An outboard gas engine needs air, fuel, spark, compression, and unobstructed exhaust – in the right amounts and at the right ime. As the piston travels up and down in the cylinder, the pis- ton rings seal against the cylinder wall so the air/fuel mixture can be compressed at the top of the cylinder on the upstroke.
If the piston and rings can’t compress the air-fuel mixture in the cylinder so the
spark can ignite it, the engine won’t run well, if at all. In addition, if the rings
aren’t sealing properly against the cylinder walls, they’re not doing another
job of transferring the heat from the pistons to the cylinder walls. Without that
transfer, the piston would grow in size from the heat and “stick” inside the
cylinder, causing damage.
A compression check can indicate that the piston rings are working properly and
that the cylinders are in good condition. The test measures how much pressure is built
up by the motion of the piston inside the cylinder, given in pounds per square inch
(PSI). Even if you’re not a trained technician, you might be mechanically inclined
enough to do this diagnostic test. And even if you’d rather not attempt this yourself,
understanding how the test works will let you evaluate what a mechanic tells you.
There are other diagnostic tests, such as a leak-down test, in which you measure
the rate of air pressure leaking from a cylinder while the piston is in top-dead-center
position. This also can help you determine whether an expensive teardown is needed.
In principle, the compression test is simple, but the many different types of outboards can add numerous important complexities. The following steps are only gen-
Checking Compression On Your
Outboard Is A Numbers Game
A compression check can tell a lot about the health of your outboard engine.
Here’s why and how it’s done
eral guidance. Your engine may require
different steps depending on factors such
as whether it’s a two- or four-stroke, has
fuel injection, has computers onboard, is
hand cranked or has a starter motor, and
how its ignition can be disabled. These
variations and other issues mentioned
below will bear on your ability to do this
Pros tend to use good compression
gauges from high-end companies like
Snap-On and Mac. But you can also buy
a reasonably good compression gauge
at almost any local auto-parts store. It
should work fine for occasional use. Get
the right adapters to fit the spark plug
holes in your engine.
There are advocates for checking
compression on a cold engine, and those
that advise doing it only on a warm
engine. My vote is for the latter; it gives
a reading that’s accurate and relevant to
By John Tiger
Checking your engine’s cranking compression is a quick task that can reveal
a lot if correctly performed.
There are four distinct phases in a four-stroke cycle: induction, compression, power, and exhaust. A lot can be learned by checking the engine’s ability to compress the gas/air mixture.