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'91 3/4T, 4x4 Burb Update

Discussion in 'Engine & Performance' started by Daveo91Burb, Jul 9, 2020.

  1. Daveo91Burb

    Daveo91Burb Full Access Member

    Posts:
    597
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    145
    Joined:
    Jul 6, 2014
    Location:
    Vancouver, WA
    First Name:
    Dave
    Truck Year:
    1991
    Truck Model:
    V2500 Silverado Suburban
    Engine Size:
    Vortec 383, modified TBI/4L80e
    Hi all, it's been a long time since I've posted, probably since last fall. But I haven't done much to the burb since then - plans in the work for some upgrades.

    Warning: this will be a long post, but my idea in posting is to help those that may be wanting to bump up displacement or HP on a 350 TBI, especially if they want to use an electronic trans such as 4L80e (or 60e)

    Refresher: I built up a 383 from a 4 bolt 350 from a '91 3/4 ton burb. Eagle crank, stock rods, KB pistons. Up top it's all stock vortec: heads, roller cam, valvetrain, etc. The tricky part of my swap has always been the fact that I stuck with TBI fuel system: 4 barrel GM intake for vortec heads with adapter plate for TBI, 46mm bored TB, 454 injectors, adjustable fuel pressure regulator, Walbro fuel pump. The very non-stock TBI setup forced me into the world of pre-OBDII tuning. I bought chip reader/writer, datalogging equipment, etc. and learned some basics. I've done a fair amount of tuning since I got it running, and it runs decent, but it still has some glitches, like some very obnoxious surging during decel and not enough enrichment during hard accel. And pretty crappy mileage.

    The central issue is the PCM for '91-'93 3/4 ton trucks with 4L80e is an enigma. It wasn't used in any other light-duty trucks and not much work on the "hacks" or mask/binary files has been done for it. The trick is to convert to the '94-'95 PCM - tons of work has been done for that one because the half tons went to 4L60e trans then, and then all the automatic trucks used the same PCM: Tahoes, pickups, burbs, etc. Sounds easy, right? Tons of those PCMs in junk yards all over the country. Unfortunately, it's not that simple. The '91-'93 4L80e is also an enigma. Its force motor requires a cleaning cycle every 10 seconds or so and that cleaning cycle is controlled by the PCM. Switch to a newer PCM without doing anything to the trans and you are looking at certain death in short order. In '94 the 4L80e was revised to use a new force motor that does not require a cleaning cycle. So....there are basically two options, assuming you want to stay with 4L80e and TBI: upgrade the existing trans or go to a new trans.

    Until a few weeks ago my plans involved going into the trans and doing some upgrades to make it work with the newer PCM. I was going to have a shop knowledgeable in this area do the work so I don't know all the details. But then I became the proud owner of a salvaged '97 3/4 4x4 burb that I got at auction. Now my plan is to refresh the 4L80e in that (same shop) and use it in my '91. I'll have to convert the cable linkage to the older style but I think it's possible to do that. The other issue is the cooler lines are in a different spot, but I'm hopeful I can use the lines from the '97.

    So once the trans is upgraded I can swap in the new PCM and tune it. Easy? Nope. Of course the connectors on my existing harness will have to be re-pinned to match the new PCM. So I'll be learning a new skill for that. (and I just bought a new Lisle tool specifically for taking apart GM wiring connectors) And making a very detailed spreadsheet - there are 64 available slots in the new connectors (although they're not all used). Then the tuning can begin, probably will be many data logs. But another benefit of the newer PCM is it can accommodate an input for wide band O2 sensor, which I added when I did the exhaust system. Wide band is apparently highly recommended for tuning of this nature since the stock narrowband really just tells you if you're rich or lean but not by how much.

    When I bought the '97 I had visions of just bringing the entire fuel system over from it to my '91, PCM, harness, intake and injectors, etc. I've since been advised against doing that though - the vortec computer is a black box and not very tunable and that system had some issues that would need to be resolved. There may be a way to convert the TBI system to multiport and use the '97's spider injector assembly, but that's a ways in the future, if I ever do it.

    I know many of you are not interested in this fuel injection and computer crap. I get it - I have a '76 Corvette that still has a q-jet and it will stay that way. But I'm hopeful there are a few folks out there that may benefit from my trial and error approach. I'll try and keep this thread up to date for the few that are.
     
    Craig 85 likes this.
  2. yevgenievich

    yevgenievich Full Access Member

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    1,620
    Joined:
    Mar 1, 2014
    Location:
    Texas
    First Name:
    Viktor
    Truck Year:
    1991
    Truck Model:
    V2500 burb
    Engine Size:
    5.7
    I am just jealous now because I sold my 91 3/4 4x4. If there is not much support for vortec ecm, then sticking with tbi might not be a bad plan. Unless it can be moved to 411 ecm.
     
  3. Daveo91Burb

    Daveo91Burb Full Access Member

    Posts:
    597
    Likes Received:
    145
    Joined:
    Jul 6, 2014
    Location:
    Vancouver, WA
    First Name:
    Dave
    Truck Year:
    1991
    Truck Model:
    V2500 Silverado Suburban
    Engine Size:
    Vortec 383, modified TBI/4L80e
    Sad to hear you sold it - you had done quite a bit with it. I was tempted to get rid of mine and go with a GMT400 454 ('96-'99) but I decided to just put some more effort into what I got.

    411 PCM is a thought, it definitely seems to be a better way to go than the vortec black box. I've heard of people doing that swap on Gearhead EFI. I think my long range plan is to figure out a way to run eight injectors to get some efficiency gain. But gotta get the basics right first.
     

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