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Thread: SBC 5.0 to 5.7 Change

  1. #11
    Ultra, sorry to hear that your boat is still not fixed. TBI's rely on manifold vacuum to control just about all engine parameters. This relays engine load to the ECM. I've seen folks do engine swaps, and hook the vacuum line from the MAP sensor to "ported" vacuum instead of "manifold" vacuum. This would cause a hell of a drivability problem, and it's easy to do. The TBI has ports for both vacuum signals, and this would be an easy mixup for anyone. Not saying this is your problem, just something I've seen happen that pissed people off trying to figure out. Crank the motor up and pull the MAP vacuum line off the TBI, stick your finger over the port and make sure it's pulling vacuum at idle. Ported or timed vacuum only pulls when the throttle plates are cracked open. The manifold vacuum port is "usually" the lowest, or closest to the base of the TBI unit. something to check anyway.

  2. #12
    Thanks for all the inputs! I wouldn't have guessed that the 5.7 would be pumping that much more then a 5.0 but makes sense.
    Fogger, there is a small sensor at the base of the TBI with an electrical connector with 2-wires. When I pulled it with the timing light on the crank, the idle smoothed out and the timing mark stopped bouncing around and was as solid as a rock close to 0 degrees. I don't know what the sensor is since I don't have a book but could this be the vacuum control module? I'll check the connections and make sure I didn't get anything crossed.
    Right now the plan is to drive out to AZ speed and marine and have them reprogram the computer and get it set up. Unfortunately it's a six hour drive one way but they seem to be the only ones who can do it for under $1100. I could always pull the 454 out of the jet boat. . . Gotta love big carbs and HEI
    Should have stuck with the 5.0 :hammerhea

  3. #13
    Ultra, it sounds like you may have unplugged the TPS (throttle position sensor). was it directly opposite the throttle linkage(other side of TBI)? The MAP is usually mounted to a stand-off bracket and connected to the TBI via a molded black plastic vacuum line. There are a few things there, so I'm not sure what you unplugged, (pics?). There is a temp sensor, Idle air controller, MAP, TPS etc. Also there are probly a few grounds that break out of the main harness that sometimes don't get hooked up after an engine swap, this causes grief also.
    let us know, I'm curious.

  4. #14
    Spoke to AZ speed & Marine and what I unplugged was the diagnostic module (I think that's what he called it) that sets the computer into diagnostic mode so you can set the timing. He said that even the slightest mods to a motor that's computer controlled will require reprogramming the computer. Unlike an automobile that has O2 sensors (boats don't) and can compensate for minor changes, the boats are programmed pretty tight and even a slight cam change could require re-flashing. I re-checked connections and everything looks to be right. I'll post again when it's done so that someone who is looking into doing this type of swap will know what they're getting themselves into. This is a great lessons learned since I'm in the process of putting an LT1 into a 39 Ford. Can't wait to go through this again!

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