Help with Carb on computer truck (now wont start at all)

Discussion in 'Engine Performance + Maintenance' started by TacosnBeer, Nov 28, 2020.

  1. TacosnBeer

    TacosnBeer I'm Awesome

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    Ok, did some digging on it more the last few days... the dizzy is HEI and is not hooked up to the computer.

    As for the pressure regulator, it isn't pissing out or anything like that. I will try and post some pics later today when I can get in the garage. Maybe that will spark some thought.

    I will figure out a way to monitor the voltage at the dizzy, thanks SKYLARK
     
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  2. gearheadE30

    gearheadE30 I'm Awesome

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    the gauges bounce when it happens? That is odd and would strike me as a grounding problem or something rather than a fueling issue.

    this is a 1998 truck...you said it originally had TBI? It should have originally had an MPFI engine after 1995. If it has been swapped that many times, you might have a few layers of issues.

    What is controlling the engine's ignition? is it a straight distributor with no electronic control, or is the ECM controlling it? If you have a MAF fault, then it definitely still has the Vortec/MPFI ECM in there doing something. TBI didn't have a MAF. If the ECM is controlling spark without a MAF/TPS, I expect you have some very strange ignition timing.
     
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  3. TacosnBeer

    TacosnBeer I'm Awesome

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    The gauges bounce a lot anyway but... I did notice, the speedo does bounce with the miss under load too. Hard to explain.

    Yes, its a 98... I miss-spoke, it was never a TBI... and was a 305 MPFI then the 5.7 was dropped in and converted to carb/HEI. From what the previous owner (also the ASE Mechanic that did the swap) told be, it is straight HEI.. not connected to the computer. That said, the ECM is still in there. It seems like it is changed over to a more basic set up but, something is clearly not right. He (the PO) said he changed the dizzy and carb twice to solve the issue but neither solved the issue. Not sure if that includes the ignition module or he just changed the lower portion of the dizzy as it has the same cap and rotor as the last dizzy I saw in it 2-3 months ago when I first looked at it. PO was supposed to get the issues resolved but, had no time and I ended up buying it AS IS... thus inheriting the bugs left over from the swap/carb conversion.

    Issues:
    Gauges bounce
    Weird miss under load (heavy throttle or uphill)
    A/C not working at all (everything is new) might be a ground issue?
    No return line on the fuel regulator
    MAF code
    Service Engine Light on
    ABS light on (all brakes and master cyl are new)

    Other than that, the truck has NO RUST (and that is odd for a Hawaii truck). It is HARD to find an OBS RCSB here... overall, it is in decent shape and the price was right, especially when you consider the engine is brand new. Body has 180k on it and all the brakes are new along with the peripherals on the motor. Was worth it to me. Just have to figure out the bugs.
     
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  4. TacosnBeer

    TacosnBeer I'm Awesome

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    Could it be the ignition module in the HEI?
     
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  5. skylark

    skylark I'm Awesome

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    Nope. The module fires the coil from input from the pick up coil in the bottom of the distributor. Weak spark isn't caused by a signal issue.
     
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  6. skylark

    skylark I'm Awesome

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    The maf code is because the pcm is looking for a maf that isn't there. They have a tps somewhere and the pcm hooked up to drive the transmission and the gauges.
     
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  7. Erik the Awful

    Erik the Awful I'm Awesome

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    What is your fuel pressure reading?
     
  8. gearheadE30

    gearheadE30 I'm Awesome

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    Okay, so you have a standalone ignition then. I was kind of wondering if it was an ignition misfire, but HEI is standalone. I still wouldn't rule out a misfire caused by something. Does it ping at all or have other drivability issues?

    AC won't work without the ECM working - the ECM controls the compressor clutch if I remember right. To get that working, you would need to do some wiring to bypass the body electronics while keeping the safety pressure switches.

    No return on the fuel regulator sounds like low hanging fruit to fix... I'd suggest doing that first.

    MAF code is because there is no MAF and it is looking for one.

    SES light will be on forever with the setup you have. You could tune it out using EFILive or something, or just remove the bulb. It will always have faults for things your engine doesn't have (knock sensors, oxygen sensors, TPS, MAF, etc). Seems like your truck only still has the ECM so that it can run the gauges.

    ABS light may or may not be related to everything else. It could be caused by the ECM, or could just be a bad sensor or sensor unplugged somewhere. I'm not sure what signals ABS would take aside from the wheel speed sensors.
     
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  9. TacosnBeer

    TacosnBeer I'm Awesome

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    gearhead30 - yep. stand alone HEI... I will try and pick up e fuel pressure regulator with a return port this week. I have two fuel lines that run up the firewall by the dizzy. One is hooked up, the other isnt... I assume that is the return line. Should be an easy swap. I need to do that anyway. I am also redoing and cleaning the grounds with new straps and wires to rule out grounds as an issue for the gauges as well.

    No other pinging or drivability issues. Runs good till you get in it and it falls on its face or its under load.
    Not as worried about the dummy lights for now. Prob deal with that later.

    I do need to handle the A/C though... I looked for threads with a HOW TO rewire the a/c out of the computer but came up empty. it is still run through the computer so, what would make it not work if the computer is still in there? is there a connection with what the motor is doing to the A/C for ECM management? Is that why it needs to run stand-alone?

    Thanks so far... gets me on the right path. I hate computer-controlled cars but... never liked the dash in the earlier trucks. Now I gotta deal with it on a whole different level with this thing.
     
    Last edited: Dec 1, 2020
  10. TacosnBeer

    TacosnBeer I'm Awesome

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    Erik - 7psi
     

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