Leak down test


Old 12-10-07, 11:00 AM
Thread Starter
Join Date: Dec 2007
Posts: 1
Leak down test

Hi All;
I recently got a new leak down tester and I experimented with it a little
today on a new tecumseh 9hp engine. I got the piston at TDC and
aired it up to 100psi. I had a drop to 85psi. Iím not sure I was at
the exact tdc, but the piston would stay up without holding the pto
or flywheel. I put low pressure in the cylinder at first and moved the piston
up and down a little till it would stay up without holding it. Piston was
on the compression stroke.
I could hear air coming out the muffler. Is it normal for some air to escape
through the exhaust valve. The engine starts and runs fine. Help will be appreciated
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Old 12-10-07, 03:03 PM
Join Date: Jul 2005
Location: Texas
Posts: 205

This is what a tech told me how to use a leakdown tester.You will have to put the piston on top dead center which you did,on the compression stroke,lock the flywheel so the engine cannot turn.Screw the tester into the sparkplug hole and slowly dial about 30 to 40 pounds of air while listening where it come out.More than that can be dangerous and is not necessary.Be sure isolate the breather hose from the carburetor,so you are not mistaken where the air is coming from.Here's some areas to listen and feel.

Muffler-leaky exhaust valve.
Carbuertor air horn-leaky intake valve
Breather hose-worn or damaged cylinder/rings
Head area-leaky head gasket or warped head

Hope tis helps
Old 12-10-07, 11:48 PM
cheese's Avatar
Forum Topic Moderator
Join Date: Jul 2001
Location: USA
Posts: 16,817
Most small engines nowadays have some sort of compression release. This allows air to escape through the exhaust on many of them with the piston at TDC. If it's running good, I wouldn't worry.

Just a note...even a brand new engine will leak air during a leakdown test. The issue is how fast the air is leaking, and where from.
Old 12-13-07, 02:54 AM
puey61's Avatar
Join Date: Apr 2004
Location: Upstate New York
Posts: 3,224
Furthermore, to properly remove the compression release from the equation, you need to place the piston 1/4" PAST TDC before you take any readings on the gauge. At TDC the MCR (Mechanical Compression Release) is still activated. In other words, the piston must actually be on the power stroke with the piston 1/4" down the stroke. Also, it is best to take readings with a warm engine so that all internal parts have expanded, through heat, to their proper tolerances.

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