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Old 05-12-2022, 11:09 PM   #1
Join Date: Apr 2019
Posts: 72
Default Newbie Guide to Compression Check

Hi Everyone,

I have to change the spark plugs this weekend and bought a compression tester out of curiosity ..

When testing.. Do I take all the spark plugs out first then do each cylinder?
Is it safe to set the kill switch to off to stop any damage to the ignition system and also my safety?

I read a cylinder shouldnt go below 75% of your highest reading.. From reading online the figures don't really matter just that they are all around the same?

Any advice for a complete novice would be very much appreciated.


Proud owner of a Maxum 1800SR year 2001.
Engine: Mercruiser 4.3 LH 1996 carb V6 engine.
Outdrive: Alpha1 Gen2 with 1.8gear
Keen interest in anything electronic or gadget related!
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Old 05-13-2022, 12:40 AM   #2
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Join Date: Feb 2012
Location: Essex, Maryland
Posts: 9,301

Yes turn the kill switch to off to disable the ignition circuit. Yes remove all of the spark plugs. All cylinders should be within 10% of each other. Compression numbers around 125 psi is an indication of a worn engine, lower numbers are a sign itís time to consider repowering.
1997 2400 SCR
5.7 Vortec / Bravo 2

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