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  #1  
Old 11-18-2010, 01:41 PM
87xjmike 87xjmike is offline
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Default what is the average cyl compression for the 5.9?

I'm trying to trade my bmw 525 http://www.dezertrangers.com/vb/we-trade/127986.htm for a truck. A guy is offering me his 2000 1500 w/ a 5.9 4x4 new tranny 210k on engine. He said compression was between 110-135. That is low and it has me worried, he continued to say that these engines are notorious for low compression but run fine with such. Is this true? Do you all have compression in these ranges?
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Old 11-18-2010, 06:00 PM
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My engine is 180 but I have a high performance build. My last engine was 167-170, still a tight grouping. I'm worried that the truck you mention has low compression AND I'm worried the grouping has a spread from 110-135. That means you would have cylinders that are up to 25psi different. That's too big in my opinion.

I don't want to make a judgment for you but I personally think it has wear and will probably burn a little oil, need to be refreshed soon. Although the Dodge specs says anything above 100 is fine I've seen 5 engines for me, three for friends and none of them ever were below 160ish.

Here's some good reading also. If you are really interested in the truck go down the page and he tells you how to test for bad rings. He also informs you how to perform a static, running and snap compression test. You should be able to get a good sense of the engine with these tests.
http://www.**************/forums/showthread.php?t=60502
Quote:
RUNNING COMPRESSION TESTS

This is a summary of the responses to a question about a "Dynamic
Compression Test" sent out via the i-ATN e-mail list and posted on
Compuserve's "For Techs Only" forum. It seemed to ring a bell with the
most techs as a "running compression test," so I will use that name
here. Call it what you will, this test is an accurate if slightly
esoteric and time-consuming test of cylinder breathing. It is in fact
recommended by Detroit Diesel instead of a traditional static
compression test, it is included as part of Delmar's ATTP program, and
several instructors use it as part of their state emission training
programs.

HOW TO PERFORM A RUNNING COMPRESSION TEST

1. Start with a normal ("static") compression test. To eliminate rings,
valves, holes in pistons, that sort of things.
To do this: Unscrew all sparkplugs, disconnect fuel injector plugs,
disconnect coil primary plug, and have another battery nearby in case the
cranking of this engine runs its battery down. Prop open the thottle bores
being carefull not to drop anything in them. Carefully screw in your
compression tester (the press on type gauges are vastly inferior) to one
spark plug hole. Crank the engine over. Record the compression tester
reading on the first revolution of the engine and the fifth revolution.
Write these values down and go onto the next cylinder.
If you suspect you have bad rings, repeat the entire test above but
this time put an eyedropper full of SAE 70W gear oil in each cylinder
before screwing in the compression tester. 30 psi higher readings on a
cylinder on this 'oil-seal-assisted' second test indicates that cylinder
has worn out or broken rings.

2. The Dynamic Compression Test:
Remove prop from throttle bores. Put all spark plugs but one back in and
reconnect all electrical plugs taken off in step one above.Ground that plug
wire to prevent module damage. Disconnect that injector on a port fuel
system.

3. Carefully thread your compression tester into the empty hole. The test
can be done without a Shrader { press to release ** valve, but most people
recommended leaving the valve in the gauge and "burping" the gauge every
5-6 "puffs".

4. Start the engine and take a reading. Write it down

5. Now goose the throttle for a "snap acceleration" reading. Reading
should rise. Write it down NOTE: Don't use the gas pedal for this snap
acceleration. The idea is to manually open then close throttle as fast
as possible while without speeding up the engine. This forces the engine
to take a "gulp" of air.

6. Now write down your readings for at least the bad cylinder (if there
is a single bad cylinder) and maybe 2-3 good ones. Make a chart like
this:
..........STATIC COMPR /IDLE -RUNNING COMPR / SNAP
Cyl1 ...........150 ..................75 ..........................125
Cyl 2 ..........175 ..................80 ..........................130
Cyl 3 ..........160 ..................75 ..........................120

7. ANALYSIS: Running compression at idle should be 50-75 psi (about half
cranking compression). Snap throttle compression should be about 80% of
cranking compression.

EXAMPLE 1 - RESTRICTED INTAKE
CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP
Cyl 1 150 ...................75 ...............................80
If Snap reading is low (much less than 80% cranking
compression), look for restricted intake air- severely carboned intake
valve, worn lobe on cam, rocker problem, "shutters" mispositioned in the
runners. (Toyota, Vortec etc. with variable runner length) Comparing
measurements between cylinders is important.

EXAMPLE 2 - RESTRICTED EXHAUST
CYL STATIC COMPR IDLE -RUNNING COMPR - SNAP
Cyl 1 .......150 ...........75 .............................180
If snap measurements are significantly higher than
80% of cranking measurements, look for restricted exhaust on that
cylinder-such as worn exhaust cam lobe, or collapsed lifter. Or, if they
are all high, look for a clogged cat converter.

