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Unread 10-08-2009, 11:18 AM   #1
sterfry2004
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Quick Compression Check Question

I am putting a new head on a 1997 4.0L Jeep. I was told I should do a compression check to make sure that all of the pistons and block are still OK since I'm pretty inexperienced. How much do I HAVE to put back in to do this test? Do the push rods, rocker cover, manifolds, serp belt, etc. need to be completely reinstalled?

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Unread 10-08-2009, 11:30 AM   #2
justanotherjpr
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Push rods, rocker arms, head should be sufficent. No need for intake/exhaust manifold or serpentine belt or any accessories except the starter, which I don't see being removed for a head job anyway. I would set the valve cover on top of the head with a bolt or two - just to keep the valve train from flinging oil everywhere. You would have to properly install the head for your compression check to be accurate. You might be able to do this using the old gasket if it is not destroyed.
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Unread 10-08-2009, 11:30 AM   #3
Wiskey
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Push rods, valves, and starter have to be in for sure,.. I would put the valve cover on, because if you don't oil will go everywhere.

You can leave the manifolds, and serp belt off.

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Unread 10-08-2009, 04:00 PM   #4
sterfry2004
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Thanks for the tips guys. Guess I'll find out how good of a mechanic I am tonight! Crossing my fingers...
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Unread 10-09-2009, 02:34 AM   #5
Wiskey
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Here's to good luck for you!

How did it go?

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Unread 10-09-2009, 10:23 AM   #6
sterfry2004
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Well, I didn't get it done last night because I ran into some mystery components in the head gasket set. I don't remember taking any of these things off... Any idea what they are??
http://www.flickr.com/photos/jminter...7622363505147/
http://www.flickr.com/photos/jminter...7622363505147/
http://www.flickr.com/photos/jminter...7622363505147/
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Last edited by sterfry2004; 10-09-2009 at 12:50 PM..
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Unread 10-09-2009, 12:51 PM   #7
sterfry2004
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Oops, pictures didn't work the first time!
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Unread 10-09-2009, 08:33 PM   #8
Wiskey
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Was there just one of each of those? Or multiples?

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Unread 10-09-2009, 11:17 PM   #9
sterfry2004
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Quote:
Originally Posted by Wiskey View Post
Was there just one of each of those? Or multiples?

Whiskey
Number of each item from pictures:
1 - 5 hole gasket - I was able to figure out from a catalog that this was an EGR valve gasket.
1 - 3 hole gasket (Maybe for the EGR again?)
12 - small cylinder thing with coil

Compression Test results:
Every cylinder was above 150, except for cylinder 3, which held at about 90 psi. I heard that you should add oil through the sparkplug hole and retest, but I didn't have time to try that tonight. What would cause bad compression on only one cylinder?
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Unread 10-10-2009, 02:32 AM   #10
Wiskey
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The 12 small cyinder thing with the holes are very likely valve guide gaskets,.. if you took the valve spring off, and removed the valve, you could pop this off and replace it.

If your not burning oil when you start the motor after sitting for a day or two,.. and especially if you already have the head back on, I wouldn't worry about it.

Ummm,.. something's wrong. The cylinders should be within 10-15% of each other,.. so either the gasket is not seated right on cylinder 3, or the head is cracked/warped,.. or the block is damaged like my cylinder #3 was. Was there any visible scoring on the cylinder wall? Was there any damage to the piston? Did the valves seat properly? Did you give it a chance to pump the valve lifters up, did you give them a chance to bleed down? Was the compression gauge in securely? What is the history on this head?

Try it again tomorrow without the oil,.. then try your oil idea and report back,.. if the compression goes way up with the oil then we know it's low for a ring related issue,.. if it stays the same we know it's top end some where (Valve, head gasket, head)

Sorry it didn't come out perfect first try,.. but I'm sure you can get it worked out
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Unread 10-10-2009, 02:33 AM   #11
Wiskey
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BTW,.. here is a picture of Cyl# 3 on my jeep,.. This is badly scored, and you can see the piston is damaged,..



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Unread 10-12-2009, 07:01 PM   #12
sterfry2004
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Quote:
Originally Posted by Wiskey View Post
Ummm,.. something's wrong. The cylinders should be within 10-15% of each other,.. so either the gasket is not seated right on cylinder 3, or the head is cracked/warped,.. or the block is damaged like my cylinder #3 was. Was there any visible scoring on the cylinder wall? Was there any damage to the piston? Did the valves seat properly? Did you give it a chance to pump the valve lifters up, did you give them a chance to bleed down? Was the compression gauge in securely? What is the history on this head?

Try it again tomorrow without the oil,.. then try your oil idea and report back,.. if the compression goes way up with the oil then we know it's low for a ring related issue,.. if it stays the same we know it's top end some where (Valve, head gasket, head)

Sorry it didn't come out perfect first try,.. but I'm sure you can get it worked out
Whiskey
The head is brand new and came with the valves already seated. I didn't see any scoring on the cylinder wall, but that doesn't mean there wasn't any. The compression gauge was in pretty snug, and I did the test a couple times to be sure of that.

On the dry test, cylinder three compressed to about 100 psi, so a little better than the 90 before. After I added the oil, the compression shot up over 300 psi... I don't have time tonight, but I'll be taking the head off and taking a look at that cylinder. Is it safe to assume that the piston will need to be removed? From what I understand, the culprit is probably the rings, a damaged cylinder wall, or damaged piston. I'm guess rings since I didn't see any damage before I replaced the head. Thanks for all your help through this!!
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Unread 10-13-2009, 01:11 AM   #13
Wiskey
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It jumped from 90, to 300 with the oil?

How much oil did you put in? It's only supposed to be a very small amount, measured in drops,.. putting a couple ounces in for example would really throw off the test because air compresses, and oil does not.

I ask this question because that is a Huge jump,.. Adding a little oil usually makes a cylinder with bad rings about match the others,.. the fact that it doubled the other cylinders makes me wonder

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Unread 10-13-2009, 09:12 AM   #14
justanotherjpr
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Definitely - how much oil did you put into that cylinder? Best to pull the gauge or spark plug and crank it a few times to clear it, then add a few drops and try it again. You can run on 90psi, it just won't run as well as it could.
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Unread 10-13-2009, 09:11 PM   #15
sterfry2004
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Yeah, I figured I must have done something wrong. I'll clear it and try it again.

Quote:
Originally Posted by justanotherjpr View Post
You can run on 90psi, it just won't run as well as it could.
If one cylinder is running on 90, is that bad for the engine? If the repair will need to be done eventually, I might as well do it now, but if it's not a huge deal, I'm not opposed to just leaving it.
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