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Technical 95% leakdown

Discussion in 'The Hokey Ass Message Board' started by Nich, Sep 12, 2019.

  1. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    Engine: SBC, 327CI built in 2003 by professional and reputable builder

    UPDATED 9/13/19: Redid the leakdown, confirmed no leak through exhaust or intake valves. I've got the valve cover off so detecting air flow into crank was a little more difficult because there are so many places for it to escape, but I'm confident the air is passing the piston rings. Now my question is, what could I have done that would have caused the rings on the one cylinder to go bad? Engine has 25-30k miles on it, compression was great on that cylinder before heads were redone, engine sat for about 1 month while heads were out to machine shop.

    Backstory: I noticed exhaust on even bank sounded off. Compression tested all, all about 160-165psi except #8 was 0psi. Found #8 exhaust valve to be bent. Took heads into original builder and he discovered springs had lost rate and allowed valve to hang. Replaced all valves, springs, resurfaced, etc. I reinstalled the heads and it runs great. However, I noticed that after heavy deceleration my next use of throttle would send very light cloud of smoke out driver side exhaust. (Exhaust is true dual, no h or x pipe.) Ran compression test again and now #5 cylinder is only building 35-40psi and spark plug is dark.

    Today, warmed up engine, removed all spark plugs, removed rockers from #5 and did leakdown with regulator set to 100psi. I got it as close to BDC as I could but still expected the crank to turn a little bit. There was no movement and leakdown showed about 96% loss. It did not even seem to pressurize. Ive got no bubbles in coolant and don't seem to detect any air through carburetor or tailpipe. Thought I'd run a test on #1 for contrast and it moved the crank, fully pressurized and showed 1% loss, perfect.

    Again, pre-head removal/replacement the driver side was great, 160-165psi. Ideas?
     
    Last edited: Sep 13, 2019
  2. Doublepumper
    Joined: Jun 26, 2016
    Posts: 447

    Doublepumper
    Member

    If you use some oil to seal the rings/cylinder and do the test again, it may be possible to determine whether the problem was there or something in the head.
     
  3. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    I did my test after I had ran the engine for about 10min. I did try that trick when I did the compression test and I didn't see a noticeable difference.
     
  4. drtrcrV-8
    Joined: Jan 6, 2013
    Posts: 1,164

    drtrcrV-8
    Member

    You know what needs doing....You gotta pull that head..... How radical of a cam you got? If you aren't constantly "Red-lining" the engine & it's the valves &/or springs again, I'd suggest calling the cam vendor for his recommendations for springs, 'cause you DEFINATELY have a problem here!!
     
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  5. I hope you meant to say at "tdc" for the leakdown. You can not accurately leak an engine at bdc.
     
    X-cpe, Montana1 and 57 Fargo like this.
  6. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    With the rocker arms removed, why not? Cylinder #1 tested just fine.
     
  7. The cylinder wears the most at the top where the rings push out the hardest. That is where you need to test for leak down.
     
    kidcampbell71 and Truck64 like this.
  8. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    Ok, so I'll grant that TDC is more accurate than BDC, however, I don't think that my issue is cylinder position. In fact, I would expect a worse result at TDC if rings were the issue.
     
  9. X-cpe
    Joined: Mar 9, 2018
    Posts: 693

    X-cpe

    95% leakage means a lot of air is moving. Only place left is the rings. Listen through the oil drain back holes if the valve cover is still off or the oil fill if put back on.
     
  10. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    Agreed, a ton of air is moving, like it's almost just passing straight through the cylinder. I'm having trouble pinpointing where the air is going, and I'm also trying to figure out, if it is the piston rings, what happened... the only cylinder I had any issues with prior to heads being rebuilt was #8.
     
  11. When leak down test is taken at BDC, the piston skirt partially comes out of the cylinder and cocks a little bit, letting the rings become unsealed.

