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I have a 1998 GMC Sonoma SLS with a 4.3 l V6 Vortec.

It used to purr, now it growls, sounds like a small diesel engine. Vibration can be felt (mostly in the dead pedal) and also vibrates/growls in park and neutral.

Just came back from usual oil change, but has never sounded like that before, usually a nice quiet motor. Also significant vibration at 90 kph, smoothing somewhat at 100 and up..Any ideas?

  • Any check engine lights on? Is the vibration engine rpm specific (raise lower with engine speed) or is it vehicle speed dependent? – Pᴀᴜʟsᴛᴇʀ2 Mar 27 '16 at 13:23
  • It sounds as if you might have a cylinder that the spark is not firing on. That would make the engine feel rough and vibrate. Have you checked all of your spark plug leads and ensured you have spark on all cylinders? – DucatiKiller Apr 4 '16 at 16:42
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    Thanks for the replies all-I thought of these things too- as it turns out its sometimes good to recheck your mechanics assessment so I got under the truck and the catalytic pipe joiner had disengaged almost completely from the joiner plate for the pipe leading to exhaust. It would stay more or less closed together at speed but when I slowed it would separate more lol. I've drilled out the bolts, put in new gasket and bolts, and she purrs like a kitten again. That said, will need new catalytic and exhaust in near future- any ideas on prices? – Thor Apr 6 '16 at 4:25
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    Nice! Can you post that as an answer and award yourself the answer? You'll get a badge for it too. Good lookin out on the update. Cheers! – DucatiKiller Apr 22 '16 at 1:19
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    Thor you need to do as @DucatiKiller says and 'write your own answer'. This will make the question classed as 'answered' and remove it from the unanswered question section. – Bevan Jun 14 '16 at 20:21
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Per OP in comments (submitted to remove this question from the 'unanswered list' - please upvote Thor's comment, not this answer!):

As it turns out it's sometimes good to recheck your mechanics assessment so I got under the truck and the catalytic pipe joiner had disengaged almost completely from the joiner plate for the pipe leading to exhaust. It would stay more or less closed together at speed but when I slowed it would separate more lol. I've drilled out the bolts, put in new gasket and bolts, and she purrs like a kitten again. That said, will need new catalytic and exhaust in near future.

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