How to properly run a compression test

Discussion in '1994 - 1995 Specific Tech' started by DanG, Oct 28, 2011.


  1. DanG

    DanG Founding Member

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    Been a while since I've run one of these, however, I wanted to make sure I'm not missing anything before I begin.

    1) disconnect coil wire from distributor
    2) disconnect fuel pumps ( I have externals ) power supplies
    3) put throttle body blade in wide open position, or simply put pedal to floor when cranking
    4) make sure battery is charged fully for accurate reading
    5) pull one plug at a time and insert gauge into spark plug hole
    6) crank motor over until gauge needle stops moving. Should take a few cycles

    Do these above items look correct? Also, would it hurt to keep a battery charger on the battery while conducting this test?
    #1
  2. VibrantRedGT

    VibrantRedGT "STANGNET'S PENGUIN SMACKER" Super Mod

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    Yeah, that'll work.

    I just disconnect the coil wire, install my battery charger for the entire test, one plug at a time, install the threaded compression line, have someone turn over the ignition with the throttle wide open. The gauge will stop after 3 or 4 cycles. Now move onto the next cylinder.
    #2
  3. DanG

    DanG Founding Member

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    Great! Thanks for the confirmation!
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  4. BIGDATOWN

    BIGDATOWN Member

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    Another addition also is if you hold the throttle completly wide open and crank it over. It will cut off ignition and fuel.
    #4
  5. JonezySN95

    JonezySN95 Member

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    Oops... I did it wrong when I checked mine. I didnt disconnect the fuel pump or hold it WOT. I also didnt hook up the charger :nonono:. I still got 145-155 on all, but im curiouse if it will make a differance if I actually do it the correct way? :shrug:
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  6. revhead347

    revhead347 I have face herpes.

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    I would disconnect the pigtail from the coil instead of the ignition wire. I never bother to disconnect the fuel. It's so little fuel, it hardly makes a difference. Crank it over a few times to get an accurate reading.

    Kurt
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  7. DanG

    DanG Founding Member

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    Kurt,

    In order to verify if there is a head gasket leak, should I be completing the compression test when the motor is warm or cold? I'm not so concerned about the rings or valve seals, as the motor only has about 600 miles on it.

    Thanks again.
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  8. revhead347

    revhead347 I have face herpes.

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    I guess warm would be preferable to check for head gasket leak. Usually a head gasket leak doesn't show up on a compression test. You have to run that pain in the ass coolant test for that.

    Kurt
    #8
  9. DanG

    DanG Founding Member

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    What's interesting, is that I purchased a block tester kit, which has a blue liquid that turns yellow when coupled with combustion gases. I ran the test and found nothing which indicated combustion gases in the coolant. I also rented a radiator pressure tester however I unfortunately couldn't get any of the fittings to fit the radiator in order to complete a full seal, however I may give it another shot.
    So, now I'm down to performing compression tests on the cylinders as I can still smell the scent of coolant in one of the tail pipes after getting into boost, although I still am not losing coolant.
    One more thing to mention, that I never retorqued the head studs on the AFR heads after initial install. I'm running an MLS gasket which was installed by the shop who built the motor. I'm wondering whether or not a retorque would be too late at this point, if it is needed?
    #9
  10. revhead347

    revhead347 I have face herpes.

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    The head gaskets are probably only leaking when the compression pressures get high under boost. I doubt that will show up on a compression test. If the intake is over torqued, it can cause the heads to lift. Even stock torque setting on the intake can cause head gasket priblems.

    Kurt
    #10
  11. DanG

    DanG Founding Member

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    That's a good point. I have the lower intake torqued to stock specification, and did not retorque the head studs.
    So, I'm assuming that torquing the lower intake bolts to 1 or 2 lbs lighter than their recommendation wouldn't be out of the question?
    This whole process is certainly a learning/re-learning experience for me.
    #11
  12. DavidEwers

    DavidEwers Member

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    what are trying to check for? I would do a leak down test instead. It will give you a much better reading of the condition of your engine than a compression check. It takes a little more work but the results will be much more precise.
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  13. DanG

    DanG Founding Member

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    So, the bottom line, is that I can smell coolant in the tailpipes after driving the car around for a while. I'm not losing any coolant that I can see, and the coolant is clean via a block test that I performed with testing fluid, however, I almost have to be leaking coolant into the combustion chambers or else I wouldn't be able to smell it from the exhaust. ( strangely only one exhaust pipe and not the other, even though they are crossed over in the middle )

    I'll have to do some research on how to perform a leak down as well.
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  14. DavidEwers

    DavidEwers Member

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    A leak down test will tell you if you have leaking valves, bad rings and even a blown head gasket.

    You have to make sure that both valves are completely closed. With the adapter installed. 100lbs of air pressure is done on that cylinder.

    If you have a bad intake valve you will hear air escping into the intake

    If you have a bad exhaust valve you will hear air from the exhaust pipe

    If you have bad rings you will hear coming from the air breather or dip stick tube

    A bad head gasket maybe just heard or you will see bubble in the radiator.

    I do not know how many miles that are on your engine but a normal car can see maybe 20% and still be healthy. When I ran my duster in stock eliminator if I got close to 8% the engine was pulled and re-ringed.

    A compression check is good easy check, but it really does not give a good reading of an engine.
    #14
  15. DanG

    DanG Founding Member

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    Thanks for the info, very helpful. Unfortunately I do not have compressed air, therefore I may need to use someone else's until I can get a compressor.
    The motor has about 600 miles on it total with all new parts including heads, valves, rings, etc. I'm thinking the folks who assembled the long block either didn't prep the block and heads correctly for the MLS gaskets which are on it, or perhaps I'm somehow lifting the heads slightly under 9-10lbs of boost, even though the heads have studs. I'm thinking the issue is mostly scenaro #1 however.
    #15

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