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post #1 of (permalink) Old 05-29-2002, 08:43 PM
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what should the compression be on a 258 78 year jeep cj.
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post #2 of (permalink) Old 05-29-2002, 10:23 PM
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Re: compression

What it more important is the difference between cyls. I don't like to see more than about 5%.
post #3 of (permalink) Old 05-29-2002, 11:56 PM
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Re: compression

Lets see,

The correct answer would be 150 psi.

If your engine is old and tired,
Look for about 120 dry and about 135 after you shoot a little engine oil or 3-in-1 oil in the cylinder.
This is acceptable, but you are getting close to needing a rebuild.

(There are a lot of 258's running around out there every day with less than 120 psi. It isn't recommended, but they still keep plugging along!)

NEVER USE WD-40 or Penetrating oil!
Those are solvents and will attack your bearing surfaces and cause scratching and galling of the cylinder walls and piston rings.

You are looking for about 10% between cylinders at the rated compression.
That means 15 PSI between the highest and lowest gauge reading is within normal limits.

You can tell a lot more if you first do a dry compression test of all cylinders, one at a time, with all the other spark plugs in.

Then do a wet compression test, the same way, one at a time.

Then do the test again, but remove the plugs from the cylinders on either side of the cylinder you are testing.
This will give you a good idea if the head gasket is leaking between cylinders.
Don't forget to do your ignition a big favor and disconnect the coil connector before you start cranking the engine over or forget and leave the key on!
That is real hard on the coil and module!
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