    However, 95% leakage is from a valve hanging open from either:
    1) a worn valve guide letting the valve flop around on the seat, or
    2) a guide that is too tight for some reason like carbon interference or
    3) overheating, thus crushing the guide and grabbing the stem or
    4) bent valve stem from piston/valve interference or
    5) too tight a rocker arm/valve adjustment or
    6) something caught between the valve and seat

    Listen for air escaping to the crankcase, intake port or exhaust port...
     
    Nich likes this.
  12. saltflats
    Joined: Aug 14, 2007
    Posts: 9,526

    saltflats
    Member
    from Missouri

    Put the leak down tester back on an take a soft hammer that tap on the valve stems and see what happens.
     
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  13. dana barlow
    Joined: May 30, 2006
    Posts: 3,516

    dana barlow
    Member
    from Miami Fla.

    Low cly psi test;,but your not hearing hiss in carb or EX=broken rings and maybe broken piston............................ether way must take out an replace
     
    Montana1 likes this.
  14. RidingMechanic
    Joined: Jul 31, 2017
    Posts: 84

    RidingMechanic
    Member
    from Cincinnati

    What about a blown head gasket?
     
    Montana1 likes this.
  15. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    This makes the most sense to me, given how the air flow seems to pass right through. Would a bad intake valve explain the low compression, high leakdown, AND the light colored exhaust smoke (maybe excess fuel burn off?) that happens right after a hard deceleration?
     
    Montana1 likes this.
  16. Here's another reason a leak down test might be fooling you. If the rings are new, they will leak compression and oil until you run at full throttle several times and press the rings hard against the cylinder wall. That will seal the rings. We call it "break-in". I have seen 90% leak down on brand new rings, and after break-in they will go up to 98%!

    New rings have a slight barrel face on them from the factory and they won't seal real good until you blast full throttle a few times. I know that scares some guys, but that's how it's done!

    As the rings wear in they become flatter on the face and seal good, but then they gradually loose tension and the gaps widen. However, they will test 98% leak down, but there will be more friction on the cylinder wall as they wear, because of a wider sealing area on the ring. Surprisingly, they live a long time under these conditions, but performance will gradually decrease, because the friction increases.

    Depending on how much smoke and what color (blue-gray), when you decelerate from a high speed run with the motor slowing the car, it will suck oil through the intake guides, if the guides and seals are going away. However, it can suck oil through the rings too, if the oil rings are going away.

    Also, its NOT good to decelerate from a high speed run on an old worn out motor, because it will stretch the rods (with NO compression cushion) and take up clearances in the bearings to the point that the sharp upper ring edge will bump the ring ridge in the cylinder and break the ring!!! Yikes! I've seen broken rings cause a lot of leak down, to the point that the cylinder won't fire the mixture!

    It appears to me that you are going to have to pull the motor down and find out what's going on! ;)
     
  17. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    Yeah, it sounds like the head has got to come off at least. The engine only has about 25 or 30k on it. The bent valve/valve spring issues that started this whole adventure was a bit freak. Then to have the #5 drop compression and driver side blow light smoke after doing heads is weird to me. I only rebuilt both heads because the multiple springs on passenger side were sagging to some degree. Driver side had no signs of issues (I think the springs on that side may have had a little sag but weren't showing symptoms), now the driver side has the only issues.
     
  18. Also at BDC, (depending on your cam profile and position) the valves will be open. That's why you test at TDC on the compression stroke. Don't mess with taking the pushrods out - it makes room for another place to error. ;)
     
  19. Was the motor sitting for a long time? What cam and springs were in it? What springs did he put back in? You might have more going on than we can tell from here. o_O
     
  20. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    To be honest, I don't know specific specs. I've asked him to get me a more detailed build sheet but I haven't gotten it yet. It has a mild cam and is fairly lobey (or lopey). I will say that I trust this guys ability to put the correct parts together. He's been building race engines for more than 30yrs, maybe 40, now and is well respected. I do feel like I'm going to ultimately find something odd, unusual at the root of this.
     
