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post #1 of (permalink) Old 01-23-2001, 05:43 AM
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Low cylinder compression

I just got done putting together a spare parts 81 CJ7, 258 I6 999 trans. The engine was rebuilt approximately 10k miles before, but hasn't run in about 2 yr. It fired up with no problem and sounded perfect. After about an hour of idling and reving to fine tune and check for leaks, it started to missfire. Cylinders 3 and 4 are the culprits, plugs are firing, plenty of fuel. The lifters are makin allot of noise, compression is low at aprox 120psi on the 2 cylinders, all other cylinders are over 150 (don't remember exact numbers). Oil pressure seems to be good, even changed the oil just to be sure. A mechanic suggested I add a qt of trans fluid and run it 20 min to loosen any stuck valves or lifters. I think it may be a head gasket due to the low compresion in adjoining cylinders, Its not overheating, and dosen't appear to have coolant in the oil. I could use some suggestions before tearing into it.

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post #2 of (permalink) Old 01-23-2001, 09:02 AM
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Re: Low cylinder compression

120 psi doesn't sound too low. You may have a scored cylinder, but the stick valve syndrome sounds like a better bet. If you have a bad head gasket, you should see percolation (bubbles) in the radiator. Check this when the engine is cool, as you will need the radiator cap off.

If you feel funny about putting transmission fluid in the crank case, your local parts house should carry something called "top oil." This will do the same thing. I once had a sticky valve train, and poured top oil over the rockers while at idle. That did the trick.

Good luck!


Visit my 2000TJ at http://www.backcountrytrailers.com/T...ers/bcrigs.htm
post #3 of (permalink) Old 01-23-2001, 10:51 AM
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Re: Low cylinder compression

Since the lifters are loud they are probably too loose. Them being too tight would cause low compression though.
Maybe you could to some type of leak down test, see how long they are holding the 120, also check the 150 cyl's. If it is a bad head gskt they shouldn't hold press too long.
It sure sucks to have to tear apart an engine right after you build it! been there done that, don't want to do it again!

[img]/wwwthreads_images/icons/cool.gif[/img] Big Ed
<font color=red>'88 YJ, 4" susp,3" body,35's,283 Chevy V8,TH350,4.11's,D30,D35c</font color=red>
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post #4 of (permalink) Old 01-23-2001, 11:26 AM
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Re: Low cylinder compression

Good ideas! The whole problem is strange in that it happened immediatelly after a high rev. Lifters weren't making any noise before, then, supprise thur it was. What puzzles me is, if it was valve train related, what's the chances of neghboring cylinders wackin out at the same time?

post #5 of (permalink) Old 01-23-2001, 11:55 AM
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Re: Low cylinder compression

If it's a lifter problem, one or more may have "flattened." This assumes you have hydraulic lifters, not solid lifters that require tappet adjustments to keep the push rods from flopping around.

When hydraulic lifters flatten, they stick in the compressed position, and won't allow oil to "pump up" and fill the lifter cavity. You can spot this problem with a timing light. Point the light toward valve train (valve cover off) with the engine at idle. Look for the offending valve set. It will be obvious as good valve performance shows a nearly steady rocker. A flattened lifter will cause the rocker bounce erratically.

Typically, bad lifters can be pulled and replaced in no time at all. Your parts house can supply a lifter puller, which can snag the offending part and pop it right out.


Visit my 2000TJ at http://www.backcountrytrailers.com/T...ers/bcrigs.htm
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