98 Silverado RPMs randomly start jumping around, running horribly

Discussion in '96+ Vortec Performance + Computer Swaps' started by cptcrash80, Sep 11, 2019.

  1. cptcrash80

    cptcrash80 Newbie

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    So within the last couple months my 98 Silverado 2500 (350 Vortec) started having an intermittent issue after driving it throughout the day. It will startup and run fine first thing in the morning but the more I drive it during the day the worse it runs. The RPMs will start bouncing up and down and it will act like it isn't getting enough fuel. The only code it has thrown was codes for O2 sensors (which I replaced last year). I also just replaced the exhaust flange gaskets and that didn't make a difference. I have also replaced the fuel filter and spider harness.

    Testing the alternator from a cold start I was getting 14.6 volts. The truck has right under 289,000 miles on it so I'm trying to figure out what could be causing this kind of issue without it being consistent. I haven't had any codes come up in over a year until just the other day and once I replaced the exhaust gaskets and cleared the code it hasn't come back.

    Thanks in advance for any ideas.
     
  2. Eveready

    Eveready I'm Awesome

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    Possible bad ground that shows when things heat up, or the recent favorite a fuel pump issue. I would start by cleaning and shining the connections at the battery end and the starter end of the battery cables with attention to the chassis ground. That would be the cheapest/easiest fix if that is the problem. Good luck and let us know what you find.
     
  3. tahoe.mkIII

    tahoe.mkIII ...

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    When is the last time the distributor cap was changed and did you use a delco?..I have to change mine every 2ish years because the terminals get really crusty.if I don't I get those exact symptoms.the non delco caps usually last even less time before acting up
     
  4. cptcrash80

    cptcrash80 Newbie

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    I just pulled the cap and sure enough there was some corrosion (light, nothing super heavy) and some carbon buildup on the inside of the contacts (also light). I'll replace it tomorrow and see what happens then.
     
  5. tahoe.mkIII

    tahoe.mkIII ...

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    Same thing happened on the limited not too long ago. Cap hada few years on it but not many miles, pulled it and it didn't look too bad, had good spark, so I put it back on and started diagnosing elsewhere. Had a Delphi pump on the shelf, did that and the fuel filter, ran great for a day or two then the drive home shuddering/missing started again...pulled into the wawa parking lot and put the spark plug tester on.#s 2 & 5 were missing at totally random times, sometimes no spark for a few seconds then good solid spark for minutes. Gimped it home via the scenic route and swapped the cap and rotor and she's running like a champ again.(having an issue with it dropping out of od once and awhile, still working on that one)
     
  6. cptcrash80

    cptcrash80 Newbie

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    Gonna replace the cap, rotor and plugs tonight. Got all ACDelco components. I'll report back with what I find.

    My buddy swears it's a sensor (TPS or CPS). I guess we will see.
     
  7. cptcrash80

    cptcrash80 Newbie

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    Was out of town over the weekend so didn't get to drive the truck much. My wife drove it some and it seems to be doing good. Hopefully it will keep going :D
     
  8. robinhood2112

    robinhood2112 Newbie

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    i believe distributor related issue if tac is acting erratic
     
    PlayingWithTBI likes this.
  9. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    That or the ICM/coil. I had an ICM in my 88 (I know they're a little different) where I was cruising at ~1800 RPM and my tach in my data log would show it jumping as high as 4800. My dash tach was too slow to pick it up but I could feel the surging.

    ICM Surging.JPG
     

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