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post #1 of (permalink) Old 11-24-2002, 12:31 AM Thread Starter
 
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cyl. pressure test, what to results mean? CJ

hey all, before I start breaking down this engine, I wanted to check the psi in each cyl. the result are the following:
#1=108psi
#2=108psi
#3=108psi
#4=125psi
#5=125psi
#6=115psi

Now this is a 258 engine with about 60k miles on it. all the plugs looked slight brown, no oil on them. however, I do have some oil in the air cleaner. it runs decent, needs work on vacume lines. but anyways,
What do these numbers tell me if anything about the necessity of an engine rebuild?
thanks
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post #2 of (permalink) Old 11-24-2002, 05:36 AM
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Re: cyl. pressure test, what to results mean? CJ

Now do the test again, putting a teaspoon of oil in each cylinder just before testing it. That's how to tell whether it's rings or valves leaking.
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post #3 of (permalink) Old 11-24-2002, 08:57 AM
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Re: cyl. pressure test, what to results mean? CJ

I wonder if your results are accurate on such a new engine?

To get accurate meaningful results from a compression test
you have to do several things first.

Make sure your battery and starter are in good shape - cranking
speed does affect your readings.

1. Remove ALL spark plugs so the engine turns freely.

2. Block the throttle open so air can get in the intake.

3. When spinning it over, you can feel the pulses, do each
cylinnder the same number of pulses - usually 3 or 4.

Watch the gauge as you spin it - Somewhere around 80% of the final reading should happen on
the first pulse.
The second pulse should come very close to the final
reading.
- That's on a fairly new engine.
- An older worn one may build up a little slower but should
still reach max in about 5 pulses.

Then, after you have good readings but you have some low ones,
then do the "with oil" test:

If you are not 100% sure of your reading, do it again, and
even again 'till you are.

One cylinder at a time - the low ones - squirt a shot of oil
in the chamber - 30 wt preferably. Just one or 2 shots, not a
whole bunch. (Oil takes up space, too much will artificially
raise the compression leading you astray.)
Do not squirt oil in the other cylinders until it's their
turn to be tested.

Spin the engine over 5 or 6 times to distribute the oil around
the rings.
Do the compression test on that cylinder, using the same number
of pulses as you did before, throttle open etc.

Now do the next one. Do it one cylinder at a time.

Doing it that way will give you meaningful results.

There's been lots of engines "rebuilt" or worked on when the
only problem was the test wasn't done right.

And - if you have been having a misfire for any reason, like
defective ignition or mixture problems, even the above procedure
is questionable.
Reason being the misfire washes the natural amount of oil off
the rings, lowering sealing, causing your first test to be
inaccurate. Again, lots of engines have been rebuilt due to a
bad plug wire.

The cheapie "push in and hope it holds" type compression
gauges are really difficult to use, hard to hold in, hard to
watch while cranking, and the leakage around the rubber can
mislead you.
Best kind is a screw-in type with a hose.

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post #4 of (permalink) Old 11-24-2002, 09:07 AM
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Re: cyl. pressure test, what to results mean? CJ

By the way - your readings -- 3 front low ones in a row.
And - you said vacuum lines need work.

Is it possible the front 3 are running leaner than the rear 3,
maybe to the point of some lean misfire? Due to a vacuum leak
in front of the carb? Lean misfire leaves a little fuel in,
washing oil off the rings, affecting compression readings.

Or - the leak is to the rear of the carb - the carb's been
richened so it idles right, making the rear cylinders run OK,
but putting the fronts in a rich condition - again washing
sealing oil off.

Fix those vacuum leaks before drawing serious expensive
conclusions.

As my grandmother used to say:
"If it ain't broke, Don't fix it."

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post #5 of (permalink) Old 11-24-2002, 12:04 PM Thread Starter
 
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Re: cyl. pressure test, what to results mean? CJ

Thanks for the info, heres the new readings with all the plugs out(didn't really even think of that before)
cyl normal after oil
1 125 125
2 125 125
3 125 135
4 127 145
5 140 150
6 125 145

checked it dry twice, checked it oiled twice. the one that seems strange is #5 being about 15 psi above all dry. So, what does this info tell you?
You said that its a new engine with 60k miles, It is orginal never been major worked on (to anyones knowledge) but it had alot of idle time, it used to be a Lifegaurd jeep and I don't think the were super concernd about care.

I am doing a frame swap with this jeep, so the engine is being pulled and Iwas going to rebuild it and clean it (oil leaks no one ever took care of) so I was figureing on replaceing at a min, rings bearings cam and lifters, but was wondering about rebuilding the head with new springs and valves.

thanks for your help with these results
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post #6 of (permalink) Old 11-24-2002, 05:37 PM
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Re: cyl. pressure test, what to results mean? CJ

See how it straightened out?

Looks good to me. Possibly a little carbon in the rear ones,
but nothing to worry about. (Probably because of the vacuum
leak that had been "adjusted out."

I'd use it as it is.

Yes, 60,000 is barely broken in.

Today's engines, as long as you don't overheat them, and you
change the oil properly, can go 200,000 + no problem.

Most of my 8 vehicles have way over 100,000 miles and are great,
one even has close to 300,000 and has never been touched.

Fix the oil leaks and vacuum lines while it's easy to get to.
Make sure the PCV system is free too, not plugged. A plugged
PCV will pressurize the crankcase slightly, causing oil leaks
everywhere.

I certainly wouldn't be afraid of it as long as the oil pressure
is normal.
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post #7 of (permalink) Old 11-25-2002, 12:26 PM Thread Starter
 
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Re: cyl. pressure test, what to results mean? CJ

I was planning on running all new vac lines and rerouting them so the are alittle more out of the way and orginized.

I will replace the oil pan gasket and the rear main seal while I have everything out, should I leave well enough alone and not change bearings and rings? what about cleaning the carbon out of the cylc?
Oil pressure at idle when cold is 60psi warm 20psi
at speed 55psi roughly

thanks
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