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Unread 04-16-2012, 06:36 AM   #1
warrior1670
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1986 CJ7 
 
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Low Compression - help!!! How to do the test???

Ok, so I got my new carb on, the motor runs great. 258 with new motorcraft 2150 no emissions crap. It was missing on #4, replaced the plugs last night, miss gone.

So, before that, the guy working on it said it had very low compression on #1 and #2, he didn't check the rest. It fires up great, runs, little sluggish, blew very little smoke. I'm not sure if this is from #4 missing or not. He did the compression test cold.

So, I did a cold compression test, between 58 and 61 psi on all cylinders. I know this is very low but should this thing be running with it that low? What's the correct procedure for a compression test? I pulled a plug, put the compression gauge in it, started it, cleared the gauge, it went to 60 in three pulses and sat there. If it opened the throttle, it went up dramatically over 100 but didn't think this was right.

So, what's the correct way to do it?

Secondly, is there a way to tell if it's the rings or valves? Since all cylinders are dramatically low, could it be something else?

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Unread 04-16-2012, 07:01 AM   #2
hotrod351
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ive never held the throttle open and have never, and i mean never, had any problem in getting a good reading. if you have a air compressor you can connect the hose to the compression tester and shoot air into the cylinder. if you hear air coming out of the valve cover where you put oil then its valves, if you hear it coming out the exhaust pipe then rings.
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Unread 04-16-2012, 07:11 AM   #3
h2ogun
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Pull all your plugs

Wire your carburetor wide open

For each cylinder , Crank your engine until you get about 3 compressions
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Unread 04-16-2012, 08:15 AM   #4
Centerfire
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The Manual

Compression Test Instructions
Page B-19

http://www.civilianjeep.info/84-86FSM/CJ-FSM-84to86.pdf


In addition;
There is also a "Wet" compression test. Squirt a couple teaspoons of oil into the cylinder and test the compression. If the compression came up on the wet test, your rings are probably bad. If it stayed the same its probably a valve.

Then use the compression leak down test to verify ...
compression-test.jpg  
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Unread 04-16-2012, 07:29 PM   #5
warrior1670
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Thanks for the help everyone. Got good news and bad. Good news, I got the test done dry and wet. More good news, it's not 60psi/cylinder. Bad news, still 100-110 psi. Eh better than 60 I guess. Thanks. Starting a new thread for opinions.
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Unread 04-16-2012, 08:07 PM   #6
Trapshooter
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Quote:
Originally Posted by warrior1670 View Post
Thanks for the help everyone. Got good news and bad. Good news, I got the test done dry and wet. More good news, it's not 60psi/cylinder. Bad news, still 100-110 psi. Eh better than 60 I guess. Thanks. Starting a new thread for opinions.
How many miles on the engine? How much oil is the engine consuming between changes? What kind of values where you hoping for?

Your results of 100-110 psi isn't bad for a high mileage engine. Consistency between cylinders is more important than high readings. This ensures the cylinders share an equal amount of work.
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Unread 04-16-2012, 09:36 PM   #7
BagusJeep
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The spec for a 258 is 120-150 psi with max 30psi variation between cylinders.

If you have an even 110psi, it is a little low but not catastrophic.

What did the dry and wet tests show? Is it valves or rings?
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Unread 04-17-2012, 06:01 AM   #8
warrior1670
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The dry check resulted in 100-110 across the board, did the wet check on #1 cylinder and it went to 150psi. Does that show if it's rings or valves? Everyone says the wet test shows it's rings. 130K on the engine.
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