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Doctors use many tests to check an individual’s health.  A thermometer can show a fever, but lack of a temperature does not necessarily mean good health.  The same is true with automobiles.  The lack of diagnostic trouble codes does not mean the engine is healthy.  A cylinder compression test can give much useful information.

For a gasoline-powered engine to run properly, four things must be present.  Fuel provides the energy to run.  A spark ignites the fuel.  We must compress the fuel-air mixture so that it is explosive, and all of this must occur at the proper time.  Compression is a result of internal engine components and a good indication of their condition.  Properly testing compression teaches a great deal about the condition of an engine.

What is engine compression?

cylinder volume over combustion chamber volume equals compression ratio

A piston is at the lowest point of travel, at bottom-dead-center.  The volume of the cylinder, from the top of the piston at this position, to the bottom of the head is displacement.  Fuel and air fill the cylinder, following the intake stoke.  The piston compresses this into the combustion chamber, as it moves to the top of its travel.  For example, they squeeze cylinder displacement of 500 cubic centimeters or CC into a 50CC chamber.  We also include the volume of the chamber with the cylinder, so the volume is 550CC.  This means the approximate compression ratio is 11 to one, 550CC divided by 50CC.

Compressing fuel and air makes it explosive and vastly increases engine power.  Most standard engines operate with compression, around eight to one.  Increasing the ratio means more power but requires a heavier design.  Higher compression also makes the fuel-air mixture explode spontaneously.  When this occurs, the mixture ignites, before the spark plug fires.  This results in spark knock or detonation and damage to the engine.  We need higher octane fuel to lower the point of spontaneous combustion.  This is why performance engines use 93-octane fuel.

The compression gauge and adapters

a typical compression gauge

To measure engine compression, we thread a gauge into the spark plug opening.  The gauges with a rubber tip, held in the spark plug hole, are useless for serious testing.  For gasoline engines, the gauge will need to read at least 225 pounds per square inch or PSI.  Diesel engines run much higher compression.  This article pertains only to gasoline-powered engines.

a few compression test adapters

Because many spark plug designs exist, we need several threaded-adapters to fit various engines.  Thread pitch, diameter and length vary greatly.  Using the wrong adapter can damage the cylinder head or engine.  For instance, if it extends into the cylinder, the piston may strike it.  Along with a good gauge and adapter, a technician will need the proper procedure.

What should compression read?

Within limits, the actual compression reading is less important than the relationship between the cylinders.  Many factors influence the number that results, for instance, cranking speed and time.  Engine design, barometric pressure, and several other things also change the readings.  Most engines read between 125 and 175 PSI per cylinder.  Readings about 135 PSI are adequate, and the highest and lowest cylinders should be within 15% of one another.  Any cylinder more than 15% under the high reading suggest a problem.

How to check compression

Remove all spark plugs from the engine.  We need a good battery and connecting a charger is best while testing.  We must disable the fuel injectors.  Cranking an engine with the fuel-injectors firing, can cause catalytic converter damage.  Some engines have a flood-clear mode and holding the accelerator to the floor, while cranking, will stop fuel flow.  On others, we can remove the injector-relay, or unplug the injector-harness.

An unrestricted flow of air, into the engine, has to be available.  With cable-actuated throttles, holding the accelerator down will work.  This may be difficult on drive-by-wire vehicles.  If possible, remove a large vacuum hose.  This allows air to flow into the engine, with the throttle closed.

block a drive-by-wire throttle open for compression test

Another method is to block the throttle open.  Use great care with this method.  Turn the ignition switch off and unplug the throttle body.  It will remain unplugged throughout the tests.  Block the blade with a soft object.  This may set a diagnostic trouble code in memory.  The code has to be cleared after testing is complete.

Thread the gauge into the first cylinder and crank the engine through six revolutions.  Watching the gauge will give additional information.  The needle should jump to about 40 PSI on the first turn.  Each successive revolution will produce less pressure until the sixth.  Record this reading and move to the next cylinder.  Each cylinder must receive the same cranking time.  If cranking goes over or less than six turns, reset the gauge and tests the cylinder again.  Unequal crank times will skew the results.

Interpreting the compression readings

record and compare each cylinder's reading                          

All readings should be above 100 PSI and nearly equal.  Low readings show a problem within a cylinder.  For example, seven cylinders near 140 PSI and one at 100 PSI means there is a problem.  Possible causes include a valve that is not opening or closing fully or leakage from the cylinder.  Broken or stuck piston rings or a blown head gasket are potential sources of leakage.  A bent, sticking or a burned valve or a bad camshaft will show as valve problems.

A further test can help isolate leakage from a valve issue.  Squirt about one tablespoon of 5W30 oil into the cylinder with low pressure.  Crank the engine over to distribute the oil.  Repeat the test on the low cylinder and again record the reading.  If the reading rises more than 10 PSI, cylinder leakage is likely, rather than a valve issue.  A reading that remains the same suggests a valve problem.  We need a cylinder leak-down test to narrow the results more.

Cylinder pressure should rise quickly and equally between cylinders.  Pressures, which do not rise quickly, suggest worn piston rings.  Adding oil, as above, and retesting will clarify the problem.  Slow rising cylinders, that rise significantly when wet, means leaking piston rings.  This is more certain if the engine also consumes oil.

The compression test gives a technician useful information on engine condition.  Combined with other methods, this will greatly improve the accuracy of diagnosis.

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