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2G Low compression

Posted by John Miller, May 24, 2020

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  1. John Miller

    John Miller Proven Member

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    Juneau, Wisconsin
    Hello fellow DSM addicts!

    I recently tried starting my 1997 eclipse GS-T for the first time since swapping in a used motor. The short block has 120k and the cylinder head around the same. I replaced all the gaskets in the engine, but left all the internals untouched. I couldn't get it to start so i went down the list of things to check. When I was checking compression, cylinder #3 was at 50psi whilst cylinders 1, 2, and 4 where at 150psi. I can't afford to rebuild an engine so it will either be sold or I will store it until I can afford to. My question is: Is it odd that only one cylinder is low on compression? Is there anything I can check without pulling the head off? Does this engine ultimately need a rebuild?

    Thanks
     

    302  1

    1997 Mitsubishi Eclipse GST
    fwd · manual · 2G DSM
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  2. luv2rallye

    luv2rallye DSM Wiseman

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    Minneapolis, Minnesota
    One cylinder low on compression is typically a blown head gasket on that cylinder. Did you replace the head gasket?
     

    3K  0

    1999 Mitsubishi Eclipse GST
    14.200 @ 95.000 · 2G DSM
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  3. John Miller

    John Miller Proven Member

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    Joined Jan 27, 2020
    Juneau, Wisconsin
    Yes, has a fresh oem headgasket propery torqued down with new oem head studs.
     

    302  1

    1997 Mitsubishi Eclipse GST
    fwd · manual · 2G DSM
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  4. Sjd6795

    Sjd6795 Proven Member

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    Joined May 14, 2014
    Wilsonville, Oregon
    Also needs to be sure the surface the head gasket is sealing on is nice and flat.
     

    Street Build 539  11

    1997 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
    Sjd6795

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    1991 Eagle Talon TSi AWD
    manual · 1G DSM
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  5. 2gmitsueclipse

    2gmitsueclipse DSM Wiseman

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    Joined Jul 13, 2005
    Auburn Hills, Michigan
    2 questions:

    1. Did you use a MLS or composite head gasket?

    2. Were either head or block resurfaced?
     

    Autocross Build 20K  47

    1998 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM

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    1999 Mitsubishi Eclipse GST
    manual · 2G DSM

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    1990 Plymouth Laser RS Turbo
    manual · 1G DSM
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  6. John Miller

    John Miller Proven Member

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    Juneau, Wisconsin
    Composite head gasket. Neither were surfaced, but I didn't see a need to
     

    302  1

    1997 Mitsubishi Eclipse GST
    fwd · manual · 2G DSM
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  7. John Miller

    John Miller Proven Member

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    Juneau, Wisconsin
    Surfaces were inspected and cleaned so the seal should be good.
     

    302  1

    1997 Mitsubishi Eclipse GST
    fwd · manual · 2G DSM
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  8. pauleyman

    pauleyman DSM Wiseman

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    oklahoma city, Oklahoma
    I see several things here.
    Used engine....sometimes you get what you pay for.
    Surfaces were inspected. Inspected how?
    Did you bleed the lifters down and have you done a leakdown? Find out for sure why its low and from where.
     

    Street Build 3K  1

    1997 Eagle Talon TSi AWD
    awd · manual · 2G DSM
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  9. John Miller

    John Miller Proven Member

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    Juneau, Wisconsin
    Inspected with that one gauge you use to measure crank play. I did not bleed the lifters and I have not done a leak down test.
     

    302  1

    1997 Mitsubishi Eclipse GST
    fwd · manual · 2G DSM
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  10. pauleyman

    pauleyman DSM Wiseman

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    oklahoma city, Oklahoma
    I would recommend a leakdown and a caliper is not going to cut it for inspection. Leak down will show you where the problem is then you can assess why.
     

    Street Build 3K  1

    1997 Eagle Talon TSi AWD
    awd · manual · 2G DSM
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  11. John Miller

    John Miller Proven Member

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    Juneau, Wisconsin
    Okay I don't have a leak down test kit, but maybe I'll buy one soon.
     

    302  1

    1997 Mitsubishi Eclipse GST
    fwd · manual · 2G DSM
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  12. pauleyman

    pauleyman DSM Wiseman

    5,859
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    oklahoma city, Oklahoma
    check local auto parts stores for rental. Not sure they have that. Alternately you can build one.

    Saw your other post about surface prep. In my opinion you're putting the cart before the horse. Find out why you have a problem. Then find a solution. If you just want to take it apart anyway then by all means go ahead. If you elect to take it apart you need to check all possible sources of low compression. If you have it apart I would suggest a thorough inspection and subsequent repairs by a qualified machine shop. If that also involves new valves. Pistons. Rings etc then that's what you do.

    Have you run this motor yet? Doubled check that one cylinder? I've had a comp tester oring roll on me. I'd hate to see you chasing a ghost
     
    Last edited by a moderator: Jun 7, 2020

    Street Build 3K  1

    1997 Eagle Talon TSi AWD
    awd · manual · 2G DSM
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  13. 2G 95 GSX

    2G 95 GSX Probationary Member

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    Joined Jan 21, 2017
    Montgomery, Illinois
    Start with a leak down test and go from there, if you’re lucky you’ll just have a valve that isn’t sealing properly which is a quick fix and then you’ll be good. Also you probably have another issue if it won’t start. I had low fuel pressure plus 2 melted valves on cylinders 1 and 2 and I still managed to get it started. Check for injector pulse and spark too.
     

    Street Build 321  6

    1995 Mitsubishi Eclipse GSX
    awd · manual · 2G DSM
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