[Shop-talk] leak down test

[Eric J Russell]

More than a compression gauge - two gauges and a source of compressed air. 
One gauge is connected to the cylinder being tested via the spark plug hole 
to measure how well it holds pressure. The other gauge measures input air 
pressure. The engine is positioned at TDC on compression (valves closed) and 
the cylinder then pressurized (being careful as the engine could suddenly 
move). By comparing the input pressure vs the cylinder pressure one can 
compute how well the cylinder holds pressure. ex: if input pressure is 100 
psi and cylinder pressure is 95 psi then it is said the leak down is 5%. A 
really tight engine might measure 2%-3% leak down.

One of the main advantages of a leak down test is that with the cylinder 
thus pressurized one can listen at specific areas and pinpoint where the 
cylinder is leaking. Listen at the carb throat to check if the intake valve 
is leaking. Listen at the exhaust pipe for a leaky exhaust valve. Listen at 
an adjacent cylinder, in the cooling system, and around the outside of the 
block for a failed head gasket. Listen at the oil sump (via the oil fill 
usually) for leaky rings. Rings do typically leak a little bit so it is a 
judgment call to decide if they are OK or not.

Since I am frugal (OK, cheap...) I have made an eL-Cheapo version. No 
gauges, just a fitting that screws into the spark plug hole with an adapter 
for my air hose. Position the piston at TDC on compression, apply 80-100 psi 
(again being careful as the engine could suddenly move) then listen as 
described above. Usually for checking a used engine or diagnosing a problem 
I am less concerned about a specific leak down percentage and just want to 
know if/where there is a problem.

Eric Russell
Mebane, NC
http://home.mebtel.net/~ejrussell
----- Original Message ----- 
From: "john niolon" <jniolon at bham.rr.com>
To: "shop-talk" <shop-talk at autox.team.net>
Sent: Wednesday, August 24, 2011 7:55 AM
Subject: [Shop-talk] leak down test


> Hi guys... got a ch20 Kohler engine on my cub cadet... replaced the head
> gasket last year in August and I suspect it's bad again... it was 
> suggested by
> some Cub guys that  before I tear it apart I want to try a leak down test.
> exactly how do you do that ???   compression gauge ??