about a compression test...

Barista · 10-07-2007, 10:28 PM

I was planning on running a compression test on my 68' Mustang, and I was wondering, does it make a difference if i pull all of the plugs and then test one at a time, or should i just pull one at a time and test as i go... Is there a difference at all?

curtis73 · 10-08-2007, 04:34 AM

Always test with all plugs out. Take them all out, screw the gauge into one and test. Move it to the next one and test.

There is a big difference. The starter would be fighting the compression of the other cylinders and giving you a weak reading.

Barista · 10-08-2007, 09:48 AM

Ah ok, that sounds about right. Thanks a lot!

534BC · 10-08-2007, 10:36 AM

don't forget to open throttle (at least slightly)

Barista · 10-08-2007, 11:18 AM

why is that?

UncleBob · 10-08-2007, 04:32 PM

as long as you test all the holes in the same manner, it doesn't matter. You're mainly concerned about the % difference between holes. all the other tricks are to get higher compression psi than you would otherwise.

If one hole is down 50psi, it'll show up no matter how you test it

bobss396 · 10-12-2007, 01:06 PM

The throttle being open is an insurance policy that the engine will turn more freely. I go by the 10% difference rule between the highest and lowest readings. If you get one cylinder reading low, squirt in some oil into the spark plug hole, an ounce or so will do it. Then run the test again. if the number comes up, you ruled out a valve problem and probably have a busted ring or piston land.

Bob

534BC · 10-12-2007, 03:40 PM

Opening the throttle ensures that any/each intake valve can get a slug of "atmospheric pressure" equally regardles of how/when the intake is designed or what the cylinder firing order is. Opening just a touch above idle (screwdriver in throttle) will guaruntee zero vac at cranking speeds.

Most compression tests on identical engine can vary mostly because of cranking speed and number of hits. These are the to largest variables I've found on any given set-up.

Torch · 10-13-2007, 02:04 AM

Make sure your battery is fully charged too, low battery voltage will give low PSI readings.

maxwedge · 10-13-2007, 03:34 PM

Cylinder leak down test is a more effective diagnostic tool as you can pinpoint a loss of cyl pressure thru the intake, exhaust or crankcase.

534BC · 10-13-2007, 05:57 PM

Or out the radiator.

maxwedge · 10-13-2007, 07:01 PM

Correct.

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