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I have 98 GMC Sierra 1500 RWD with 4.3 L engine and automatic transmission.

Have replaced the following within this year:

  • distributor cap
  • spark plugs and wires
  • valve cover gaskets
  • catalytic
  • thermostat and radiator flush
  • oil change and filter, and fuel filter as of yesterday.

Problem started out of no where hesitation, sputtering and rough like mild clogging and progressively getting worse. NO CHECK ENGINE LIGHT AT ALL DURING THIS TIME STILL.

Things that I had planned to get done in next 2 weeks before this:

  • Intake manifold gaskets (both upper and lower)
  • Muffler

It's been suggested to double check spark plugs, wires settings and connection to distributor cap. Distributor cap under warranty from shop that did job so told don't touch it.

Now can anyone give me any suggestions as to what else to look at or for? Really, this truck's engine has less than 85000 miles on it (3rd engine) and almost all components have been replaced or serviced. At a loss here and need help figuring out what it could be so I can fix it or get a shop to fix it.

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Listen to cranking the engine when cold. If uneven, compression is bad (unlikely, but you must rule it out before spending any more money). Check vacuum lines, especially the PCV valve hose and grommet. Replace the PCV valve itself. Replace the EGR valve. Once removed, spray carb. cleaner through the PCV and EGR ports in the intake manifold (they often clog). Replace those intake manifold gaskets. Test each spark plug boot with a tester to be sure ignition is firing properly. Do you hear any tapping sounds? If so, your oil change may have dislodged sludge within the valve train. If all else ok, remove each spark plug wire at idle with insulated pliers and listen for rpm drop; if one is dead, it has a faulty fuel injector. Finally, mix 2 oz. per gallon of Marvel Mystery Oil into the gas tank with 1 oz./gallon acetone and downshift the truck to 3,200 rpm in stop-n-go traffic for 30 minutes to clear fuel lines, injectors, heads, cat., etc.

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  • You realize pulling spark plug wires while the engine is running can fry the distributor?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 14, 2018 at 12:47
  • That sounds like a lot of work that may or may not yield results. OP should start by using a scantool to check for codes regardless of whether the lights on or not. Checking fuel trim and o2 sensor activity. Also since its a GM it will give misfire counters.
    – Ben
    Feb 14, 2018 at 16:06
  • Paulster2 - I didn't say pull the plug wires while the engine is running. I said connect a tester to each spark plug boot.
    – Carguy
    Feb 15, 2018 at 4:56
  • Ben - Yes, it is a lot of work but has yielded fantastic results in my real world experience. The poster asked for suggestions, and did not post any codes. Fuel trim, o2 levels, and misfire counters don't point to any specific problems or solutions, so the best possible resolution is to do as I say.
    – Carguy
    Feb 15, 2018 at 4:59
  • If the two of you have a better answer, why don't you post it?
    – Carguy
    Feb 15, 2018 at 5:00

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