89 350 TBI problem

Discussion in 'Troubleshooting & Diagnosis' started by Chuck78, Jan 6, 2007.

  1. Chuck78

    Chuck78 Veteran Member

    Oct 22, 2000
    Columbus, OH, USA
    the early 700R4's had some issues, the later ones were halfways decent, but you really need to install a reprogramming kit to make them a great trans. The TransGo "systems correction/calibration" kit for these has a ton of changes in spring pressures in the valve body (and an entirely new TV plunger and assembly), and lots of other tweaks and drilling of the shift plate to correct close to a dozen factory shortcomings of the way these trannies shift and other operational issues that wear out the clutches prematurely.
    Also I see lots of these rebuilt with the "corvette servo" installed.
    The rebuilt one I just blew up seemed to shift a lot better than the stock/used 70K mile unit I just installed. I'm sure it had something to do with the shift kit used in the previous rebuild.

    All the TBI consist of is basically the throttle shaft, and two injectors that are servos that open and close whatever amount the computer dictates based on the readings from the throttle position sensor, manifold absolute pressure sensor, and O2 sensor, etc. Then the only other part is the diaphragm and regulator spring. The fuel pump pushes gas into the tbi, and builds pressure against the regulator and injectors. If the injectors are wide open, the gas takes the path of least resistance. If they are closed down more, then more pressure builds until it starts to overcome the regulator spring at 9-13 psi, and that allows the unnecessary quantities of fuel to flow back to the tank in the return line.
    Then I suppose the MAP sensor and TPS also give input to the Electronic Spark Control to alter ignition timing advance. That's the TBI in a nutshell.

    The wiring diagram in the Haynes for the early TBI units like this is three pages long, not too complicated. The wiring diagrams for the later multiport injection and TPI's are all 4 extensively cluttered pages long... that's a bit too much for me! I'd rather deal with a Quadrajet than that much wiring and computer stuff. The TBI doesn't seem all that bad now...
  2. Knuckle Dragger

    Knuckle Dragger Administrator Staff Member Lifetime Gold Member

    Nov 2, 2002
    Waddell AZ
    I'm glad to see you got it worked out.
  3. danbrennan

    danbrennan Veteran Member Lifetime Gold Member

    Mar 13, 1999
    Brighton, MI
    I've always thought TBI was a very cost effective way of getting a fuel injection system - it replaces a rather complicated mechanical device - the carburater - with a few sensors, simple injectors, and some simple electronic controls. Good bang for the buck, and they don't require very high fuel pressures. I would think the major drawback is that they still have a wet manifold like a carburater, so under transients its hard to predict how much fuel is on the manifold wall versus how much is actually in suspension in the air. I have to think that's why the OEMs went to the more expensive port fuel injection systems, with their relatively dry manifolds - so they could get more precise transient fuel control and thus better emissions. It seemed like it was after everybody went to port FI that we started seeing all the "tuned" intake manifolds on the OEM engines - I'm guessing they couldn't get very good drivability and emissions with those manifolds if they still used TBI.
  4. cold Z28

    cold Z28 Veteran Member

    Sep 12, 2001
    Anchorage,AK USA
    Glad you figured it out. TBI's are so simple and seem to last forever. My truck is a breeze to work on when needed and easy to figure out. I like the simplicity. I think it's one of GM's best long running motor combo's with relative few problems.
  5. Chuck78

    Chuck78 Veteran Member

    Oct 22, 2000
    Columbus, OH, USA
    240,000 hard miles and the chassis keeps on takin' it! On it's second replacement trans (3rd trans including the original), and the body has about had it, but the truck runs and drives like new still! When the body falls apart, I'm yanking the drivetrain, computer, entire wiring harness, and possibly front suspension to save for an old Suburban/Panel Truck project in the future.

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