How does my compression test rate ? - JeepForum.com
 
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post #1 of 10 Old 10-30-2013, 11:12 AM Thread Starter
krazybob
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How does my compression test rate ?

I have a '88 YJ with the 4.2 engine which has about 165K miles. I did a compression test the other day, and all of the cylinders were right at 130#. I did the check with all the plugs out and engine was cold. I'm wondering if the compression is in the bad, good, or other range? I don't know what a reading should be with an engine with this many miles should be.


Krazybob,, Mtn. Home, Idaho
My goal, is to die young,,
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post #2 of 10 Old 10-30-2013, 11:14 AM
bpounds
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I think 130# is very good. And if they were all close to that, that is even better.

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post #3 of 10 Old 10-30-2013, 11:53 AM
Diode Dynamics
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As long as it is consistent in all cylinders and your getting those numbers you should be ok!

This video is very helpful:

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post #4 of 10 Old 10-30-2013, 11:58 AM
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Quote:
Originally Posted by krazybob View Post
I have a '88 YJ with the 4.2 engine which has about 165K miles. I did a compression test the other day, and all of the cylinders were right at 130#. I did the check with all the plugs out and engine was cold. I'm wondering if the compression is in the bad, good, or other range? I don't know what a reading should be with an engine with this many miles should be.
I would be happy if all of the cylinders in my motor were all like that. The biggest thing to be concerned with is consistency. The numbers can all vary by up to 10%, anything more and the motor will need to be rebuild, re-ringed, re-gasketed, etc, what ever it took to bring them all back into range.

Robert.
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post #5 of 10 Old 10-30-2013, 12:42 PM
Siva283
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Those numbers are great. Especially cold. The should raise a little when warm. From the factory they IIRC only 150 psi.

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post #6 of 10 Old 10-30-2013, 08:38 PM
c5wagner
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That's really good, mine are at 165 +or- 5psi but I'm rebuilt with .030 bored over stock

1989 Base YJ TBI 350, 4l60, Np231 SYE/6 pin planetary/Wide chain, D30 ARB, 8.8 ARB 4.56
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post #7 of 10 Old 10-30-2013, 09:15 PM
Old4X
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Good numbers.

Check timing chain slack, replace gears and chain if over 6 degrees.

If you are feeling industrious, drop the pan and plastiguage the rod bearings. You want 2 1/2 to 3 thousandths clearance there (IIRC). The factory maint manual details inspecting the bearings and field clearance specs.

Properly treated, your engine is not at the half way point in it's life yet.

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post #8 of 10 Old 10-31-2013, 08:47 AM
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Nick,
Thanks so much for that video! That guy is awesome and made it really easy to understand both how and why.

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post #9 of 10 Old 10-31-2013, 09:23 AM
Diode Dynamics
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I'm always trying to learn more myself, having more learning resources never hurts!

Plus I like that guys laid back attitude lol

Nick C.


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post #10 of 10 Old 10-31-2013, 03:49 PM Thread Starter
krazybob
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Quote:
Originally Posted by krazybob View Post
I have a '88 YJ with the 4.2 engine which has about 165K miles. I did a compression test the other day, and all of the cylinders were right at 130#. I did the check with all the plugs out and engine was cold. I'm wondering if the compression is in the bad, good, or other range? I don't know what a reading should be with an engine with this many miles should be.
Thanks a ton for the replies ! It always helps to go to the forum for info on the questions I have Now I won't have to wonder how my little 6 banger is doing

Krazybob,, Mtn. Home, Idaho
My goal, is to die young,,
at a very old age
1988 YJ 4.2 5sp
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