Compression test, and now tapping
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500, 472, 425, 368 Discussion, Compression test, and now tapping in Cadillac Engine Technical Discussion; Well, finally got around to doing a compression test on my '70 SDV. All came back good, in the 165-185 ...
  1. #1
    BluEyes's Avatar
    BluEyes is offline Cadillac Owners Enthusiast
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    Compression test, and now tapping

    Well, finally got around to doing a compression test on my '70 SDV. All came back good, in the 165-185 psi range that my old Chiltons says it should be except for #1 which was high, around 200psi.
    But, then I put all the plugs back in and fire it up again to hear a disturbing clicking/tapping from the engine. One of the plug wires had come loose from its connector when I was removing them, so I figured I just hadn't gotten the connector crimped back on well. Got new wires, but no change.
    It is definately on the passengers side, and using my stethescope it really seems like it is that #1 cylinder that had the high compression.
    The engine has never made any strange noises before, always Cadillac-smooth for 2000 miles that I've driven it.

    So, any ideas? It's Christmas break, and I want to be driving my fun cars!

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    JTraik is offline Cadillac Owners Enthusiast
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    Re: Compression test, and now tapping

    Quote Originally Posted by BluEyes
    Well, finally got around to doing a compression test on my '70 SDV. All came back good, in the 165-185 psi range that my old Chiltons says it should be except for #1 which was high, around 200psi.
    But, then I put all the plugs back in and fire it up again to hear a disturbing clicking/tapping from the engine. One of the plug wires had come loose from its connector when I was removing them, so I figured I just hadn't gotten the connector crimped back on well. Got new wires, but no change.
    It is definately on the passengers side, and using my stethescope it really seems like it is that #1 cylinder that had the high compression.
    The engine has never made any strange noises before, always Cadillac-smooth for 2000 miles that I've driven it.
    So, any ideas? It's Christmas break, and I want to be driving my fun cars!
    Sounds like you dropped a chunk of something in the cylinder.

    A: Remove the spark plug from the suspected cylinder, then turn over the engine and immediatley turn it off, maybe whatever was dropped in there will shoot out the hole.

    B: Start soaking head bolts!

  4. #3
    BluEyes's Avatar
    BluEyes is offline Cadillac Owners Enthusiast
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    Re: Compression test, and now tapping

    Well, whatever it was, it went away.
    I started it for the second time since I did the test, and noticed that the sound was alot less. Decided to take it for a drive since I had some errands to do anyways, and she's back to her usual smooth, quiet self.
    I'm wondering if I wasn't getting proper oiling to the topend during all that cranking and it just needed some time running to get oil up those pushrods.
    Either that, or whatever was in the cylinder made its way out the exhaust!

  5. #4
    JTraik is offline Cadillac Owners Enthusiast
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    Re: Compression test, and now tapping

    Its more likely that whatever you dropped in there got all mashed up and yes... did find its way out the exhaust.

  6. #5
    jk1000 is offline Cadillac Owners Member
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    Re: Compression test, and now tapping

    It's possible you have carbon build-up in that cylinder. This can cause a tapping sound which is much louder than lifter noise. Also, since the carbon occupies space, it will raise compression. If the carbon breaks loose, it will be munched by the exhaust valve and blown out with exhaust. There are products you can pour down the carb (while revving the engine) that remove carbon.

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    JTraik is offline Cadillac Owners Enthusiast
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    Re: Compression test, and now tapping

    Quote Originally Posted by jk1000
    It's possible you have carbon build-up in that cylinder. This can cause a tapping sound which is much louder than lifter noise. Also, since the carbon occupies space, it will raise compression. If the carbon breaks loose, it will be munched by the exhaust valve and blown out with exhaust. There are products you can pour down the carb (while revving the engine) that remove carbon.
    Very true however his symptoms started directly after he monkeyed around with the compression tester. Pouring ATF down the carb will break up the carbon buildup. Seafoam also makes a good product for this situation.

  8. #7
    dkozloski's Avatar
    dkozloski is offline Cadillac Owners 10000+ Posts
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    Re: Compression test, and now tapping

    Get a cheap plastic spray bottle and fill it with tap water. Take the intake loose so you can get at the intake. With the engine running about 2000 RPM spray a mist of water into the intake and carbon will come flying out the exhaust. This mechanics trick has worked successfully for nearly 100 years. The chance of doing damage is nil.

  9. #8
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    caddeville is offline Cadillac Owners Enthusiast
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    Re: Compression test, and now tapping

    Won't the carbon shit clog the catalityc converter? My caddy makes that tapping sound upon start up and has gone much quieter, and my car is still a little bit sluggish upon acceleration when warm, the cat rattles and is most likely clogged, plus it has 94,000 miles. When i got the car, it would rattle for a couple days upon idle and then it wouldn't, then it would start a little bit again. I replaced the cap, rotr wires and plugs, MSD everything + Bosch platinum plugs. I got the 6a ignition too, it worked perfect, but because of the fact that it is used, it developed a problem and i sent it to MSD ignition for repairs. I still have to replace the coil and ignition control module.

  10. #9
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    davesdeville is offline Banned
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    Re: Compression test, and now tapping

    ^^Not on a 70 it won't clog the cat.

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