Compression test - how to

[Mark Jones]

Can someone please explain to me the proper method for performing a compression test? Hot/cold, plugs in/out, throttle open/closed?

Also what is the difference between a dry and wet compression tests?

Thanks.

 

[aeronca65t]

Plugs out and throttle wide open.
Generally, the engine should be warmed up to get max readings.
Crank the engine over until the needle won't go any higher (this should take no more than 5 or 6 crank-turns on most cars).

Note: Here's something I do which is a bit more work.
I crank the engine as above but cold and record the readings.
Then I crank the engine after it is warmed up and repeat the compression test.
I have found that if the rings are bad, there will be a significant change in PSI from hot to cold.
If measured PSI is lower than spec and not too much different from hot to cold, the problem is valves, cam lobes, cam timing, head gasket or cracked head/block/piston.

Be sure valves are adjusted prior to checking compression. An alternate idea which can provide more info is a "leak-down" test, but many of us may not have the equipment to do this.

 

[AltaKnight]

As Mark said above, make sure you jam the throttle wide open, it makes a big difference.
You can chart the readings dry then rechart them wet with a teaspoon of oil injected thru the plug hole.
The readings will improve with the oil, if any cylinder comes up a way more than the dry reading then you've probably located some bad rings that are getting "sealed by the oil.
Make sure your battery is in good shape for all this, if it's grinding slowly then the readings will be bad.
If you get the numbers all in a chart format you can see where the problem cylinders are and make a good assessment on the cause (and how much it will cost to fix)

 

[piman]

Hello all,

err?, plugs in surely?, as there will be no compression without them. (excepting, of course the cylinder into which the compression gauge is connected)

Alec

 

[Kurtis]

[ QUOTE ]
Hello all,

err?, plugs in surely?, as there will be no compression without them. (excepting, of course the cylinder into which the compression gauge is connected)

Alec

[/ QUOTE ]

That (the cylinder you're testing) is the only one that needs compression. Relieving the compression on the others by removing the plugs will make the starter's chore a little easier.

 

[piman]

Hello Kurtis,

of course, a touch of brain fade there. I've been working nights, that's my excuse.

Alec