  21. You need to be aware that the only thing that seals rings is the firing of the cylinder. If a plug goes bad or a leaky valve, the cylinder can pump oil until the fault is remedied.
     
  22. boring-hop-yard
    Joined: Feb 24, 2008
    Posts: 74

    boring-hop-yard
    Member

    I find it odd that your losing 96 pounds of air and cant find the location of the loss.
    Were not talking about a hiss from 5 - 10 pounds, but 96 pounds of air.
    If its going by the rings you should see a volume of air coming out the oil cap hole.
    I have seen a oil fog come out at 5 pounds loss.
    If your numbers are correct you should have 96 pounds of air coming out of the motor.
    100 pounds in engine, holding 4 pounds, 96 pounds of air coming out of motor
     
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  23. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 1,735

    2OLD2FAST
    Member
    from illinois

    Did you back the valve adjustment off and try the test ? If it was good before , you've probably over adjusted ( and possibly bent) a valve , or you over-revved it and did the same. Block the carb open , put a paper towel over it ,if the towel flutters , it's an intake valve , do the same with the tail pipe for an exhaust valve and finally at the oil feed hole in the the valve cover to check the rings....it ain't rocket science..
     
  24. RichFox
    Joined: Dec 3, 2006
    Posts: 9,746

    RichFox
    ALLIANCE MEMBER

    Do people read anymore? He clearly says he removed the rocker arms from the leaking cylinder. Says air is not coming out the intake or exhaust. boring-hop-yard is thinking the same thing as I am. That much air should be easy to find where it's going. Pan, water jacket, or head gasket. Got to be somewhere.
     
  25. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    I updated my post.
     
  26. RichFox
    Joined: Dec 3, 2006
    Posts: 9,746

    RichFox
    ALLIANCE MEMBER

    Don't you have an oil fill on your car that you could have listened to? Your air compressor is able to keep up with a 95% leak? You may have holed a piston. Everyone who said the head is going to have to come off were right.
     
  27. Nich
    Joined: Sep 12, 2019
    Posts: 10

    Nich

    Oil fill is through breather on driver side valve cover and it is off of the engine. I've got a 26 gallon 150psi air compressor that keeps up pretty good. After filling it, I can run the leakdown for at least a minute(?) before it kicks back on. I was planning to pull the engine to make it easier to redo the timing cover and oil pan gaskets anyway since I don't drive it in the winter. Looks like the head will come off too... eh. I'm still wondering what would have caused this when they were fine right before head work.
     
    Last edited: Sep 13, 2019
  28. RichFox
    Joined: Dec 3, 2006
    Posts: 9,746

    RichFox
    ALLIANCE MEMBER

    It is inconceivable to me that you can't very clearly hear 100 pounds of air coming out . Are you now or have you ever been an aircraft mechanic? Ever get ant detonation? Preignition? You may have holed that piston. Interested in what you find.
     
  29. The last time I saw a cylinder with 96% leakage, the piston was torched thru the ring lands due to detonation. All of that air pressure has to be escaping to somewhere and if the valves are sealing the possible escape routes have been significantly reduced.
     
    Truck64 likes this.
  30. Desoto291Hemi
    Joined: Apr 21, 2009
    Posts: 766

    Desoto291Hemi
    Member

    Rich fox is correct!
    Also,,,,how do you keep the piston at TDC with over 100 psi against it.
    Block the movement of the crank by attaching something to the flywheel?

    I don’t do leak test,,,but I air test every engine and heads.
    I apply 150 psi to the cylinders,,,easy to hear any leakage past valve or rings.
    That is why I hand lap every valve that comes back to me,,,just to be sure.
    No leaks yet!

    If his valves are not blowing air,,,it should sound like a tornado coming out of the pan area!

    Tommy
     
    gimpyshotrods and X-cpe like this.

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