $85 mask. Here's my intro thread for some more info:

    http://www.gearhead-efi.com/Fuel-Inj...ortec-head-383

    Considering my complete lack of experience with tuning, it's running decent. I know it's running rich and I'm continuing to play with the BLM table using my log files and an adjustment spreadsheet that @Dave W provided me. But other than that, I'm unsure of where to go next with tuning, e.g. spark, IAC, choke, etc. My two biggest concerns (other than still a bit rich) are,

    1. Significant surging during compression braking. I can hear it out the tailpipe, see it in the tach (300-400 rpm surge range, cycle time is about 1 second) and feel it in the seat of the pants. It might be doing the same thing, but very slightly, during steady, low speed cruise, too. But going down hills with foot off throttle and brakes, it's pretty bad.

    2. High idle at cold startup. 1200-1300 rpm, doesn't match target idle in .BIN. Takes several minutes before it will even come down to below 1000.

    Do these issues sound like they're related? Vacuum leak? I've attached all the files I'm using in TunerPro, including my two latest log files and my latest modified .bin file - just in case anyone has the desire to take a look.

    Oh, and EGR has been deleted via RPM limits (Dave W helped with that on initial .BIN creation) and no EGR valve physically installed. EGR is complicated on Vortecs - need to route exhaust gas from DS exhaust manifold to intake manifold.

    Thanks much!
    Attached Files Attached Files

  2. #2
    Fuel Injected!
    Join Date
    Apr 2017
    Location
    Oregon
    Posts
    138
    It looks like three weeks and no one responded. Do you have a wide band o2 hooked up? If so what's it reading during your decel problem? You could have a vacuum leak, the only way to tell is to check. A smoke machine is what I prefer. There are instructions online on how to make one for not much money. There are other ways, but sometimes they miss leaks.

  3. #3
    Fuel Injected!
    Join Date
    Apr 2017
    Posts
    27
    Quote Originally Posted by donf View Post
    It looks like three weeks and no one responded. Do you have a wide band o2 hooked up? If so what's it reading during your decel problem? You could have a vacuum leak, the only way to tell is to check. A smoke machine is what I prefer. There are instructions online on how to make one for not much money. There are other ways, but sometimes they miss leaks.
    Thanks for the response. I haven't been able to do anything for a couple weeks due to a severely leaking tranny, but that is resolved now so back to the tuning. I'm leaning away from vacuum leak since it's now running pretty good with the only exception being surging at deceleration. Decelerates smoothly down to about 1500 rpm and then between there and about 1100 rpm there's a crazy oscillation between the two, back and forth on the tach and audible. Then once it's down below 1100 and all the way to 650 idle - smooth as butter. I found another thread on here that may hold some insights. Going to start working through some of those issues. I'm still running pretty rich, too. I also recently did the adjustable FPR conversion (using the stock regulator and making the spring cup adjustable) -- I only had about 8 psi before that, now I'm back up to 12-13.

    No WB O2 sensor. I may go that way, but would prefer to stay stock if I can get it to work. I did get a new Y-pipe made recently and I had him put in additional bungs but not sure they'll work for wide band. Stock O2 is just after the two legs of the Y come together, but he added bungs upstream of that, one in each leg of the Y. Would that work?

  4. #4
    Fuel Injected!
    Join Date
    Apr 2017
    Location
    Oregon
    Posts
    138
    Great progress! The other thread you found has some great info. The reason the factory had the unheated o2 in the exhaust manifold was heat, when they relocated it to the y pipe in later years they installed a heated three wire o2 from the factory. The wide band is also heated and can be mounted pretty much anywhere in the exhaust. I wouldn't waste my time without a wideband. It can be done but really the tune should be more accurate with the wideband and dataloging. I see it all the time, people spend thousands upgrading then compromise the results with a shifty tune. A wide band is cheap and can be sold or rented to friends once your done with it.

  5. #5
    Fuel Injected!
    Join Date
    Apr 2017
    Posts
    27
    Thanks, I'm definitely considering the wideband. Going to start though with just converting existing O2 to three wire. I agree with the reasons you state about why they changed to 3-wire, but I don't think they had made that change yet in '91. As best I can tell, it came stock in the current location (after the Y pipe junction, before the cat) and was never heated. But it seems like a good start to just swap out that one wire unit for a 3 wire. Then, if I want to add WB, I can add it into one of the banks, before the y-junction, since I have a bung in each bank now.

  6. #6
    Fuel Injected!
    Join Date
    Apr 2017
    Posts
    27
    I think I finally got the surge at deceleration issue resolved today. Minimum idle air was misadjusted. Fortunately my re-man and bored throttle body already had the cap over the throttle stop screw removed, so I didn't have to worry about that. It made following the procedure here really easy. After getting the base idle adjusted I checked the TPS voltage at idle and had to adjust it slightly - ended up right at 0.51 volts which is pretty much optimal. (Used ALDL cable and TunerPro R/T to look at TPS voltage) Took it for a short drive and surge at decel seems to be gone. Fingers crossed.

  7. #7
    Administrator
    Join Date
    May 2011
    Location
    Lakes Region, NH
    Age
    49
    Posts
    3,065
    If Decel surge continues, try reducing spark advance by 2-3 degrees in the cells affected by decel.

  8. #8
    Fuel Injected!
    Join Date
    Apr 2017
    Posts
    27
    Quote Originally Posted by 1project2many View Post
    If Decel surge continues, try reducing spark advance by 2-3 degrees in the cells affected by decel.
    Thanks for this. I guess I was wrong - the decel surge is still there although it seems to be a little softer. Now I'm wondering if there is an IAC problem that could be related. Most of the time it will idle at or near target idle speed (650 when warm), but it will occasionally idle higher. I drove it 45 miles at highway speed a few days ago, at the end it was idling over 1000. Shut it off and restarted a minute or two later and was back to normal. IAC sticking? I have a spare IAC that I'm going to swap in to see if anything changes.

