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Diagnose This!!

Discussion in 'The Garage' started by ErikH, Jul 1, 2003.

  1. ErikH

    ErikH 1/2 ton status

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    Hiyuh,

    Gotta problem with my '71 with the 350... The noise started gradually one day when I drove up a grade. I first thought it sounded like severe pinging or detonation, but then it sounded more like a valve slapping real hard... but it only happened when I put my foot into it. After 30-minutes of driving, the sound became more constant, and then the idle got pretty rough. I also noticed the right exhaust was real rough, while the left seemed fine.

    SO THEN, I replaced the cam, lifters and timing chain, which were all pretty worn. I also re-adjusted my aftermarket roller rockers. After I set the timing, fired it up, I drove down the street only to hear the same damn noise! And now the idle is rougher than ever. Sounds horrible.

    What do you guys think is wrong. Do I have a bad cylinder head (bad valves or springs) or do I have a bottom-end problem? I'm not too experienced with these deeper problems. Please help!! Thanks in advance to all those who respond!
    -Erik
     
  2. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    Do yourself a big favor and start by pulling all the plugs and doing a compression test first. If after doing the compression test and everything is fine then your problem is not internal. When doing a compression test you aren't really looking for any given number but rather the spread between the highest and lowest numbers. You should be within 20 lbs. between the low and high numbers. Typically a V/8 compression reading would be somewhere around 140-160 lbs. with the engine at operating temperature.
     
  3. ErikH

    ErikH 1/2 ton status

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    Thanks for the response.
    If compression is bad (varies a lot), where would I go next... what would I replace? If compression is good, then what kind of "external" problem could I have?

    FYI, I don't blow smoke at start-up, which says to me that the valve guides are in decent shape... But could I have other head problems... This is so frustrating!
     
  4. doctor4x4

    doctor4x4 1/2 ton status

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    I am with 4x4 on this dbl.check timing first then compression if all checks out then ya probably got a spun main bearing or a wrist pin
    good luck
    hey mabe make a Wave file so we can all hear it
     
  5. ErikH

    ErikH 1/2 ton status

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    Thanks for the response, but I still don't know what a compression test will tell me. What failing component would produce a rough idle, knock under load, AND cause poor compression? If I have GOOD compression, what part(s) does that eliminate?

    Also, a spun main bearing doesn't make since since only the right exhaust sounds rough, but maybe a bad wrist pin is the problem. How do I diagnose this w/o pulling the engine? Thanks again.
     
  6. mastercraftkpk

    mastercraftkpk 1/2 ton status

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    Knowledgable Kung Fu Master Voice

    "it is frustrated man who get good advice but not follow. It is not what problem is..but what problem isn't".

    "Your eyes and ears will lie to you like a politician promising to care about you, a compression gage can not lie."

    /forums/images/graemlins/usaflag.gif
     
  7. ErikH

    ErikH 1/2 ton status

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    Ahh... Sank yu, Kung Fu Masta...

    Actually, however, I AM interested in using a compression gauge IF I knew what the results could tell me. Other than yielding various PSI readings, what can the gauge tell me about actual engine components? Please enlighten me, Master.

    __________________________________
    Knowledgable Kung Fu Master Voice

    "it is frustrated man who get good advice but not follow. It is not what problem is..but what problem isn't".

    "Your eyes and ears will lie to you like a politician promising to care about you, a compression gage can not lie."
     
  8. SUBFAN

    SUBFAN 1/2 ton status

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    It can tell you many things, almost to numerous to list.

    dDo the test, and 'update' with the results, and we will resume....
     

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