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  1. #1

    Question 3.4 COMPRESSION TEST HELP

    I haven't been on for awhile...but I had my mechanic find a 'good' 3.4 engine to replace mine with...supposedly it had around 82,000 miles. I did a compression test on all the cylinders just to see what I had, here's what I found out: All the cylinders had compression within 3 or 4 of 170 EXCEPT the rear far right cylinder had compression reading of 150. Another mechanic told me that from one cylinder to another it should not differ more than 10% and this is apparently VERY close to that. He also said this could be from a sticking ring since it had not been run for a bit or it could be a dirty valve or it could be starting to have a problem. I've sunk a good chunk in this engine exchange question:

    Should I be concerned with these readings? Did I buy a problem engine???

    The engine does not smoke and there are no knocks or pings internally that can be heard. All gaskets have been replaced and the original mechanic said the parts all looked good before he put it all together.

    Please respond those of you who know these engines and let me know your thoughts. Thanks!

  2. #2
    RobertISaar's Avatar
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    170-150 isn't bad at all for a motor that supposedly hasn't run for a while... you're supposed to do the test with the motor warm, at WOT with all of the plugs removed.

    oddly enough, it is cylinder 6 that sounds like it is the weakest, and IIRC, that is the cylinder that runs the hottest due to recieving coolant last.
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    All in all, those number sound pretty good to me.
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    AL's Avatar
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    Sound good to me too

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    Two words:

    Leakdown test

    Compression test is flawed due to several factors. I'm not saying it's worthless--quite the opposite. But a compression test provides less info than a leakdown test; and ideally you'd have BOTH tests done.

    The leakdown test will tell you HOW MUCH compression is bleeding off; and WHERE it's going to. If you hear pressure escaping out the throttle body, you know the intake valve isn't sealing. You hear pressure escaping from the tailpipe, the exhaust valve isn't sealing. You hear pressure escaping out the oil fill cap, you know it's going past the rings. If the radiator explodes, the pressure is going through a defective head gasket or a cracked casting. (well, OK, you might want to remove the rad cap and look for bubbles instead.)

    You hear a LITTLE air coming out the oil fill cap; and the overall leakage indicated on the gauge isn't too high--no problem.

    As with all things, the leakdown test isn't perfect, either. The guy doing it MUST have direct experience with the leakdown tester in use; because there's no "industry standard" for the orifice size in leakdown testers; and some are just plain MORE SENSITIVE than others. So don't be fooled by some bozo telling you that "X" amount of leakage means the engine is shot. IT ALL DEPENDS ON THE TESTER IN USE. The tester I use--an ancient Snap-On MT240 (???)--is very sensitive; I don't get too worried about any cylinder showing less than 40% leakage unless it's very small-displacement.

  6. The Following User Says Thank You to Schurkey For This Useful Post:

    94vertluvr (09-29-2010)

  7. #6
    Well this is the first time I've heard of this test. How do you ask a mechanic if he's a bozo or not? What kind of questions should I ask him so i know he knows what he's doing when it comes to this test? It sounds like it would be a helpful test to do! Thanks for the suggestion and info!


    Quote Originally Posted by Schurkey View Post
    Two words:

    Leakdown test

    Compression test is flawed due to several factors. I'm not saying it's worthless--quite the opposite. But a compression test provides less info than a leakdown test; and ideally you'd have BOTH tests done.

    The leakdown test will tell you HOW MUCH compression is bleeding off; and WHERE it's going to. If you hear pressure escaping out the throttle body, you know the intake valve isn't sealing. You hear pressure escaping from the tailpipe, the exhaust valve isn't sealing. You hear pressure escaping out the oil fill cap, you know it's going past the rings. If the radiator explodes, the pressure is going through a defective head gasket or a cracked casting. (well, OK, you might want to remove the rad cap and look for bubbles instead.)

    You hear a LITTLE air coming out the oil fill cap; and the overall leakage indicated on the gauge isn't too high--no problem.

    As with all things, the leakdown test isn't perfect, either. The guy doing it MUST have direct experience with the leakdown tester in use; because there's no "industry standard" for the orifice size in leakdown testers; and some are just plain MORE SENSITIVE than others. So don't be fooled by some bozo telling you that "X" amount of leakage means the engine is shot. IT ALL DEPENDS ON THE TESTER IN USE. The tester I use--an ancient Snap-On MT240 (???)--is very sensitive; I don't get too worried about any cylinder showing less than 40% leakage unless it's very small-displacement.

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    AL's Avatar
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    He should know what that test is... If he doesn't then find someone who does lol

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    Quote Originally Posted by 94vertluvr View Post
    How do you ask a mechanic if he's a bozo or not?
    Well, if you're Patrick Jane (The Mentalist) you'd ask directly. "So...Are you a bozo?" Good luck with that; you'll get a fist in the face or be escorted back to the parking lot.


    Quote Originally Posted by 94vertluvr View Post
    What kind of questions should I ask him so i know he knows what he's doing when it comes to this test? It sounds like it would be a helpful test to do! Thanks for the suggestion and info!
    Quote Originally Posted by AL View Post
    He should know what that test is... If he doesn't then find someone who does lol
    I'd ask to SEE his leakdown tester; and I'd ask what a "typical" good "reading" is.

    He looks at you blankly...you need another shop.

    If he says they just pump air into the cylinder and listen for problems; you need another shop.

    If he shows you a well-kept, non-greasy gauge that looks like it's been used, but not abused; and if he talks about various experiences he's had with it so it sounds like he's completely familiar with "typical" results from THAT leakdown tester...go for it.

    He shows you a brand-new gauge that looks unused...you need another shop.
    Last edited by Schurkey; 09-29-2010 at 11:48 PM.

  10. #9
    LOL LMAO that's funny! I just seen this comment for some reason I missed it!!! I watch that show and I can see/hear exactly what ur saying!!! Thanks for the good advice!

    Quote Originally Posted by Schurkey View Post
    Well, if you're Patrick Jane (The Mentalist) you'd ask directly. "So...Are you a bozo?" Good luck with that; you'll get a fist in the face or be escorted back to the parking lot.




    I'd ask to SEE his leakdown tester; and I'd ask what a "typical" good "reading" is.

    He looks at you blankly...you need another shop.

    If he says they just pump air into the cylinder and listen for problems; you need another shop.

    If he shows you a well-kept, non-greasy gauge that looks like it's been used, but not abused; and if he talks about various experiences he's had with it so it sounds like he's completely familiar with "typical" results from THAT leakdown tester...go for it.

    He shows you a brand-new gauge that looks unused...you need another shop.

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