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  1. #1
    Join Date
    Nov 2005
    Posts
    326

    Default De Bugged but Low Compression on Yanmar

    I have recently taken out the tank on my Dehler as inspection of the diesel tank identified what looked like half a pound of tea leaves. I stank for a few days but was pleased with the outcome.

    Just had a compression test on my Yanmar 3gm30 resulting in cylindres 1 to 3 having 300, 260, 260 psi compression. The manual states 340 psi.

    In a previous thread I was seeking a good mechanic within the Brighton area to undertake some initial troubleshooting, I received zero replies. Obviously concerned as do not wish to pay 60 for the on-site mechanics, especially when the neighbouring yacht owner was handed a bill for 1200 pounds following a blocked inlet causing overheating. they had undertaken every check, renewing parts on the way but did not do the primary task of checking the strainer. Absolute true storey and he paid the bill.

    Anyway, can anyone suggest what my options are and do they know someone who can fix the engine at a reasonable cost.

    Thanks

  2. #2
    Join Date
    Jun 2005
    Posts
    13,126

    Default

    I wouldn't worry about your compression readings. The absolute compression reading will depend on the tester and coupling. The variation is important but might be within the bounds of a bit of carbon build up. Is there a problem?
    If I'd wanted to live in a Banana Republic I'd have gone to South America.

  3. #3
    Join Date
    Jun 2001
    Posts
    8,504

    Default

    Quote Originally Posted by yelbis View Post
    I have recently taken out the tank on my Dehler as inspection of the diesel tank identified what looked like half a pound of tea leaves. I stank for a few days but was pleased with the outcome.

    Just had a compression test on my Yanmar 3gm30 resulting in cylindres 1 to 3 having 300, 260, 260 psi compression. The manual states 340 psi.

    In a previous thread I was seeking a good mechanic within the Brighton area to undertake some initial troubleshooting, I received zero replies. Obviously concerned as do not wish to pay 60 for the on-site mechanics, especially when the neighbouring yacht owner was handed a bill for 1200 pounds following a blocked inlet causing overheating. they had undertaken every check, renewing parts on the way but did not do the primary task of checking the strainer. Absolute true storey and he paid the bill.

    Anyway, can anyone suggest what my options are and do they know someone who can fix the engine at a reasonable cost.

    Thanks
    If the two low readings are on two neighbouring cylinders, there is a possibility that you have an head gasket leak into the cooling gallery or between two cylinders.

    This is easy to confirm on an indirect cooled engine - you lose coolant - but far more difficult on a raw-water cooled engine.
    This type of leakage is probably the only real weakness in the 2-3 GM engine range as any Yanmar mechanic will tell you.

    As already suggested the drop is not really sufficient to start stripping the engine - but forewarned is fore-armed.

  4. #4
    Join Date
    Nov 2005
    Posts
    326

    Default

    Thanks for the replies. there is no evidence of head gasket problems, no oil use, no water in oil. Once started will run nicely and start on the button following 12 hours over to France, leave it two days and it will crank for 40 seconds trying its hardest to cough into life, almost like one cylider is starting then dragging the others along.

  5. #5
    BlueSkyNick is offline Registered User
    Location : Near a marina, sailing club and pub
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    Apr 2003
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    11,514

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    Quote Originally Posted by yelbis View Post
    ... it will crank for 40 seconds trying its hardest to cough into life, ....
    A bit like its owner, then !!

  6. #6
    Join Date
    Nov 2007
    Posts
    18,844

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    Other causes of poor compression are bore wear or rings - but usually leads to oil consumption. Poor valve sealing is another possibility. Outside possibility on Yanmar engines is distorted con rods caused usually by water getting into the cylinder - can't be compressed and bends the rods so reducing the compression ratio. Lifting the head will allow a check on all these. The ring/valve wear are obvious visually and the manual has the procedure for checking the rods. Slow starting is often an early sign of these problems.

  7. #7
    Join Date
    Jun 2005
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    A leak down test can be more informative than a compression test. Look it up on google. Poor starting could be a whole host of issues, the problem is that there are good and bad mechanics and you may never know which one you had on your engine until it's too late and end up with a huge bill for unnecessary work.

    Bearing in mind it runs OK once started:

    Start with the fuel supply, you need clean fuel and filters with no leaks anywhere a small amount of air in the system could cause no end of problems. If you have a little DIY skill it might be worth taking the injectors out and having the injectors tested at an automotive diesel workshop for nothing.

    Don't forget to use half throttle when starting.
    Last edited by savageseadog; 23-02-12 at 18:59.
    If I'd wanted to live in a Banana Republic I'd have gone to South America.

  8. #8
    Join Date
    Sep 2003
    Posts
    663

    Default Starting Procedure

    Unless you have a problem or have noticed a deterioration in performance and iIf once started she runs fine and starts again once warm I would leave well alone. These engines often take a bit of starting from cold. Procedure I followed when it was cold was flip the de-compressors and turn the engine until the oil pressure alarm goes out to make sure oil was spread on key surfaces. Then give her full throttle and turn her over. Often she would catch on one cylinder and then the other two would come in. Throttle down straight away.

    Make sure the sea water inlet is closed if your turning the engine for any period so you don't "backfill" the engine.

  9. #9
    RAP77 is offline Registered User
    Location : Midlands / Boat Portsmouth
    Join Date
    Jul 2009
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    46

    Default Starting Problems

    I had similar with my Yanmar - Starts Ok if run recently but leave for a day and requires churning for ever.
    The problem urned out to be a very small air leak in fuel supply - with the tank below the engine inlet it must have drained the feed pipe
    Check for air leaks filter stop tap and all connections

  10. #10
    Join Date
    Oct 2003
    Posts
    2,881

    Default

    Quote Originally Posted by yelbis View Post
    ... leave it two days and it will crank for 40 seconds trying its hardest to cough into life, almost like one cylider is starting then dragging the others along.
    I concur with the air leak suggestions, our 2GM20 was similar. Especially check the bleed screw on top of the primary fuel filter and the serrated ring that holds the primary filter bowl in place. This can bottom-out on its threads before the o-ring seals properly. Filling a thicker o-ring cures this (the one around the filter bowl - there have been previous posts about this on here) and was the key in our case.

    Basically track down and cure any tiny diesel leaks around the engine, as they will become points of air ingress when the engine is off if the fuel level is below the level of the primary filter.

    I think you also need to give ~3/4 throttle until the engine from from cold to help it start (throttle back once it 'catches').

    Andy
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