Re: [Chrysler300] Compression pressure , 300H

[Michael Moore <mmoore8425@xxxxxxx>]

Thanks Rich
Mike Moore  
On Nov 8, 2012, at 10:47 PM, Rich Barber wrote:

Individual compression pressures on a long non-operated, 85,000 mile, cold,
ram K block ranged from 100 psi to 140 psi. I found this interesting text
on analyzing compression pressures. According to the 75% criterion, the
engine failed, although a properly operated and warmed up engine with
properly adjusted solid tappets would have probably tightened the range and
marginally passed.

C300K'ly,
Rich Barber

COMPRESSION TESTING 

Note: although this is written primarily for 4cycle gas engine testing,
2cycle gas and diesel follow pretty much the same sequence of procedures.
Also, many diesel engine companies do not publish specific compression
readings so the values, in the chart, at the end of this section will be too
low for use with a diesel, you'll have to do the calculation.

1. Be sure the crankcase is at the proper level and the battery is properly
charged. Operate the engine for a minimum of 30 minutes at 1200 rpm, make
sure that the choke has opened, and the engine is at normal operating
temperature. Putting a load on the engine for at least 10 minutes, can
increase the accuracy of your test. Turn the ignition switch off; then
remove all the spark plugs (injectors on the diesel). If the engine has
closed cooling, (green anti-freeze) relieve pressure in the cooling system.
(carefully)

2. Set the throttle plates and choke plate in the wide open position. On 4
cycle gas engines: Try not to pump raw fuel into the intake with the
accelerator pump.

3. Install compression gauge in No. 1 cylinder.

4. Install an auxiliary starter switch in the starting circuit. Ground the
coil wire on gas engines. Next, using the auxiliary starter switch, crank
the engine (with the ignition switch off) at least five compression strokes
and record the highest reading.

Count the approximate number of compression strokes required to obtain the
highest reading.

5. Repeat the test on each cylinder as was required to obtain the highest
reading on the No. 1 cylinder.

If the readings degrade from front to rear, re-check the battery and
possibly re-run the test starting at cylinder #1. In this way you can have a
set of two tests to compare an average. 

TEST CONCLUSION

2 Cycle Engines: 
Your test results need to be within 15 psi range to be considered an engine
worth using. For example; a 3 cylinder with readings of 130, 135, and 120
would be considered a good engine. Some might argue that there should be no
more than 5psi difference between high and low cylinders. But, obviously,
the closer the readings are to each other, the better.

4 Cycle Engines:
The indicated compression pressures are considered normal if the lowest
reading cylinder is within 75% of the highest. Refer to the following
example and chart.

Seventy-five percent of 140, the highest cylinder reading is 105. Therefore,
cylinder No. 7 being less than 75% of cylinder No. 3 indicates an improperly
seated valve or worn or broken piston rings.

If one, or more, cylinders read low, squirt approximately one (1) tablespoon
of engine oil on top of the pistons in the low reading cylinders. Repeat
compression pressure check on these cylinders.

1. If compression improves considerably, the piston rings are at fault.

2. If compression does not improve, valves are sticking or seating poorly.

3. If two adjacent cylinders indicate low compression pressures and
squirting oil on the pistons does not increase the compression, the cause
may be a cylinder head gasket leak between the cylinders. Engine oil and/or
coolant in the cylinders could result from this problem.

Note: An engine compression test is easy to do but, your results need to be
tempered with the knowledge of testing conditions that reduce the accuracy
of your test. As the battery gets weaker, starter gets hotter, and the
engine cools down, your results from any one cylinder will change. It is
important to do the test a quickly as possible, with as little cranking as
needed to complete the test.
Circumstances arise where a more accurate leak down test should be utilized
before you start tearing the engine down. Readings that are at the extremes
of the chart, (meaning over-all very high or very low readings), may
indicate conditions inside the cylinders that a compression test won't
depict properly.
You can do the math yourself but, the following quick reference chart should
be used when checking cylinder compression pressures. ( to avoid
mathematical errors) The chart (below) has been calculated so that the
lowest reading number is 75% of the highest reading. 
Special note: Although the "actual" compression readings are not all that
important, keep in mind that very few engines will run with a compression
under 100 psi. However, there are some engines that will test with readings
in the 90 psi range on a regular basis. If you're not sure if you have one
of these engines, contact your servicing dealer before you get excited. 

Although 75% range is tolerable for an older motor, keep in mind that most
modern engines are designed at much closer tolerances. Also, on a V8 engine,
if 7 cylinders test at 150psi and only one tests at 113psi, this is not a
good sign...

-----Original Message-----
From: Chrysler300@xxxxxxxxxxxxxxx [mailto:Chrysler300@xxxxxxxxxxxxxxx] On
Behalf Of Michael Moore
Sent: Thursday, November 08, 2012 11:13 AM
To: 300
Subject: Re: [Chrysler300] Compression pressure , 300H

Sorry, that would be engine compression.

Mike Moore

On Nov 8, 2012, at 11:00 AM, Michael Moore wrote:

Does anyone have available the pressure I should read for a stock 300H when
cranking (with all plugs out) ? 

Thanks, Mike Moore 

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