compression test

[mariner61]

On purchasing a used boat, what are thw thaughts on using oil analysis vs compression testing. And whisch is more reliable? And second why do some surveyors not consider the compression test as informative as the engine oil analysis?

Thanks

 

[Gary001]

Compression testing is better. Doing both is probably the best way to go. Oil analysis is good when the oil is used for 25 hrs or so. If they changed the oil before sale, it will show as good. First start the engines when cold and see how they start and idle. Then do a sea trial to see how they run, meaning getting up to WOT, with no smoke, vibration, etc.

Gary Washienko
Sound Marine Survey
Mamaroneck, NY

 

[Al_Prisco]

Mariner61,

Live data testing: performing a clyinder balance test with a scope and oil analysis is the best way to go. If there is a clyinder thats a problem it will show itself right away. Then go to that clyinder and perform a wet and dry compression test. Wave data can also be checked on the vessels charging system to check the condition of the alternators . With the late model vessel a complete scan on the engine with a scanner that can read trouble codes and show live data is the way to go.

With an oil analysis today, a surveyor can test new or old oil with out a problem. The lab just has to have this information to determan the correct information to reflect back to you. This is why the sample is always taken right after sea-trial. It will pick up any excessive wear or contamination that no other testing can do.

 

[newportdave]

I have to disagree that a cylinder balance test is better than (or just a replacement for) a conventional cranking compression test.

While a cylinder balance test done under load can identify a cylinder with low output, it isn't specific enough. The low output can be caused by things other than low compression, such as plug, wire, injector, ect. and the results can't always be verified by another tech. I also believe that the information you gain from examining the plugs (oil, rust, ect) is critical to evaluating engine condition, and pain in the butt it may be, it must be done.

Dave

 

[PascalG]

are we talking diesels or gas ?

on a gas boat, a compression check is so easy to do that you'd have to be crazy not to.. diesels are a different story depending on the engine...

oil analysis is good but there are many variables incl. how many hours are on the oil. Also, a one time analysis may not give you the big picture, the trend.