difference between leakdown and compression tests?

[boatster]

Long story short I ran a motor without oil for a mile and it scored the pistons. Limped home and emery clothed the walls and ran her again. She still reaches the same top speed (maybe 1 mph slower) and runs smoothly. Thing is a leakdown test showed a good amount of leaking in 2 of the 3 cylinders. If it were leaking wouldn't my boat run alot slower? This leads to my question, what does a leakdown test tell you? How does it differ from a compression test information wise? I've heard you can get high compression even with a bad leakdown. I don't understand what this implie s

 

[Northern Eclipse]

Re: difference between leakdown and compression tests?

A compression test is just a measure of pressure for a cylinder and will give you a rough idea on the condition of that cylinder, a Leakdown test is the process of inducing compressed air into the cylinder via spark plug hole, with piston at tdc and on compression stroke, a leakdown test will inform you were a cylinder may be leaking resulting in a loss of compression and is usally done after a low compression reading has been found. The results of a leakdown are diagnosed as follows. Air hissing from exhuast exhaust valve or seat damage, air leaking up into intake manifold up into carb or throttle body, damaged intake valve or seat. Air leaking up thru dipstick tube or valve cover could be worn or broken rings damaged piston or cylinder. Air bubble's appearing in a cooling system could be a bad head gasket or cracked head. Any results found in a leakdown my not also be true to what actually is the damage, but will almost always require a teardown to inspect further. So in a nutshell a compression test will not tell you whats broken, but will indicate whether or not a leakdown test is needed. Hope this helps.

 

[JB]

Re: difference between leakdown and compression tests?

Moving to Outboard Non-repair.