Rough Idle Miss-tery

Discussion in 'Engine Performance + Maintenance' started by Biggun, Jan 27, 2020.

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  1. Biggun

    Biggun Newbie

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    Howdy, folks.

    My newly-acquired '98 Chevy 350 (which is off up in in a Chevy K3500) has a miss; rough idle, vibration, no power. The truck does not surge, stutter, or stumble, it just has a consistent miss. But she starts right up as soon as you bump the key.

    • All cylinders are getting spark.
    • New cap, rotor, wires, iridium plugs -- all AC Delco
    • The old plugs looked OK, except for one which was pretty carbon-fouled (#3 or #5 cylinder...I had them sorted by cylinder and then of course I knocked them all on the ground...)
    • I haven't checked cylinder compression, but there's no oil in the water, no water in the oil, no antifreeze vapor out of the tailpipe so I don't think its a head gasket.
    • OBD isn't throwing a code.
    I'm not at all familiar with the type of injection system in these trucks, but I suspect that as the culprit. The truck sat for a long time; could one of the poppets be stuck? I put a can of SeaFoam in the tank hoping maybe that'd knock something loose.
    Any ideas or suggestions?
    I was reading about the MFI upgrade--which I'm planning to do--but hopefully not this weekend.

    Thanks
     
  2. Schurkey

    Schurkey I'm Awesome

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    Consider a power-balance test. Connect a tach, or a vacuum gauge to manifold vacuum. Short out the spark to each cylinder in turn. Each cylinder "should" drop the rpm or the vacuum the same amount. When you short a cylinder and the vacuum and/or RPM doesn't drop much, that cylinder is weak. If vacuum/rpm don't drop at all when a cylinder is shorted, that cylinder is dead.

    When you know which cylinder(s) are the problem children, you'll have a better idea what to look for.
     
  3. Biggun

    Biggun Newbie

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    OK, I pulled the plugs when I got home and all of them looked good except for #5 (two shots of the same plug):
    upload_2020-1-27_21-23-32.png upload_2020-1-27_21-23-50.png
    I checked the compression in that cylinder, and it pumped up to 150lbs.
    I smelled of the plug, and it didn't really have a super-strong gasoline scent, however, the crankcase dipstick is oddly showing more "oil" than it did the other day...:uhoh2:
    I'm leaning toward an injector stuck open.
    Any suggestions or alternate diagnoses?
     
  4. honkon

    honkon OBS Enthusiast

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    Use OBDii dongle and app to check fuel trims. That will point you in the right direction.
     
  5. Schurkey

    Schurkey I'm Awesome

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    Plug out. Jump the fuel pump so it runs. Give it a minute or so. If the injector is stuck open, when you crank the engine, fuel should gush out the plug hole. Might wanna have a fire extinguisher or three handy.

    Mind you, I'd slap the upgraded spider in there on general principles; especially if it hasn't been replaced previously.

    I have no faith in Seafoam as a cleaning agent. You want results, you buy Techron Complete , or you pay a pro to do it with "real" tools.
     
    Last edited: Jan 28, 2020
    HotWheelsBurban likes this.

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