Compression Loss -- Cause

Jukka Majanen jiipm at
Tue Dec 12 22:20:38 EST 2000

Hi Again Shawn

Some comments among Your listing.

>I was thinking about the possible causes of lower compression readings in a
>cylinder and came up with the following.  I will list from what I guess to
>be the most to least common.  I would love to know what other causes there
>might be or what changes should be made to the order.
>1. Worn Rings
Or cut/coked

>2. Worn Valve guides
Only if they stuck the valve open.

>3. Worn Cylinder Walls
Yup, but usually it´s the piston.
Inject oil to the cylindert through the plug hole, 
if compression raises, fault should be #1 or #3

>4. Worn Valves
Or burned, or with wrong cap

>5. Leaking Head gasket
Not in the compression easily, other symptoms

>6. Worn Valve Seats
Yup, or wrong valve (cam belt) timing

>7. Cracked Head
Head cracks are usually so small, you don´t find
them by compression or cyl by-pass metering,
and -I don´t mean fully blown head

>8. Cracked Block
Oh, this belongs to the series: bent connecting-rod,
or hole on the piston top...

Well, You talk about readings. I guess You mean the situation, 
when You´re metering the compression by cranking the car with 
the starter motor. Then You can forget the glogged exh-line, 
loosen plugs, or bad turbo etc...

Low values can be then also gaught:

-by too low cranking speed, take all the plugs off and make sure it cranks well
-by keeping the throttle closed while cranking, it should be fully opened 
-You left the hood open and the compression did run away ;>)

My best

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