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Unread 09-14-2013, 11:23 AM   #1
Neo1130
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Proper way to do compression test

What is the proper way to do a compression test on a 3.8 JK? When I changed my oil for the first time, I found a ton of VERY find metal shavings along with a couple large ones... About 1mm wide by 3-4mm long... I am hoping that this is from the threads of the oil filter or something like that, but I somehow doubt it. Oh and yes, the shavings were only present in the oil filter. Not so much the pan... So I was going to do a quick compression test while I changed the plugs today. I've got a slightly rough idle (probably the throttle body) as it dips down to around 550 RPM before it gets rough...

So, do I pull the fuse for the fuel pump only or the ECM?! I know a few of my previous vehicles needed very specific things done, which if not done correctly, could blow the ECU or ignition controls...

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Unread 09-14-2013, 12:13 PM   #2
RockyClymer
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I would pull the fuse on the fuel pump, connect the pressure guage to the plug hole and crank the engine over once or twice. My gauge has a needle that records the high reading so I do not even have a remote starter like the old setups had. You will probably get a code of some sort, but it would go away when reconnected. Metal pieces that large would concern me, but probably not related to rings/valves (compression) IMO.
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Unread 09-14-2013, 12:18 PM   #3
Neo1130
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Yeah, I was thinking rings or crank bearings. It was definitely a hard metal. Very hard to bend... I have heard that with the 3.8's if it gets low on oil, the cam bearings can shred away... But I want to do the compression test while I'm in there. I've done it on every one of my vehicles just to tell the condition of the motor. I don't burn any oil. Like literally NONE in the 2k miles I have put on the oil change... Even with the 5w-20...
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Unread 09-14-2013, 03:07 PM   #4
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Everything checked out good... Eventually if the motor blows, I'll just end up doing a hemi swap...
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Unread 09-14-2013, 06:25 PM   #5
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Here are the steps for a 3.8L compression test:

CYLINDER COMPRESSION PRESSURE TEST
The results of a cylinder compression pressure test can be utilized to diagnose several engine malfunctions.

Ensure the battery is completely charged and the engine starter motor is in good operating condition. Otherwise the indicated compression pressures may not be valid for diagnosis purposes.


1. Check engine oil level and add oil if necessary.

2. Drive the vehicle until engine reaches normal operating temperature. Select a route free from traffic and other forms of congestion, observe all traffic laws, and accelerate through the gears several times briskly.

3. Remove all spark plugs from engine. As spark plugs are being removed, check electrodes for abnormal firing indicators fouled, hot, oily, etc. Record cylinder number of spark plug for future reference.

4. Remove the Auto Shutdown (ASD) relay from the PDC. [fuse box]

5. Be sure throttle blade is fully open during the compression check.

6. Insert compression gauge adapter into the #1 spark plug hole in cylinder head.

7. Crank engine until maximum pressure is reached on gauge. Record this pressure as #1 cylinder pressure.

8. Repeat the previous step for all remaining cylinders.

9. Compression should not be less than 100 psi and not vary more than 25 percent from cylinder to cylinder.

10. If one or more cylinders have abnormally low compression pressures, repeat the compression test.

11. If the same cylinder or cylinders repeat an abnormally low reading on the second compression test, it could indicate the existence of a problem in the cylinder in question.
The recommended compression pressures are to be used only as a guide to diagnosing engine problems. An engine should not be disassembled to determine the cause of low compression unless some malfunction is present.
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Unread 09-14-2013, 07:13 PM   #6
73azbronco
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only 100 psi? What's the compression ratio on the 3.8? or 3.6 if anyone knows?
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Unread 09-14-2013, 07:34 PM   #7
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Quote:
Originally Posted by 73azbronco View Post
only 100 psi? What's the compression ratio on the 3.8? or 3.6 if anyone knows?
The 100 psi is only a guide line minimum. I think an actual good reading will be 125 or more.

According to both factory service manuals....

The 3.8L compression ratio is 9.6:1

The 3.6L compression ratio is 10.2:1
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Unread 09-14-2013, 11:46 PM   #8
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Quote:
Originally Posted by ronjenx View Post
Here are the steps for a 3.8L compression test:

CYLINDER COMPRESSION PRESSURE TEST

9. Compression should not be less than 100 psi and not vary more than 25 percent from cylinder to cylinder.
25 percent surprises me, 15 percent has always been the standard. If I had 150 psi on one cylinder and 112 psi on another I'd be awful nervous.
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Unread 09-15-2013, 04:56 AM   #9
ronjenx
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Quote:
Originally Posted by VEGASROBBI View Post
25 percent surprises me, 15 percent has always been the standard. If I had 150 psi on one cylinder and 112 psi on another I'd be awful nervous.
Seems like I remember seeing 10% quite often, years ago.
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Unread 09-15-2013, 05:22 AM   #10
michiganmaveric
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The 100PSI is "not less then" and not the standard PSI that you would get.
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Unread 09-16-2013, 11:17 AM   #11
73azbronco
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I would hope to see in the 165psi cold crank plus range for normal. At 100 I'd be putting in a hemi.
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