QUESTION - What does compression
ANSWER - Compression testing
is an important way to check the general health of your petrol engine.
Your engine was designed to provide optimum horsepower at a given
level of compression. There are a number of factors that can reduce
an engine's compression some of which can be specifically tested
for without disassembling the engine.
Compression is very easy to check for. Remove all the spark plugs
and use a compression tester to test the compression of each cylinder.
Basically, you connect the compression tester to a cylinder and
crank the engine over on the starter motor for a couple of seconds.
Compression meters are not generally all that accurate. They are
best used to compare cylinder's pressures.
Compression naturally decreases over time as an engine wears. So
unless the compression number for all your cylinders are really
low, what you are looking for is one or two cylinders with very
different compression from the others. Ideally you would like them
to all read exactly the same. I have never had that happen. It is
quite common for there to be a few pounds difference between cylinders.
I generally am not concerned of a range of numbers that can be up
to 10 pounds between the highest and lowest cylinders. When it gets
that high I start thinking about putting money aside for a valve
job or engine rebuild if I think the rings are worn. But I would
consider the engine to be healthy enough to drive hard for some
What causes compression loss:
1. Blown head gasket -
Head gaskets tend to stay intact unless the engine has overheated.
Early failure from incorrect torquing, dirt on a mating surface,
dirt in the holes the head bolts are torqued into, or uneven head
or block surfaces are common. Sometimes a head gasket will fail
immediately because on interference from another part. For instance
the water pump sits under the lower thermostat housing integral
to the head. If the head is milled, the front of the head could
be resting on the water pump and everything would look and torque
down properly but the gasket will not seal properly. Once I had
a wire from the generator get caught in that narrow space between
the lower thermostat housing and the water pump housing. The thickness
of the wire kept the head form torquing correctly.
Overheating an engine causes the head to expand then contract.
This can cause a head gasket to fail. This failure is most commonly
seen in the thin section that goes between two adjacent cylinders.
This shows up on the compression test as adjacent cylinders having
low compression. A major gasket failure between two cylinders can
cause the explosion in one cylinder to leak over and ignite an air
fuel mixture in its adjacent cylinder. You will not see the oil
in water symptom if the gasket is just broken between two adjacent
2. Burnt valve -
Valves exist in a very hostile environment with high pressures
and explosions happening thousands of times a minute. It survives
because it is made out of a hard material and it can transfer the
excess heat to surrounding surfaces. Some heat is dissipated up
the stem but most is dissipated through contact with the valve seat
in the head. The head is cooled by the coolant and the heat flows
from the edge of the valve to the head when it is closed.
The job of the intake valve is to let air fuel mixture into the
cylinder at a given time then keep it from escaping. The intake
valve seldom burns because the cool air and the fuel droplets cool
down the valve.
The job of the exhaust valve is to keep the air fuel mixture from
escaping until after the explosion then to vent it out through the
exhaust system. This valve deals with the explosion like the intake
valve then it has the hot exhaust gases flow along past it while
it is open. The exhaust valve relies upon a head cooler than the
valve to keep from burning.
The valves are constantly withstanding the forces of thousands
of explosions a minute and they are pounding against the head surface
thousands of times a minute. They wear over time and the seal they
Worn valves will cause a general compression decrease. Since they
do not wear at exactly the same rate you will see greater variation
in compression among the cylinders as valve seat wear increases.
The symptoms of worn valves are a general very slow decrease of
power over time from an engine that runs fine and otherwise seems
If a valve is unable to cool itself it will burn and parts of it
will dissolve so that it no longer seals. If the valve does not
seat properly against the head, it will burn faster.
If a valve is adjusted too tight, it will have less time to stay
seated against the head and will eventually burn. If the cylinder
is running too hot from pre ignition, retarded timing or the wrong
kind of fuel, the valve can get hot enough to try and spot weld
itself to the head. This creates a ragged edge that keeps the valve
from seating and the valve rapidly burns because it can not cool.
Since the exhaust valve has the hottest job it usually is the one
When you do a compression test on a head with a burnt valve you
will find one cylinder with very little or no compression.
