P

COMPRESSION? CHECK!

Testing and understanding an engine’s compression

readings

By John Bellah

erhaps your car’s engine has begun to idle in a rough manner. Maybe
you found the collector car of your dreams, but are unsure of the
condition of its engine. When in doubt, a cylinder leak-down test or
compression test is in order. While a leak-down test may be more
accurate, it requires compressed air, whereas a compression test can be
done almost anywhere. In this story, I will walk you through the
compression test.
A properly conducted compression test will provide a good indication
of engine condition. An improperly conducted test will give erroneous
information. Unscrupulous used-car dealers may insist on a compression
test on vehicles it takes in trade. Their mechanic will jerk the plugs and
immediately conduct a compression test. This is a problem, however,
because removing spark plugs will loosen bits of carbon, which can
become stuck between valves and valve seats. A compression test made
using this method will usually indicate lower readings and may not show
the true cylinder condition. Readings may also vary far more than
specifications allow. The unscrupulous dealer will point out the low
readings and deduct the cost of a valve grind, or even an engine
overhaul.
Prior to beginning the test, roll the engine over several times using the
starter, but with its ignition primary wire disconnected or the coil wire
removed. (Consult your vehicle’s shop manual for the proper procedure
as it relates to your specific vehicle, because later electronic ignition
systems can sometimes be damaged by improperly removing the
secondary coil wire.) If the engine turns over evenly, this preliminary
test indicates that compression is similar between all the cylinders. An
uneven rhythm indicates one or more cylinders has a problem.