    If IAC does not solve problem, I will try the timing reduction in the affected cells - thanks!

  9. #9
    Fuel Injected!
    Join Date
    Apr 2017
    Location
    Oregon
    Posts
    138
    Quote Originally Posted by Daveo91 View Post
    Thanks for this. I guess I was wrong - the decel surge is still there although it seems to be a little softer. Now I'm wondering if there is an IAC problem that could be related. Most of the time it will idle at or near target idle speed (650 when warm), but it will occasionally idle higher. I drove it 45 miles at highway speed a few days ago, at the end it was idling over 1000. Shut it off and restarted a minute or two later and was back to normal. IAC sticking? I have a spare IAC that I'm going to swap in to see if anything changes.

    If IAC does not solve problem, I will try the timing reduction in the affected cells - thanks!
    I am not sure if it helps but there is idle overspeed spark retard and idle underspeed spark advance tables at least with the 95 tbi computer. My cam is very mild but even with it, the timing was bouncing all over the place like a yo yo with the factory settings and the idle had a little lope/instability. I neutered the tables for advance or retard up until a 50 rpm difference then after that decreased the adjustment range slightly. The idle even with the almost stock ramjet cam smoothed out and the timing is very stable at idle now. Even the definitions list the problem " Idle overspeed/ underspeed corrections .... This is why timing is constantly moving around at idle." Not sure if it will help your problem or not but since your IAC adjustment seem to help. I figured I would mention it. I did not remove all the correction, just toned it down a bit.

  10. #10
    Fuel Injected!
    Join Date
    Feb 2015
    Location
    New Zealand
    Age
    49
    Posts
    198
    I've been wondering for ages what was causing my spark timing to bounce around at idle so much!
    It would be much appreciated if you could give me an example of how your tables vary from my stock ones (shown in the attachment).
    I'm keen to try this out :-)
    Attached Images Attached Images

  11. #11
    Fuel Injected!
    Join Date
    Apr 2017
    Location
    Oregon
    Posts
    138
    Idle Overspeed Spark Retard vs. RPM Error 0.0 0.0 0.0 0.0 1.1 2.1 3.2
    Idle Underspeed Spark Advance vs. RPM Error 0.0 0.0 0.0 0.0 2.1 2.1 3.2
    It's not my tip, someone that is very familiar with the later LS computers told me to check that when I was wondering why the timing was so unstable at idle. My $0D computer has it, not sure about the earlier ones.

  12. #12
    Fuel Injected!
    Join Date
    Apr 2017
    Posts
    27
    Quote Originally Posted by donf View Post
    I am not sure if it helps but there is idle overspeed spark retard and idle underspeed spark advance tables at least with the 95 tbi computer. My cam is very mild but even with it, the timing was bouncing all over the place like a yo yo with the factory settings and the idle had a little lope/instability. I neutered the tables for advance or retard up until a 50 rpm difference then after that decreased the adjustment range slightly. The idle even with the almost stock ramjet cam smoothed out and the timing is very stable at idle now. Even the definitions list the problem " Idle overspeed/ underspeed corrections .... This is why timing is constantly moving around at idle." Not sure if it will help your problem or not but since your IAC adjustment seem to help. I figured I would mention it. I did not remove all the correction, just toned it down a bit.
    Thanks for the tip, but, yeah, I don't think my older ECM has those tables. Searched for overspeed and underspeed with the parameter finder and nothing. Searching for "decel" gives me two: Decel Enlean Multiplier Map Diff (Scalar) and Decel Enleanment Coolant Factor vs Coolant temp. I've been looking at what DFCO is set at - looks like it's a pretty narrow RPM range that is a little outside where I'm seeing the surge, so not sure that would be the problem?

    I just found two more exhaust leaks upstream of the O2 sensor tonight. I had a custom y-pipe made but have found three pinhole leaks in two different welds so far. A little bummed. Going to get those fixed before I do anything more with tuning or chasing out the surge. If surge still there after exhaust tightened up, I'll swap IACs and see if that changes anything. Then I'll dial back timing just a little in cells affected by decel. Thanks for the help!

  13. #13
    Fuel Injected!
    Join Date
    Apr 2017
    Location
    Oregon
    Posts
    138
    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

  14. #14
    Fuel Injected! PlayingWithTBI's Avatar
    Join Date
    Dec 2018
    Age
    65
    Posts
    68
    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
    Great read!

  15. #15
    Fuel Injected!
    Join Date
    Apr 2017
    Location
    Oregon
    Posts
    138
    Quote Originally Posted by PlayingWithTBI View Post
    Great read!
    It's one of the better sources of info I have found. Its missing the last part, the tuned examples according to the index. Even so it's worth saving a copy.

Similar Threads

  1. Squarebody with TBI and Vortec Heads
    By brandon6469 in forum Introductions
    Replies: 0
    Last Post: 09-29-2016, 09:07 PM
  2. Modified Vortec heads
    By old truck guy in forum Buy - Sell - Trade - Wanted
    Replies: 0
    Last Post: 02-15-2016, 09:17 AM
  3. 4.3 tbi with vortec heads need chip help
    By ls1 in forum GM EFI Systems
    Replies: 9
    Last Post: 12-23-2015, 10:09 AM
  4. WTB 062/906 vortec heads
    By Tylerz281500 in forum Buy - Sell - Trade - Wanted
    Replies: 0
    Last Post: 01-04-2015, 01:05 PM
  5. tbi into 76 vortec heads ?
    By BIGWORM76 in forum GM EFI Systems
    Replies: 33
    Last Post: 04-18-2013, 09:24 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •