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How to perform a
COMPRESSION TEST
INTRODUCTION:
Compression testing is a good way to measure the condition of the rings,
cylinders, and valves. You will need a helper to do this
test. It can be done alone, but is more difficult, and you cannot see how
the cylinder pumps up.
TESTING (DRY)
The engine must be warm, the ignition disabled, and all sparkplugs removed.
Caution: Wear protective clothing and gloves while working around a hot
engine. While doing the test, the throttle and choke plates must be fully
open for an accurate test. Have your helper fully depress the accelerator
while he is cranking the engine. Caution: Avoid any flames around the engine
during the test because of the air/fuel mixture being pumped out of the
open sparkplug holes.
Connect the tester to a cylinder and turn the engine over about 6 to 8 compression
strokes. You will be able to hear the cranking speed slow as the tested
cylinder comes up on it's compression stroke. Note how fast the compression
increases and jot down the highest reading. Test all four cylinders the
same way with the same number of compression strokes.
INTERPRETATION OF THE DATA
There is more than one school of thought on what the compression of the
average engine should be. As a general rule a compression of 135 PSI or
better is excellent, and a compression of 85 PSI or lower is extremely bad.
The most desirable situation is that all cylinders give the same or close
to the same reading, and that reading is above 135 PSI. Uneven readings
are not uncommon on worn engines, and as long as the difference between
the highest and lowest readings are no more than 21 PSI and the lowest reading
is 100 PSI or better, then the engine is acceptable. Greater differences
between cylinders indicate worn or broken rings, leaky or sticky valves
or a combination of all.
When you were observing the gauge during the cranking of the engine, you
should have noticed the way each cylinder pumped up. Usually, a cylinder
will produce about 40 PSI on the first stroke and about 35 PSI on each additional
stroke until the maximum compression is reached.
Problem cylinders may have trouble pumping up and may increase by only something
like 10 PSI per stroke. You may be able to crank these cylinders enough
times to come close to the other cylinder's total PSI, and this is one reason
to limit the number of compression strokes. Poor rings usually cause this
condition. Be aware that a cylinder suffering from excessive oiling, even
from bad rings, can yield high compression test results because the excess
oil in the cylinder seals the rings. Other symptoms may give you a clue
to the problem (a smoking engine).
There are some variables that affect the readings obtained from compression
testing. They are cranking speed, altitude, temperature, worn camshaft lobes,
and high performance long duration profile camshafts. The cranking speed
needs to be maintained the same for each cylinder. This may mean jumping
your battery to maintain the speed. The effect of the two camshafts above
is the same, one of lower readings. Compression data is usually based on
14.7 atmospheric pressure and 59F at sea level. There are factors to compensate
for the different altitudes and the corresponding temperature differences.
These are as follows: 1000' = .9711, 2000' = .9428, 3000' = .9151, 4000'
= .8881, 5000' = .8617, 6000' = .8359, 7000' = .8106, 8000' = .7860. The
equivalent compression reading for a cylinder that should be 135 PSI by
the data at 5000' would be 135 x .8617 = 116.33.
TESTING (WET)
Many automotive books describe a dry and a wet compression test. Usually
these tests must be interpreted together to isolate the trouble in cylinders
or valves. Unfortunately, the wet compression test is unreliable when performed
on engines where the cylinders are horizontal (like the old VW). To perform
the wet test, a tablespoon of oil is poured into the cylinder though the
sparkplug hole before checking the compression. On horizontal cylinders,
oil does not spread evenly over the piston crown, a necessity for this test.
With the above said, you squirt the tablespoon of oil (SAE 30W is fine)
into a cylinder that is reading low. Crank the engine two revolutions or
so to spread the oil then retest the cylinder. If the compression comes
up markedly, 40 PSI or more, the trouble is poor ring to bore sealing. If
compression doesn't increase much, about 5 PSI, then the problem is probably
with the valves. It could also be pulled head studs or a warped cylinder
head.
RECAP:
- Charge the battery if the battery is not fully charged.
- Disable the ignition system.
- Disable the fuel injection system.
- Remove all of the spark plugs.
- Block the throttle plate wide open.
- Start with the compression gauge at zero, and crank the engine through
6 compression strokes (6 puffs).
- Make the compression check for each cylinder. Record the reading.
- If a cylinder has low compression, inject approximately 15 ml (one
tablespoon) of engine oil into the combustion chamber through the spark
plug hole, recheck the compression and record the reading.
- The minimum in any one cylinder should not be less than 70% of the
highest cylinder. No cylinder should read less than 100 PSI. For example,
if the highest pressure in any one cylinder is 150 PSI, the lowest allowable
pressure for any other cylinder would be 105 PSI.
Interpreting the readings.
- Normal: Compression builds up quickly and evenly to the specified
compression for each cylinder.
- Piston Rings Leaking: Compression is low on the first stroke. Compression
builds up with the following strokes but does not reach normal. Compression
improves considerably when you add oil.
- Valves Leaking: Compression is low on the first stroke. Compression
usually does not build up on the following strokes. Compression does
not improve much when you ad oil.
- If two adjacent cylinders have lower than normal compression and injecting
oil into the cylinders does not increase the compression, the cause
may be the head gasket leaking between the cylinders.
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