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Compression Test

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Adub, Aug 4, 2003.

  1. Adub

    Adub 1/2 ton status

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    Sorry guys, this one's going to be a little long.
    89 Jimmy TBI 350 vortec heads milled .030 running about 9.5:1 compression. Bottom end has about 135k on it, heads have about 3k on em. Truck was using oil when I replaced heads, oil consumption went down, but its going back up. About a quart every 150 miles, leaks some, but it does smoke at high rpms. Did a compression test tonight
    1) 146 2) 146
    3) 146 4) 150
    5) 150 6) 156
    7) 140 8) 142

    Plugs 3 & 5 are constantly covered in oil. I even run plugs 2 steps hotter in those cylinders, but they only last about 1500 miles or so. Plugs 7 & 8 were a little tan, but looked good, the rest looked new. The truck has to make a 500 mile trip home in a couple weeks before I can rebuild the bottom end. What do you guys think? The numbers aren't bad, but I expected worse because of the plugs and oil consumption. Any advice would be appreciated.
    AW
     
  2. the professor

    the professor 1/2 ton status

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    A couple things here...
    --
    The compressin is good, but the compression test only measures the valve sealing and the piston compression ring sealing...

    It unfortunately does nothing to check the oil-rings

    Again, unforutnately, bad oil-rings tend to smoke the most at hi-rpm, not so much at idle and lo speed...
    --
    Fortunately, oil rings do not tend to "oil-up" plugs.as much as some other things...and tend to wear "evenly" so all or most cylinders will give trouble

    There are two other possabilities...

    You said "you replaced the heads", but you didn't say if they were "reconditioned" or not

    In case they were not, the possiblilty is the "O" ring seals on the valve stems are "hardened" and leaking, also the valve seals themselves might be "hardened"

    This again, tends to happen to all or most of the cylinders, so unless there was an assemply problem with the heads, I do not think this is the problem...

    --------------------------------------------------
    The very best possiblity, and most probable is that the intake gasket is leaking at the 4/7 port lower edge...

    This is very common in ALL V engines, 6 or 8, that cover the "valley" directly

    GM and Felpro, now use a very thick and soft gasket, OR a hard fiber gasket with traces of orange silicone around all the ports..

    Either of these gaskets will work.. Just DO NOT use the "hard-embossed" gaskets..(there ones with raised ridges around the ports) they leak....

    DO NOT use the rubber end seals if supplied, instead use Ultra Black, O2 sensor safe Silicone..smear some on the manifold and on the block first, then put a bead on the block....

    use NO sealer around the ports..
    .
    but DO use Hi Tack or some other gasket glue, between the ports on the head and gasket to hold the gasket in place for assembly

    Be EXTREMELY careful to center the manifold, keep the gaskets from sliding out of position..

    Be very careful to tighten the bolts in many stages and evenly to prevent the maifold from being pulled off-center.

    The later heads/maifolds with the near verticle bolts in the center, on either side of the heat passage, shold have those snugged up first alternate side to side.. the alternate and work ot to the ends..OR follow the factor recommended tigheng sequence if you have it

    This applys to ALL manifolds, but the Aluminum manifolds, because they are more flexable tend to leak worse..

    What happens is the manifold "cocks" slightly, or gets "off-center" this makes ports, usualy in pairs, to leak at the lower edge.. sucking oil from the "valley" into the port, using oil, smoking and fouling plugs.
    .
    The wrong gaskets, again more so with the Aluminum manifold, will cause this too...

    Since you recently had the motor apart.. I suspect this is the problem...

    On a related topic, some Ford engines, most notibly the V6 "Gernan" engines used in Rangers, Explorers and Mazda Navajos.. should have the head gaskets and bolts replaced when replacing the intake gaskets... and they are all torqued together... /forums/images/graemlins/cool.gif

    .. /forums/images/graemlins/cool.gif /forums/images/graemlins/truck.gif.
     
  3. Adub

    Adub 1/2 ton status

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    Heads were brand new GM factory pieces. Manifold used stock vortec gaskets and silicone on the ends. Was torqued properly as well. I have an Edelbrock performer vortec on it and I used the recommended intake gaskets. What do you think?
     
  4. Adub

    Adub 1/2 ton status

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    TTT
     

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