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Thread: 1991 TBI Setup Need tech HELP!

  1. #1
    Electronic Ignition!
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    1991 TBI Setup Need tech HELP!

    I installed a 1991 GMC 3500 7.4 TBI system in a 1972 402 powered truck for a friend. it's a little rich at idle (open loop) figured it would be. but it lacks power and the 91 donor truck had a knock sensor in the block but no wire on the factory harness on B7. so we didn't install the knock sensor and WINALDL is reporting a ton of knock and fouling the timing advance. We installed an MSD older HEI and used the tach lead to fire the injectors and it ran great. We would love to have the advantages of closed loop operation. The 402 is completely stock accept for headers by the way. It has the small cap separate coil dizzy. No esc or wiring for it in dizzy.

    Thanx in advance for any help!

  2. #2
    EFI GearHead ! EagleMark's Avatar
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    Lots of 7.4L did not come with knock sensor or ESC and any scantool will report loads of constant knock counts this way. But if you use the chip/bin that came with a no knock system it also has no knock retard set in chip/bin!

    The reason your HEI distributor did so much better was the spark advance curve in it! From GM the same engine, without knock sensor, spark advance is set SO SAFE, it will never have spark knock. This was built that way so a one ton truck with wife hauling her four horse trailer... for example... could just drive and put her foot to floor and motor was always safe!

    Another flaw in GM stock chip/bin for these trucks was fueling. Most are Async fuel and during Async the BLM Learn is disabled, even in Closed Loop. Open Loop idle is also the norm so when at idle it was smooth and not Stoich AFR of 14.7 to 1 AFR, this helped them cool down at idle. Also at idle and WOT the Air Pump is on to help heat up the CAT for emissions.

    Emmissions was not really an issue on BB, 7.4L engines back then as the standards were different then passenger cars and light trucks. Remember that same engine with TBI wa the same engine HD trucks had with a carb. Hence the adapter for TBI.

    What I do when tuning these early 7.4L TBI engines is add the correct knock sensor and ESC, start the tune with a 5.7L Sync fueling bin and change spark advance table to one that is simalar to what a HEI distributor with mechanical and vacuum advance would be. POOF! A good running BB TBI like it should have been from factory!

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  3. #3
    Super Moderator dave w's Avatar
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    Can you post the .bin file your using?

    I'm thinking most of the problems your having can be fixed in the .bin file.

    How are you set up to burn chips?


    I abandoned WinALDL years ago, as did most of us here. For the money, TunerPro RT is SUPERIOR to WinALDL!!!!

    dave w
    Last edited by dave w; 09-14-2013 at 02:56 PM.

  4. #4
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    Thanks for the info Mark. What confused me was that there was a knock sensor on the 454 and factory wiring to the third (now removed) plug at the ecm. I would love to experiment with tuning and burning but I don't have the equipment for that and this is my friends truck so I think I will let someone else have that job. If it were up to me I would have got something like a Megasquirt ecm. Dave, I have no clue which bin is in there. It is the stock ecm from the 1991 GMC 3500. This is what I do have: ecm = 7747 other numbers on ecm are 86ANTY K383480987 and the software reported the PROMID = 8221 hope that helps and if you guys do any tuning by mail let me know. This is not a max effort deal but rather looking for something like Mark described. Lean the idle a bit, I think the injectors are a little large for the 402 in open loop. Advance etc...

    Thanx again!

  5. #5
    Super Moderator Six_Shooter's Avatar
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    ANTY is the bin.

    It's a 1987 7.4L Auto bin.

    3 plugs at the ECM? The '7747 only ever had 2 plugs. Do you have any pictures of the harness? What was the colour of the 3rd connector?

    The '7747 uses an external EST module that is usually mounted to the firewall, or a bracket on the intake of the engine ('90 to '94 Full Size).

    The Delco ECM is a great ECM, and in many ways better than a MegaSquirt ECM, though the '7747 is one of those ECMs that while it has been thoroughly hacked and used in many non-stock applications and set-ups, it is an early ECM and therefore leaves a lot to be desired. Many people that really want to tune the TBI set-up will swap to the later TBI PCM found in the '93 to '95 automatic trucks. There are a few different service numbers that were used, most common is the 16197427 PCM that has been documented quite well, and even patches out there to add E-fan control and MAF control. There are other service numbers that are basically the same PCM. The other semi-common one that comes up is the 16147060, that was used on many 454/4L80E trucks. I have tuned a couple of these in friends vehicles but they are not as well documented and a little harder to find than the '7427, if a replacement is ever needed, though it is unlikely.
    The man who says something is impossible, is usually interrupted by the man doing it.

  6. #6
    Electronic Ignition!
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    Pretty sure the third plug that was in the harness went to the cluster\fuse box. The harness had no wire just an empty hole for the esc at ECM but did under the hood and that's what caused the confusion, the bracket that held the MAP is where the esc should have been but it was never there. There was also no plug anywhere in the harness for the ESC. Strange I know.

  7. #7
    EFI GearHead ! EagleMark's Avatar
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    ANTY is a factory no knock system. Error 43 is unchecked and knock retard set to 0.

    Look at the picture of the spark advance table. See the dot in the hole? That's where your driving most of the time...

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    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  8. #8
    Electronic Ignition!
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    Downloaded the rt software were going to try to tune it ourselves. Should be an adventure...lol Will be a couple of weeks to get a burner and the time to get back to it. Thanks for all the info everyone.

  9. #9
    Electronic Ignition!
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    Ok ordered the flash conversion adapters and a burn2. This may be a noob question but were all the 747 ecu's identical? We have one from a 4.3 s10 and he would be more comfortable modifying that one.

  10. #10
    Super Moderator Six_Shooter's Avatar
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    Yes, only the cal EPROM and the NETRES (the other small removable chip on the daughter board) were different between applications.
    The man who says something is impossible, is usually interrupted by the man doing it.

  11. #11
    Electronic Ignition!
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    Cool so we also have a back up :)

    Thanx again!

  12. #12
    Electronic Ignition!
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    Update: The burn2 showed up in record time from Moates, read the chip (after the addressing problem was solved via forum search) then it was easy stuff. Still waiting for the other parts in the snail mail (different company). Then I will get on to tuning or de-tuning then starting over.... :) Learning is fun stuff until parts hit the wall...lol

  13. #13
    Electronic Ignition!
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    Well, the G2 install went well. Will start tinkering in the morning. Truck fired up first try. Big relief.
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    Last edited by Trentec; 09-27-2013 at 07:42 PM.

  14. #14
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    Ok, humbling experience it was! Had some success and some failure. Went too rich moved a stumble around and had to punt...lol Back at it in a couple of weeks when he gets the truck legal to drive and we can collect some useable data. It was fun though!

  15. #15
    Electronic Ignition!
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    As if I needed this to get any more complicated... my buddy calls and wants to change the cam and intake uggg so I picked a cam and would very much appreciate any feedback and or experience with this one please. Any suggestions for an intake?

    COMP Cams 249-11-302-4

    LSA = 112
    Exhaust Valve Lift (in.) 0.503
    Intake Valve Lift (in.) 0.503
    RPM Range 1000-7000
    Int. Duration @ .050 in. (Deg) 212
    Exh. Duration @ .050 in. (Deg)218
    Int. Duration Advertised (Deg) 260
    Exh. Duration Advertised (Deg)268
    Engine Make/Size:Chevy Big Block (396-454)
    Lifter Type:Hydraulic Flat Tappet
    Usage:Street/Performance

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