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Leak Down Test

To: "Shop Talk" <>
Subject: Leak Down Test
From: "Simon Young & Heidi Millton-Young" <>
Date: Thu, 9 Jan 1997 20:30:05 +1100
Sorry I lost the original post.
        I made up a tester which connects to the screw in adapters of my 
compression tester.  By setting the line pressure to 100 psi then connecting 
the tester the resultant reading will tell you the leak-down as a percentage 
(ie 100 psi-new reading psi = % leak).
        All my testing was on turbo charged race engines, we didn't have a hard 
and fast figure for what is bad but good readings were below 10%.  If we had a 
large variation or a series of readings above 20% we pulled the motor.  This is 
probably excessive for a road car, 40 or 50 will probably still be fine as long 
as there isn't a large variation, depends how picky you want to be. 
        If you have one bad leak don't panic, first make sure the piston really 
is at TDC, wild cams can make it easy to  leave a valve just a little open.  If 
you are feeling game give the valve spring retainers a tap to crack and reseat 
the valve, sometimes debris will stick in the seat.  Still bad? You will 
probably be hearing a hiss, if it seems to be from the :
                carb you probably have inlet valve problems,
                 the tailpipe exhaust valve problems or
                 the oil filler or crankcase breather, ring problems or in an 
extreme case a holed piston (I hate it when that happens)

        Test at TDC for the cylinder in question to avoid the previously 
mentioned turnover problem.

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