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Go Back > General Technical Discussions > Engines & Drivetrain > Erratic Compression

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Unread 06-28-2013, 11:25 AM   #1
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1981 CJ8 
Join Date: May 2013
Location: Spicewood, TX
Posts: 8
Erratic Compression

I have an '81 model CJ with the 4.2 I6 engine (bone stock). It has started to act as if the timing chain has jumped a notch. Loud clatter from the valve cover and rough idle, then it goes away and smooths out.

I checked compression and got 50 - 55 on all cylinders; however I left the gage hooked up while it was idling and noticed at one point the compression on that cylinder shot up to 120. Tried another cylinder and the same thing, it reads 55 to start, then the rough idle starts and it goes to 120.

Can a lose timing chain cause this low compression and erratic reading?

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