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4.3 vortec with rough idle

Discussion in 'Chevy S-10 / GMC S-15' started by stansgray03, Sep 17, 2011.

  1. stansgray03

    stansgray03Well-Known Member

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    My sons 03 S-10 is running rough at idle. He had a blown head gasket between cylinder 4 and 6 that has been fixed. Cleaned the Mass air flow sensor, new plugs, wires, distributor cap, rotor and fuel filter. idled fine at first then after a few starts it began to idle rough again. Runs fine when you give it gas. Engine light is on but it hasn't been reset yet since I replaced the head gasket.

    Anyone have any idea what else I might try?
     
  2. ramenth

    ramenthActive Member

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    Don't let the engine light being on fool you. Chances are it's throwing other codes. In this case, I'd better dollars to donuts you're going to pull a 300 (multiple misfires) code or one if not more cylinder specific codes (301-306).

    The 4.3 has a few issues to start looking for, the most notable a distributor with a bad bushing. Take the cap off and have someone turn the engine over. See if the distributor is turning true or wobbling.

    Did you get the fuel filter tight? Stupid question, I know, but I have to ask to cover all bases. I repaired a 4.3 powered 1500 not too long ago where the owner had hacked in a smaller filter because he couldn't get the fittings loose and AZ said "this will work." Drip city. Once the filter was put in properly, the truck ran fine.

    This truck should still have the ignition module. Sorry, I don't have the wiring schematics in front of me for the module to tell you what wires need what power or reistance, but you should be able to come across it online with a google search. If not I'll see what I can do to post it tonight. Same with the crank sensor.

    Now, here's one I hoping you don't have: I can't remember the years but there's a TSB on the 4.3 (and the 5.0L and the 5.7L) that states the valve guides weren't machined right and the valve are sticking in the guides causing a 300 code. Not sure if it covers your truck. I can check. The basic check is to take the valve covers off, pop the springs off the valves, and move the valves up and down by hand while turning 'em to see if you feel any bind.

    Good luck!
     
  3. stansgray03

    stansgray03Well-Known Member

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    Thanks ramenth,

    The fuel filter is tight, no dripping there. The code is in the 300's it was miss fire cylinder #4. I'll check the distributor to see if it wobbles.
    Any chance it could be one of the sensors?
    Problem seems to come and go. Was idling fine then shut it off and when restarted started idling rough again.

    Stan
     
  4. ramenth

    ramenthActive Member

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    The 304 code is specific to #4 cylinder. Check your plug, check your wire, and do a compression check.

    Just to spread the word to anyone who's reading the 300+ codes correspond to that cylinder: 300 is multiple misfires, 301 is a misfire on cylinder #1, 302 is a misfire on #2, etc.
     
  5. stansgray03

    stansgray03Well-Known Member

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    Well we took it to a shop and they did a bleed down test on it and determined it needed a valve job. So now the heads are in getting redone.
    My son overheated the poor thing real bad and I guess this is the results. Compression was down, only 70 lbs on that cylinder.

    Robert, thanks for responding.