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350 leakdown

Discussion in 'The Garage' started by Mikeyg79, Nov 3, 2002.

  1. Mikeyg79

    Mikeyg79 Registered Member

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    For some reason I replaced the valve guide seals in my '84 350.
    When both rockers for each cylinder were off, my compressor (1hp, 2gallon) could hardly keep uo with the leakdown. This happened on all the cylinders. They all make compression (125lbs) but won't hold it.
    Anyone think of what would make all 8 do this??

    Mikey
     
  2. ntaj*ep

    ntaj*ep 1/2 ton status

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    Did you have the pistons all the way at the top or bottom of their stroke? /forums/images/graemlins/grin.gif
     
  3. Mikeyg79

    Mikeyg79 Registered Member

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    I took out all the sparkplugs and both the rockers for the cylinder I was working on (I know there is an order to do it that makes it easier, but I did it cylinder by cylinder.) So the piston was forced down, but both valves were cully seated- as far as I could tell.

    Mikey
     
  4. jeffro

    jeffro 1/2 ton status

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    Where's it going? Intake or exhaust? BTW the valve seals don't fix compression leaks.
     
  5. Mikeyg79

    Mikeyg79 Registered Member

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    Duh, I know valve guide seals don't fix compression. They fix the first little puff of blue smoke at start-up.
    As far as I can tell, air was coming out the holes in the heads between the pushrods.

    Mikey
     
  6. MJB774X

    MJB774X 1/2 ton status

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    holes in the head between the pushrods??? /forums/images/graemlins/rolleyes.gif
     
  7. Mikeyg79

    Mikeyg79 Registered Member

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    If you take the valvue covers off a 350, there are square holes in the heads between the pushrods.
    This is one way to tell a 350 from a 305.

    Mikey
     
  8. CooknwithGas

    CooknwithGas 1/2 ton status

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    There are only three places for the air to go - 1. into the crankcase through the rings 2. Out the exhaust valve 3. out the intake valve. That's it (assuming your head is bolted tightly to the engine).

    Either your valves are not sealing (one or both), or it's leaking past the rings. If air is coming up from the valve stems with the air applied to the cylinders, your valves are not seating. New valve stem seals won't help this problem. You need to have the valves re-seated.

    If the air is coming out of the crankcase (holes in the head between the pushrods), it sounds like it is leaking past the rings.
     
  9. ntaj*ep

    ntaj*ep 1/2 ton status

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    With the piston going all the way to the bottom of its stroke you had to fill the whole cylinder area with compressed air. In all honesty I'm surprised the valves stayed put very well at all. /forums/images/graemlins/grin.gifBut its cool that they did. /forums/images/graemlins/thumb.gif
    The air comming from the pushrod holes means the noise was comming from the vally which is connected to the crankcase, which means the air was going past the rings. Not an issue of lost compression just the air your pumping in leaking past the gaps in the rings. Your compressor not keeping up was 'cause of the cylinder volume (piston being all the way down) /forums/images/graemlins/thumb.gif
     
  10. ntaj*ep

    ntaj*ep 1/2 ton status

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    Make that four places dude. It can go into the cooling system as well (bad head/block or gasket) /forums/images/graemlins/grin.gif
     
  11. Mikeyg79

    Mikeyg79 Registered Member

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    It shouldn't matter where the pistons are. 150psi should keep the valves closed if the piston is up or down, which it did.
    Saying the position of the piston matters is like saying your impact gun works better with a shorter air hose.
    Mikey
     
  12. CooknwithGas

    CooknwithGas 1/2 ton status

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  13. ntaj*ep

    ntaj*ep 1/2 ton status

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    Then why did your compressor not "keep up" DA! /forums/images/graemlins/angryfire.gif
    Yes the valves will stay closed, but your compressor now has the entire cylinder area to fill and keep filled, instead of the combustion chamber and top of the piston.
    If you know the answer to the questions you ask, don't waste our time!
     
  14. ntaj*ep

    ntaj*ep 1/2 ton status

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    Oh one more thing DA. With the piston at the top of its stroke if you did have a faulty valve seat surface. The valve wouldn't "drop" all the way into the cylinder when the spring is removed. Promting your DFA to remove the cylinder head. But you already knew that huh!? /forums/images/graemlins/angryfire.gif
     
  15. Mikeyg79

    Mikeyg79 Registered Member

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    0.7125 Liters < 2 Gallons. Once the cylinder filled with air, the air would leak out, my compressor would run- then stop. Less than 45 seconds later it would start to run again.
    That is one heck of an air leak for an engine that makes 125psi of compression.


    Mikey
     
  16. Triaged

    Triaged 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    Less than 45 seconds later it would start to run again

    [/ QUOTE ]

    That is a fairly long time for the compressor to stay off if you have a 2 gal tank. The ring gap on a cold engine can be fairly large but it closes up as the motor warms up. If the compression is fine then don't worry about it.
     

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