WHAT IS GOING ON?

When you do a normal compression test, you are checking cylinder
sealing, not cylinder breathing. When you check engine vacuum at the
manifold, you are looking at the breathing of the entire engine, by
checking vacuum at a common (plenum) source. You aren't testing a
specific cylinder. This test looks at the breathing of an individual
cylinder.

Say the engine is running at 18 inches vacuum. Atmospheric pressure is
about 30 inches, so the difference (30 inches - 18 inches = 12 inches)
is what the engine is sucking in. 12 inches mercury is equivalent to
about 6 psi absolute air pressure. Compressed at an 8 to 1 ratio, you
should get CONSIDERABLY MORE THAN 6 x 8 = 48 psi pressure if all the air
makes it into the cylinder and then gets pushed out. So your idle reading
on running compression is about 55 to 75 psi.
Contary to popular belief, an 8 to 1 compression ratio will give you
MORE than eight times the pressure. It would give you exactly eight times
the pressure - but only if you compressed the air and then gave the now-hot
air a few minutes to cool. Fast compression in an engine gives
considerably more pressure because the air doesn't have time to cool. This
kind of compression is called "adiabatic" which is a fancy name for "no
loss of heat."

When you snap the throttle, the manifold vacuum drops, so the absolute
air pressure going into the cylinder increases.

In fact, you can do running compression tests at various constant
manifold vacuum readings (by brake-torqueing the engine momentarily),
and the running compression should roughly correspond to the manifold
vacuum. For example, at 10 inches vacuum, engine should be breathing in
about 10 psi air pressure, so you should see a running compression
reading of about 80 psi (at 8 to 1 compression ratio).

If one cylinder reads low running compression compared to the rest it
means that the air didn't make it in. If one cylinder reads high, the
air didn't make it out (and the next pulse of air raised the pressure).
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Old 11-18-2010, 06:12 PM
87xjmike 87xjmike is offline
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not really what i wanted to hear but it is what i was expecting... thanks for that great response. I'm going to rent a comp tester and test his truck this weekend based on your write up above. i'm hoping he did it wrong.
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Old 11-18-2010, 08:01 PM
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Even if didn't have the throttle open during the test the variation isn't going to change even if the numbers go higher. My tech info for that motor is offline for the moment but if memory serves, the minimum for that engine should be 135 psi.
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Old 11-18-2010, 08:43 PM
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even worse news... ok well thanks for the response ram tech. Maybe i'll just replace the bearings and piston rings from the bottom end. this truck has new cyl heads soo... i have a sneaking suspicion the lowered compression is due to the fact that engines decay as a unit, if you tighten up the top end but not the bottom it will start pushing things out of the bottom end at a higher rate. Thus premature failure of the rings may begin. Although 210k isn't exactly pre-mature. Is it easy to take the pistons out from the bottom w/ these trucks (2000 4x4, auto)? Do they have a main bearing girdle? I'm fairly inclined mechanically and could probably do it in a weekend if its doable w/out taking the top end apart.
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Old 11-18-2010, 09:32 PM
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Might want to do a leakdown test to pinpoint where the compression is being lost.
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Old 11-18-2010, 09:37 PM
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With the mileage on this engine, I'd advise against trying a re-ring since the cylinders are likely out of round and fresh rings could actually yield lower compression numbers. The only way to go would be to have it checked with a bore gauge and at the least having the cylinders honed. My guess is that it will likely need boring out. You should also consider that the cam and lifters may have some wear and the timing chain is going to be stretched. The bearings are also likely to have a wear pattern and the crank should be polished or turned as needed. The mains have traditional style caps.
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Old 11-19-2010, 12:18 PM
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Quote:
Originally Posted by RamTech View Post
With the mileage on this engine, I'd advise against trying a re-ring since the cylinders are likely out of round and fresh rings could actually yield lower compression numbers. The only way to go would be to have it checked with a bore gauge and at the least having the cylinders honed. My guess is that it will likely need boring out. You should also consider that the cam and lifters may have some wear and the timing chain is going to be stretched. The bearings are also likely to have a wear pattern and the crank should be polished or turned as needed. The mains have traditional style caps.
yeah your right. It sounds like these engines do not last much longer than 200k?
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Old 11-19-2010, 12:33 PM
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They can but they certainly won't be running at peak performance. I did some double checking today and the bare minimum compression spec is 100 psi. Keep in mind that most engines need around 80 psi just to run. This engine is definately showing its age.
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Old 11-19-2010, 12:41 PM
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ok thx for checking that.
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