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Unread 07-14-2007, 10:01 PM   #1
willyb8
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buying a cj need advice

is it a good idea to do a compression test when checking out a CJ. i've never done one so how hard is it. does it take long? can someone please explain how to do one and what numbers should i look for with a 83 cj with a 4.2L with a GM TBI. i did a search and only got people talking about the rresults of one


thanks

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Unread 07-15-2007, 02:05 AM   #2
CaliCJ5
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Have a mechanic do it. About $100, but worth every penny.
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Unread 07-15-2007, 07:53 AM   #3
willyb8
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really, i've heard its not that hard. the tester is only like $25.
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Unread 07-15-2007, 08:29 AM   #4
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It's certainly a good idea to test out the engine. $100 seems high to me, but I've never had one done. There are several step by steps on how to do it, and if you have the tester, shouldn't take all that long. Google Engine Compression Test and you will get lots of info. I can't speak for what your compression numbers should be, but I know they should all be realtively close to each other. If you have one significantly lower, then you have problems with that cylinder.
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Unread 07-15-2007, 04:35 PM   #5
cavedweller
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It's not hard. I have a V8 and it only took like 20 minutes or so. Having another person makes it easier, then one person can crank the engine while the other checks the reading (you can count the revolutions that way).
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Unread 07-15-2007, 04:42 PM   #6
cavedweller
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BTW, I just checked my '79 service manual and a new 4.2 6 cyl. should have 140psi (keep in mind this is what a new engine should be). Also, according to the specs there shouldn't be more than a 20 psi difference between cyls. Hope this helps!
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Unread 07-15-2007, 04:49 PM   #7
willyb8
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Quote:
Originally Posted by cavedweller
BTW, I just checked my '79 service manual and a new 4.2 6 cyl. should have 140psi (keep in mind this is what a new engine should be). Also, according to the specs there shouldn't be more than a 20 psi difference between cyls. Hope this helps!

thanks that at least gives me a starting point.
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Unread 07-16-2007, 05:35 PM   #8
willyb8
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is it too much to ask to do a compression test when looking at it??
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Unread 07-16-2007, 05:42 PM   #9
Bard
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I usually just ask them to fire it up, bring a stethoscope and wrap it around your neck. That will make them think. My 258 runs 125 across all 6, it just needs to be close by ten. A $100 to do a test?? I am sure you can do it for much less. Do you need a motor or a Jeep?
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