compression specs?


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Old 02-22-2010, 05:07 PM   #1
jeeper435
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Default compression specs?

what is the cylinder compression specs(psi) for 4.0L I6?


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Old 02-22-2010, 11:33 PM   #2
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what is the cylinder compression specs(psi) for 4.0L I6?
120 psi to 150 psi, but maximum variation between cylinders is 30 psi.
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Old 02-23-2010, 05:19 PM   #3
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thanks 88fadedglory
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Old 02-23-2010, 10:31 PM   #4
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No poblem, glad to help.

Make sure the throttle body is wide open when cranking as you won't get a full reading if it's closed and use a remote starter switch for cranking. That way the engine won't start.
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Old 02-28-2010, 03:39 PM   #5
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i did a compression test today, here are specs:

cyl. #1 at 78 psi
cyl. #2 at 120 psi
cyl. #3 at 110 psi
cyl. #4 at 109 psi
cyl. #5 at 113 psi
cyl. #6 at 115 psi

my check engine light is on, code P0301(cylinder #1 misfire)
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Old 02-28-2010, 04:04 PM   #6
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Originally Posted by jeeper435 View Post
i did a compression test today, here are specs:

cyl. #1 at 78 psi
cyl. #2 at 120 psi
cyl. #3 at 110 psi
cyl. #4 at 109 psi
cyl. #5 at 113 psi
cyl. #6 at 115 psi

my check engine light is on, code P0301(cylinder #1 misfire)
#1 definitely has a problem. Could be bad valves, blown gasket or bad rings.

#3 & #4 both have low readings and are next to each other ... possibly a blown head gasket between them.

All are pretty low. You might get a old style squirt type oil can with the flexible spout and squirt a little oil into all the cylinders and crank the engine for about 30 seconds, then re-do the test. all readings should rise some but a cylinder with a bad valve will rise the least and a blown headgasket not much more than the bad valve.

A good cylinder won't change much either, oddly enough, but it won't be your #1 cylinder. Bad rings will jump a large amount as the oil will seal them.

Btw, you did pull all your spark plugs before the test didn't you? I don't think I mentioned that ... darned old-heimers got me again, I think.
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Old 02-28-2010, 06:19 PM   #7
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i didnt pull all the plugs when i tested. also yesterday when i started it up, blue smoke came out the exhaust. it doesnt do that every time i start it though just sometimes. why?
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Old 02-28-2010, 10:09 PM   #8
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Pulling the plugs and holding the throttle open, lets the engine spin a little faster and you get a better reading, so you test may not be as accurate as you need it to be.

Blue smoke is not a good sign. It means oil is being burned.

If the smoke is worse:

on acceleration: your rings are worn. Repair: rebuild engine

on deceleration: your intake valve seals are worn Repair: get a valve job done.

on cruise: the engine needs a rebuild as you have oil coming in from boith the valves and the rings.

Sorry I wasn't clear on the plugs needing to be pulled. Try the test again without the plugs (take a bit of masking tape and mark the wires, if needed) and then add the oil and try it.

I think we'll find out where the problems lies. How many miles on the engine?
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Old 03-01-2010, 06:05 PM   #9
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i've got 220k miles on the engine. i pulled the plugs and did what u said to do... i put oil in the cylinders and the compression jumped! smoke comes out on acceleration. so must be the rings. im probably jus gonna pull the motor and buy a reman. once again thanks for all your help 88fadedglory
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Old 03-01-2010, 10:32 PM   #10
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i've got 220k miles on the engine. i pulled the plugs and did what u said to do... i put oil in the cylinders and the compression jumped! smoke comes out on acceleration. so must be the rings. im probably jus gonna pull the motor and buy a reman. once again thanks for all your help 88fadedglory
Glad to be able to help.

That's what the forum is all about, helping each other ... between teasings, of course.
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Old 03-02-2010, 11:13 AM   #11
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I finished replacing my engine yesterday, I baught a remanufactured long block 4.0 from s&s motors it was timed they put a bigger cam in it and ported and polished head for $1900 delivered to the house with a lift truck the instal went smoothly had a problem with external stuff. ensure to take into account prices for distributor, ignition coil, flywheel....etc. trust me you will need new. Any questions just post I also have pics of the install if you would like to see. 1992 JEEP CHEROKEE ENGINE
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Old 03-02-2010, 11:45 AM   #12
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Originally Posted by stbiblecg View Post
I finished replacing my engine yesterday, I baught a remanufactured long block 4.0 from s&s motors it was timed they put a bigger cam in it and ported and polished head for $1900 delivered to the house with a lift truck the instal went smoothly had a problem with external stuff. ensure to take into account prices for distributor, ignition coil, flywheel....etc. trust me you will need new. Any questions just post I also have pics of the install if you would like to see. 1992 JEEP CHEROKEE ENGINE
Excellent info! Thank you and welcome to the forum!
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Old 03-02-2010, 12:08 PM   #13
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Your welcome and thanks
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Old 03-02-2010, 08:47 PM   #14
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i wanted to buy a reman. engine but now im thinking of rebuilding the engine my self. which brings me to a question: with bad rings, would the cylinder walls be damaged?
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Old 03-02-2010, 11:22 PM   #15
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i wanted to buy a reman. engine but now im thinking of rebuilding the engine my self. which brings me to a question: with bad rings, would the cylinder walls be damaged?
Possibly, but it depends on the composition of the iron in the engine block.

I know older Mopar and AMC engines were famously long wearing because of a higher-than-all-the-others nickle content in the cast iron, but i'm not sure if the 4.0 engines are the same material.

First, take a flashlight and check for rough spots, scratches and scrapes in the cylinder walls. If you find any and a finger nail catches on them, you need to get a rebore done.

You'll need to buy or rent a set of inside micrometers so you can measure the bores properly. You'll need to measure lengthwise to the engine at the top just below the mark left by the top compression ring and at the bottom of the area polished by the rings, then again crosswise at the same heights. Do this in all cylinders as the wear might be different in each cylinder.

If you have more than 0.010" diference in either pair of measurements, you'll need the block bored which will mean over size rings and pistons to match the overbore amount.

If it's under 0.005", just hone the cylinders and put on new rings.

While your at it, you might as well get a set of outside mikes and check your main and rod journals on the crank.

If the journal surface is smooth and you can't sang a finger nails in a groove and it mikes to less than 0.005" wear, just put in new stock size bearings, over 0.005' and have the journals all turned the same amount undersize, get matching bearings and you're good to go.


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