Common operating symptoms include hard starting, loss of power
and uneven running with poor idle. Basically your engine is running
without that cylinder.
3. Cracked head -
Heads tend to crack when there are large temperature gradients
within the head causing different degrees of expansion within the
head. This might be caused by a blocked coolant passage or by incorrect
fuel or timing causing the inside of a cylinder to get very hot
It is most commonly caused by overheating followed by cold water
being added into the cooling system. The safest way to handle overheating
is to park the car and let all the parts cool down gradually together.
The inside of a head where you can not see is hollowed out to allow
coolant to flow. Almost every head crack will open a passage between
one or more cylinders and the coolant. This will result in overheating
problems where you loose coolant rapidly. The gas flow into the
cooling system is easy to see if you have a radiator with the cap
on top. Start the engine with the cap off and observe the fluid
while the engines gets up to operating temperature. You will be
looking for a steady stream of bubbles after the thermostat opens.
The coolant may pick up a slight oily look. It is normal for a system
to have a few bubbles come out when the thermostat opens but it
is not normal to have a constant stream of bubbles. (A constant
stream of bubbles can be caused by a small hole between the cooling
system and the outside world but a coolant leak to the outside world
is more common in that case).
There are very few oil passages within a head. It is uncommon for
a head crack to go into an oil passage. If it does the coolant will
look like a chocolate brown mess.
A compression check on a cracked would usually show very low compression
between two adjacent cylinders. The compression is often the same
as the pressure rating of the radiator cap. You would look for bubbles
in the coolant to differentiate between a blown gasket and a cracked
The driving symptoms would be essentially the same as for a burnt
valve except that you will almost certainly have coolant loss and
4. Cracked block -
I have only had this happen once and it was long ago. A crack in
the block generally occurs between adjacent cylinders. When it happens
the oil and coolant systems become one with the insides of one or
more cylinders. The oil becomes a chocolate brown substance. Your
exhaust becomes discolored. I just remember it as one heck of a
mess. You will not need a compression test to diagnose this. It
is like an engine Armageddon.
5. Burnt piston -
Incorrect fuel and timing causes temperatures in a cylinder to
become much hotter than normal. A carburetor providing a little
too little fuel or air leak adding too much air to the air fuel
mixture leans it out to burn hotter. Usually it is the exhaust valves
the burns first but sometimes a hole is burned into a piston.
A compression test shows no compression in a single cylinder.
Driving symptoms would be the same as a burnt valve but you will
be going though a lot of oil and you will probably have a noticeably
blue exhaust colour.
6. Worn rings -
Rings go around the upper side walls of a piston. They provide
a seal between the combustion chamber and the lower inside of the
When the engine is rotating oil is constantly getting splashed
up into the cylinders to lubricate the piston's path. The rings
keep this splashed oil from reaching the combustion cylinder and
burning. The rings also keep the compressed gases from the combustion
chamber out of the central oil area. When the rings wear, or if
scratches are made in the cylinder walls they become less efficient
and more leakage occurs between the combustion chamber and the central
Scratches are caused by dirt particles entering the carburetor
with the air. The more you get, the faster an engine wears. Running
an engine in a dusty off road condition without an air filter can
cause scratches to ruin it within a hour. A hole in the hose going
between the carburetor and oil bath air cleaner will rapidly accelerate
the rate of engine wear.
Hi flow air filters work by having larger air passages in the filter
element. This lets more air in, but it also passes more dirt particles
through that will score your engine cylinder's walls. Be wary of
high flow air filters in a dusty off road environment. Always keep
your air filter system intact and clean.
A compression test will show overall lower compression. If you
squirt some oil into the spark plug hole and rerun the test, the
compression will be higher because the oil around the rings will
temporarily provide a better seal.
In summary, a compression test tests for a blown head gasket, cracked
head, burnt valve, cracked block, burnt piston or worn rings. If
you have a problem that does not have an obvious solution, a compression
test can identify or eliminate a number of possible problems that
can be very difficult to isolate otherwise.
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