mgs
[Top] [All Lists]

Re: Leak down test questions

To: wmgilroy@mtgbcs.mt.lucent.com (William M Gilroy)
Subject: Re: Leak down test questions
From: Barney Gaylord <barneymg@ntsource.com>
Date: Tue, 13 Oct 1998 01:52:45
At 10:58 PM 10/12/98 -0400, William M Gilroy wrote:
>.... about what could be causing the low compression ....
>.... leak-down test. I gave it a quick try tonight and I have some
questions. .... charge the cylinder with air .... listen at the dipstick
for worn rings, 

Easier to listen at the oil filler hole for that.

>.... So now I have the following questions.
>       
>       1.  How many PSI should I charge the cylinder to?  (I used ~40 PSI)

More than enough, I usually use about 20 PSI.  If you can hear it hissing
somewhere it's enough.

>       2.  How fast should the air leak out?

As slowly as possible, preferable not at all. <G> I don't think the speed
or the volume of the air leaking is important, just where it's coming from.
 If you have low compression you already know the leak is too much. 

>       3.  Will the volume of the cylinder affect this?

Nope.  The cylinder is a static pressure chamber.  once you have the
pressure supply connected, air flows in as fast as it flows out, no change
in pressure, no change in flow.

>If show how do I figure in the location of the piston in the cylinder?

The piston has to be at TDC.  At BDC one of the valves is open.  Anywhere
in between and the air pressure will turn the engine to BDC.  Just turn the
crankshaft until the timing mark lines up at TDC for testing #1 and #4
cylinders, and exactly 1/2 turn away from that for #2 and #3.  If the
cylinder does not hold the pressure at all, the piston is at TDC after the
exhaust stroke rather than the compression stroke.  In that case just turn
it one turn more and back to TDC for that cylinder to close the valves.

>I am going to try a leak-down test to see if I can identify the problems.
I will then pull the head and look at the head gasket.  If it is blown I
will just replace the head gasket and then run another compression check.
If the head gasket is blown how will I tell?  Will be obvious even to me :-)

If it has been leaking significantly between cylinders it will be blackened
from carbon deposits and the heat of the combustion gasses.  If it has been
leaking badly from a cylinder into the water jacket, you may see blackening
theree as well.  If it has been leaking a little from the water jacket into
a cylinder, you may see mineral deposits in the area, and maybe water stone
on the internal parts of the combustion chamber, especially on the valve
heads.  And yes, it will probably be obvious.

If you do find that the head gasket has been leaking, be sure to use a good
straight edge to check the flatness of the cylinder head and the top of the
engine block.  Any more than a couple of thousanths of an inch out of plane
means you should have the surface machined to make it flat again.  And
don't feel alone.  Lots of MG engines have low compression.

Good luck,

Barney Gaylord
1058 MGA with an attitude




<Prev in Thread] Current Thread [Next in Thread>