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Compression Test

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Floyd Hauffe
Member
Username: floydh

Post Number: 6
Registered: 06-2009
Posted on Friday, June 19, 2009 - 11:53 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

Can anyone tell me the proper way to perform a compression test on a single cylinder P6?

Thanks,

Floyd
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George Martin
Visitor
Posted on Saturday, June 20, 2009 - 08:35 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

I just used a normal car compression test guage and crnked the engine over as fast as possible, only needs a couple of turns. I got about 100lb/psi. I also have a vac/pressure guage which could probably be utilised to measure crankcase vac/press, but not tried that yet, cos I put new seals etc in it.
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Miah Gregory
Member
Username: mace

Post Number: 8
Registered: 05-2008
Posted on Saturday, June 20, 2009 - 08:23 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

I have been having some trouble with my P5 lately, and happened to do a compression test today, getting around 100 PSI, which feels about right.

However upon replacing the spark plug, i noted that there is some loss of compression from around the plug itself - does anyone have any ideas as to how to work around this?

Thanks.
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George Martin
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Posted on Sunday, June 21, 2009 - 04:53 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

Surely it must be the plug gasket. unless its a new plug badly machined.
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Miah Gregory
Member
Username: mace

Post Number: 9
Registered: 05-2008
Posted on Sunday, June 21, 2009 - 07:44 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

Seemed to have the same problem with a number of plugs; I had some limited success borrowing a rubber O ring from my compression tester and using that as a gasket but obviously not for very long as its not designed to handle heat.

Thanks.
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herbert alfaro
Visitor
Posted on Saturday, October 09, 2010 - 01:02 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

I have twin 454 cid mercruiser marine engines that show low compression on a number of cylinders in both engines when tested both hot and cold.Does this indicate a need for a major overhaul of both engines.The engines have approx. 1700+ hours of run time.They are original installation ala 1986.Comments would be welcomed.
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Andrew Menkart
Moderator
Username: andrew

Post Number: 1079
Registered: 11-2001


Posted on Saturday, October 09, 2010 - 02:51 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

Herbert,

That 454 mercruiser is a bit too new for this board. We are focused on antique style marine engines, mainly pre 1940's.

You should post that question in the board.Mercruiser category of the MarineEngine.com Forum
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Andrew Munns
Member
Username: johnoxley

Post Number: 11
Registered: 04-2010
Posted on Saturday, October 09, 2010 - 06:43 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only)

Hi - despite the Mercruiser being "modern", one day it will be an antique.

The priciples are the same for old or new engines - do a compression test and check manufacturers specs (if they exist). Remove all plugs + throttle should be kept wide open + remove coil lead to prevent sparks --> shocks/fires.

Low pressures indicates leaky valves/piston rings/head gasket. Adjoining low pressures suspect head gaskets between cylinders. If you can get a teaspoon of oil onto the top of the pistons* and retest pressures go up, the valves and head gasket are OK, but you will need big overhaul anyway! If the pressures stay low, suspect valves/head gasket --> top end overhaul.

* Getting oil onto piston will be difficult if you are testing a side valve engine or have bowl type piston (eg. Gardner diesel).

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