A compression test? Sure,very important. I just don't see where a leak down would provide any useful diagnostics. If compression is down,it's down for any number of reasons but why would knowing where in the cylinder column the problem existed be useful? Maybe detect a leaking head gasket or something but simple air pressure would do that. Besides,if you lose your concentration when doing a proper leak down,you could end up down for the count. Guys that do these know what I'm talking about.
for the count is for sure
.................................................. ...the scariest thing in life is the unknown ...................................
How would loosing concentration???
I have never done a leak down.
Engine has to be held at (or near) TDC for leakdown testing (some may test at different positions within the compression stroke for a more accurate picture).
For general testing, Compression is a good first-step (or go-to).
Leakdown provides a more accurate picture of the bore's health.
Both have their place.
Riddle This: A 3L V-6 Engine has compression down 20 psi on #4 cylinder, but runs fine up to 5000 RPM's. Seems to hit a wall around 5300 and sounds noisey.
Leakdown on #4 is within 1% of cylinder's 1,2,3,5 and 6 (all are 4% or less).
Both tests played a role in diagnosing the problem prior to teardown...
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
Obstructed reeds, Bent rod, piston skirt? seriously turned or loose sleeve or extreme crankcase leak. Got me curious
Last edited by lpugh; 02-04-2019 at 03:49 PM.
Thank You Leon Pugh
A leak down is done with air pressure applied in the spark plug hole.You must hold the piston in the cylinder in the position where you want to test at. This is commonly done with a breaker bar and socket on the flywheel nut.If you shoot the air to it and don't have a good grip on the bar or forget to grab it all together,there's a good chance the bar will swing around and nail you upside the head....down for the countOriginally Posted by moetorola
Don,my guess is water got in that cyl,it hydro locked and bent the rod. Bore and rings,piston still in good shape. Compression was down and air blew out at the down stroke on the leak down before it should have.The noise might be the piston skirt contacting the crank but can't explain the wall. Just a shot in the dark...
My very uneducated guess would be bad spark plug.
Mercury 2002 200hp Carbed Saltwater
OT566776
2002 Cape Horn 22' Bay
Bent rod was indeed the situation... oddity was that it had just been rebuilt somewhere a very short time ago.
Each test has it's place and benefits.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
Ouch,, that would be no good!
Follow my instructions,you wont be de-brained,Chris...Search leak down test.Ouch,, that would be no good!
Anyone believing that a leakdown test is not necessary on 2 stroke motors does not know what they are talking about. Compression numbers can be fine but leakdown numbers may not be. If only a compression test is done and the numbers are fine and no leakdown test is performed.....................TRUST ME....................you may find yourself having to rebuild the motors totally, possibly in excess of $10,000 if new nicasil is needed. For the record, it was $11,000 for 2 motors.
Last edited by catfish123; 02-09-2019 at 07:39 AM.
Nicasil?? That stuff is mainly for high perf applications and short lived compared to more conventional bores. If you're running Nicasil coated bores,the heads need to come off fairly frequently to check condition of the bores.In your case,if you would have done that,more engine damage probably would not have happened. With Nicasil you have to assume that it's going bad and if a leakdown confirms it,it,too late anyway. Apples and oranges here. With chrome or iron bores most motors have,most cyl wear is in the top which would be detected by compression test where a leakdown farther down the hole would show good.
You assume the motors were mine. They weren't. And they were that way when purchased. Seller had advertised them as excellent running condition with great compression numbers. $11,000 later, after the purchase, they could be run.
OUCH! Excellent example of where/why leakdown testing is critical.
Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor
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