Weird Compression Readings - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 8 Old 10-07-2017, 02:44 PM Thread Starter
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Weird Compression Readings

Folks. I just rebuilt and installed my sons 2008 3.8l engine. After it wouldn't start I ran a compression check. Numbers 1, 2,and 3 have no compression while numbers 4, 5, and 6 have a rock solid 110 psi. Given the heads are good, the pistons are new it points to cam timing. I set the cam with the #1 cylinder (passenger side front piston) on TDC. This positioned the crank gear mark on top. I aligned the cam mark on the bottom center with the arrow facing down pointing to the crank gear mark. Any help or ideas appreciated.
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post #2 of 8 Old 10-07-2017, 06:11 PM
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There is more than one mark on the cam and crank.
In addition to the marks that face each other, as you describe, each has a dot that needs to line up with a chain link that is a different color from the rest.
There will be 8 regular links between the different ones.
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post #3 of 8 Old 10-07-2017, 08:50 PM Thread Starter
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I saw the blue colored links but didn't do anything about lining up as shown in your thumbnail. Help me learn from my error, if the two main marks are aligned, what does lining up the colored links change from a valve timing perspective?
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post #4 of 8 Old 10-07-2017, 10:25 PM
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Lining up the dots with the colored links is more precise than eyeballing the alignment of the other two marks.
Lining up the two marks that face each other gets the cam timing close. Having the two dots lined up with the colored links ensures the timing is right on.

Have you had the valve covers off to see what the valves are doing as each cylinder comes up on compression TDC?

A cylinder leak-down test will show you where the leaks are; intake valves, exhaust valves, rings.

Another thing to consider... an intake valve that fails to open will result in a zero compression reading.

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Last edited by ronjenx; 10-08-2017 at 12:38 AM.
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post #5 of 8 Old 10-08-2017, 05:58 AM Thread Starter
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Thanks, I think I'll try the valve covers and leak down first, much easier to get to then the timing chain.
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post #6 of 8 Old 10-08-2017, 06:11 AM Thread Starter
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Just a thought. The engine suffered a sudden stop when it turned off the #1 rod bearing. Prior to that, it was running fine so I assumed the heads were good. A visual inspection didn't show any signs of piston contact nor obviously bent valves. If the cam is timed to allow cylinders 4,5&6 to create 150 psi of compression, logic says the cam would be equally timed for cylinders 1,2&3. I added oil to #1&2 to no effect. All of this points to valves not cam timing.
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post #7 of 8 Old 10-09-2017, 10:47 AM Thread Starter
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The cam timing was bad. Set it per your instructions. The engine started yet seemed to have too much timing. Backfired through the intake when the engine died. It would run with throttle applied but wouldn't idle. The check engine light lit which was a crank sensor not plugged in. Cleared code. Now it won't start with back firing through the intake.
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post #8 of 8 Old 10-09-2017, 07:21 PM
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After you re-timed the cam, did you perform another compression check?
The 3.8L is an interference engine. Maybe the cam timing was out far enough to cause piston/valve contact.

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