whats a leak down test & hows it performed?

[fireguy12117]

MAKG- this has your name all over it

i always see you mentioning this, as well as others. what is a leak down test exactly, and how does one carry this test out? i see it mentioned many times as a tool to diagnose engine internals a lot, and id like to have this 'tool' in my aresonal! Thanks!

 

[almostclueless]

If I can help....


A dual guage device attached to an air compressor is connected thru the spark plug hole. The piston is brought to top dead center on the firing cycle.


One guage is before a regulator in the tester, one after. The after guage reads lower because of leakage which happens in all cylinders.


If hissing is heard in the intake, it's probably an intake valve.


Hissing in the exhaust, exhaust valve.


Hissing in the oil filler opening, rings, maybe a severe head gasket.


Bubble in the cooling system, crack in head/block/head gasket.

 

[MAKG]

You can do most of that with just a spark plug/air hose fitting. Piston rings naturally have some leakdown, but all the others are faulty if there is any leakdown at all.

And it's not a regulator. It's a fixed orifice.

 

[fireguy12117]

i didnt mean to single out Michael as the only person to reply, i just seem to see you mentioning it a lot- so thanks for the post almostclueless!

so there is some hardware/tool involved, but essentially your putting compressed air into the cylinder and depending where the piston is, you can infer certain things.

 

[shadetree]

Here is one of the correct ways of doing the test.

http://www.tmossporting.com/tabid/337/Default.aspx

shady

 

[Will]

A compression test is good enough. In my new Kubota service manual, it mentions a compression test with no mention of a leakdown test. If it passes a compression test, there's no need to bother with a leakdown test. If it fails a compression test, a leakdown test is an excellent way to figure out why it failed the compression test. In fact, the tool you need is already in your compression tester. Just pop the gauge off and plug in the airhose.

Engines are simple. Once it is screwing up badly, if it is a leaky cylinder it's going to be really leaky. If it passes a compression test, it ain't leaky--find another cause.

 

[KELLY88]

Here is one of the correct ways of doing the test.

http://www.tmossporting.com/tabid/337/Default.aspx

shady

Good link shady. I've been wondering about this question for a while also.

 

[shadetree]

A compression test is good enough. In my new Kubota service manual, it mentions a compression test with no mention of a leakdown test. If it passes a compression test, there's no need to bother with a leakdown test. If it fails a compression test, a leakdown test is an excellent way to figure out why it failed the compression test. In fact, the tool you need is already in your compression tester. Just pop the gauge off and plug in the airhose.

Engines are simple. Once it is screwing up badly, if it is a leaky cylinder it's going to be really leaky. If it passes a compression test, it ain't leaky--find another cause.

Absolutely correct. No need of complicating an uncomplicated problem. shady

 

[fireguy12117]

thanks for all the input! ive been busy on re-sodding the yard so i havent gotten much time on here lately, but i will read up on everything. i think i have a compression test kit in the garage so im probably more better equiped to perform this test than i thought.