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Correct way to read compression numbers?

Discussion in 'Diesel Tech / 24 volts' started by lacruiser, Mar 5, 2007.

  1. lacruiser

    lacruiser

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    Hey folks, I have a question for you, about compression readings.

    From what I've been told over the years, the correct way to take a compression reading is to warm the engine up a bit, remove all the spark plugs, (or glow plugs if diesel) make sure the battery is fully charged, make sure the fuel is turned off, and then take the readings.

    When the engine is cranked, it encounters the cylinder being tested, and the pressure builds. You get an instant reading on the compression gauge. Then it spins around and the piston comes up for the second compression stroke, and the reading goes a little higher on the gauge. then it happens a third time, and so on. Each time it spins and encounters the cylinder being tested, the incremental increase on the gauge becomes smaller. finally, after about 10 or 12 spins, it's maxed out.

    For my diesel engine, I got numbers around the following:

    first spin: 280 psi
    second spin 380 psi
    third spin 450 psi
    twelfth spin 480 psi

    These numbers were pretty consistent in all 6 cylinders.

    So my question is this: what is the correct "compression reading" for the cylinder? I can't seem to find an answer to this question by doing web searches.

    thanks

    Dana
     
  2. Ron R

    Ron R Drive a Landruiser, enjoy life

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    You should spin the engine till the indicator has more or less a steady level.
    If that with 12 rotations, that's fine. If you need 20, that's fine as well.
    What is important is this max value. Not so much the value by itself but the values of the pressures of the different cylinders compared to each other.
    if you have a reading like 480,490,482,470. (4 cyl) ....fine. If you get 480,490,450,470 ...start worrying a bit about nr 3.
    Try to find the standard values given for this engine and compare them to what you have. Your's (normally) will be worse, but a lower limit will be given as well.
    From the info you gave, I think your engine is fine (compression-wise !! ;) )
     
  3. Japan4X4

    Japan4X4

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    Dana don't try a wet test with a good compression diesel :) Ask me why I know!
     
  4. lacruiser

    lacruiser

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    Thanks Ron for the info. Yes, I think this engine has good compression. Toyota specs a new engine at around 490 psi, so my 480 is pretty good. I have one cyl that's around 470, but thats's still good. Plus I have a couple of valves that are too tight. Gotta get that doggone SST.


    Japan 4x4, I actually did a wet test once, but i used a very small amount of oil, and after introducing the oil into the cylinders, (all of them) I cranked the engine for awhile to get the oil fully distributed into the rings and cyl walls, etc. I'm still here, so obviously the engine didn't explode, nor did it ignite the oil. I think it's pretty tough to ignite it unless it's atomized as it is through the injector nozzle.

    I don't think I'll do that again, though, as I was a little nervous when I did it. I ducked behind the fender while cranking!

    :)

    thanks again, guys.

    Dana
     
  5. Ron R

    Ron R Drive a Landruiser, enjoy life

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    Valves too tight is baaaadddd!!! Risk of burning them. Better to have some clatter.;)
    Guess y're not buying that SST from Toyo do you. Aftermarket is fine.
    Or borrow one.

    That's the way to go. A very little amount of oil in the cylinder to test.
    If you do it that way, all the glowplugs or injectors removed there is no danger to it at all. When oil squirts out, you used far too much.
    And after the test spin the engine for some time just to make shure the oil is gone.
    Should you try to start with glowplugs and injectors in place and too much oil in the cylinder(s) you can encounter hydraulic lock. This then can lead to extensive damage of your engine. :crybaby: