DodgeTalk Forum banner

Not open for further replies.
1 - 6 of 6 Posts

75 Posts
Discussion Starter #1
I need some info., please. I am doing a compression check on my 5.9 magnum, because I have a low idle miss. I think it is a burnt valve. I was wondering if anyone had the compression specs. on the 5.9 engine (stock). I've only checked one bank, but the numbers don't look good. I have two 125s, a 110 and a 100 :WHT: . I'll check the other bank tonight. I have 114000 on the truck, It may be time for an overhaul.

75 Posts
Discussion Starter #2
I checked with my local stealership (saw that in another thread and I like it), and they said dodge specs are as followes.
normal is 120-150 psi with 170 being allright
100 psi is min.
Sounds like I'll be needing some engine work.

1,733 Posts
Here's an old post of mine that may be of use:
I did a compression test on my 1995 Magnum 5.9V8 this afternoon.

This engine was broken in for the first 18,000 miles with conventional oil and
switched to Mobil One synthetic. It presently has the Mobil One 15w-50 but
has used the 5w-30 and 0w-30 in the past.

Engine has about 142,000 miles on it.

I did the compression test because about 3000 miles ago my 2nd catalytic
converter plugged and created excessive exhaust backpressure - so much so that
the truck would not go above about 25 mph for the 8 miles it took to get to
the muffler shop. This excessive backpressure ruined the EGR modulating valve
diaphram, which was replaced, as was the O2 sensor.

I was worried I might have burned an exhaust valve.

The engine has been running 'ok' but seems to get slightly less mpg than

I changed 2000 miles ago to NGK NGK ZFR5F11 (stock number 2262) just after the
bad cat, and the engine idle seems more uneven. Previous plugs were Autolite

In the figures that follow, 'Oil Added' means a squirt of Mobil One Gear Oil
75w-90 was put in the cylinder using a rubber hose on the end of the gear oil
bottle. The engine was cranked 5 revolutions and then the compression on that
cylinder was read again. It would have been better to have used a plunger
style oil can and put a consistent 3 shots into the cylinder. It might be
possible that the 'Oil Added' compression psi's are not consistent because the
amount of gear oil varied.

Engine was cool from sitting overnight.
Outside air 75 degrees.
Throttle bores were propped wide open during cranking.
All sparkplugs were removed.
Readings were taken on the 5th stroke and the compression tester had a
'shraeder valve' that held pressure on the gauge until pressed. Typical first
stroke compression numbers were around 100 psi and climbed to steady maximum
shown below by the 4th stroke.


Cylinder 1 - dry 145psi - oil added 175
Cylinder 2 - dry 145psi - oil added 165

Cylinder 3 - dry 150psi - oil added 185
Cylinder 4 - dry 155psi - oil added 185

Cylinder 5 - dry 155psi - oil added 180
Cylinder 6 - dry 150psi - oil added 170

Cylinder 7 - dry 160psi - oil added 175
Cylinder 8 - dry 148psi - oil added 165

Average dry: 151 psi
( # 7 is 6% high, # 8 is 2% low)

Average Oil Added: 175 psi
(#3&4 are 6% high, #2&8 are 6% low)

Additional information:
All 8 NGK sparkplugs looked clean after 2000 miles of mostly interstate
driving and had extremely white insulators on the relatively long 'projected
tips' of this design plug. Cylinder 6 had just bit different looking deposits
on the ground strap electrode - the other 7 plugs grounds looked normal 'dry
flaky' and this one looked slightly 'greasy'. When I changed out Cylinder 6
on the Autolite 3923's used previously cylinder 6's white insulator had more
black carbon deposits on it and looked just a bit oily. At the time I took
this to be because I had just sprayed a can of Mopar Combustion Chamber
Conditioner into the engine.

Engine does not seem to be using any coolant. Oil consumption is about 1
quart every 2000-3000 miles and there are slow drips from both front and rear
engine crankshaft seals.

Cylinder 7 may have more carbon deposits. I had previously found some time
ago that spraying Mopar Combustion Chamber Conditioner in cylinder pairs 7&8
got rid of pinging temporarily, maybe the problem is just cylinder #7.

The 1995 Ram FSM is not too helpful on what 'proper' compression readings
should be - it just says 100 psi on page 9-108 of my model year 1995 book.

This 100 psi seems to me like a 'cop out' number from DC listed to counter
angry customers who have a low reading cylinder. Or perhaps it is an
unchanged 'carry over' spec from 1978-1984 LA 360 V8s that had only 8 to 1

This 360 V8 engine is supposed to have a 9.1 to 1 compression ratio, as listed
on FSM page 9-81. {I hear later model 1996+ 5.9V8s were lowered to 8.9 to 1
compression ratio because of numerous owner pinging complaints.

On page 9-142 the 1995 FSM says the 1995 8.0L V10 with 8.4 to 1 compression
ratio is supposed to have 170-190 psi compression test numbers. Go figure?

The Engine Analyser 3.0 software from Performance Trends estimates that a 1995
Magnum 5.9V8 engine with 9.1 static compression ratio and the cam specs in the
1995 FSM should have 180 psi 'theoretical' cranking compression.

My analysis of the above compression numbers is that the cylinders look mostly
normal for a high mileage engine

1,733 Posts

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


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

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
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.

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.

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.


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

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).

Many thanks to the people who responded through both Compuserve and

75 Posts
Discussion Starter #5
Thanks HankL this should help me some, but it still looks kink of grim. On the up side I can do some internal mods I have been wanting to do. I just can't believe my reading are this low. I did buy my truck used, but it only had about 60k on it. I am very perticular with my maintainance, but who knows about the first guy.

60 Posts
Well compression will at times not always show u the answers u can have good comp while cranking but will not show a bad valve due to hyd lifters, been burnt by this twice on my own 02 with 185 and a van the valves are soft and will not take the abuse what u really need to do is pull the valve covers and run a straight edge over them looking for a valve that is "sunk" one that is up higher than the rest this is a better idea than a comp check. little more time consuming but way lots better guessing u r going to be needing a valve job in the near future from my experience
1 - 6 of 6 Posts
Not open for further replies.