Mechanics

Compression Test

Step by step repair guide on how to perform an automotive engine cylinder compression test, this article pertains to all internal combustion engines.

Difficulty Scale: 3 of 10

Begin with the vehicle on level ground, engine "OFF" with the parking brake set.

Step 1 - Before a compression test can be begin, the engine must be disabled so it doesn't start. One way to achieve this is to remove the fuel pump or ignition fuse which can be done by referencing the owners manual. Other methods include disconnecting the ignition module or coils.


Removing Fuel Pump Fuse

Step 2 - Identify the cylinder that will be tested, if all cylinders are being tested it should be performed one at a time. (Note: If all cylinders are being tested and two cylinders test low that are next to each other, the head gasket could be blown between the cylinders.)


Test Cylinder

Step 3 - Remove the electrical connector from the ignition coil or spark plug wire.


Remove Electrical Connector

Step 4 - After removing the wiring connector and mounting bolt, grasp the ignition coil and pull upward to remove.


Remove Ignition Coil

Step 5 - Using a spark plug socket, remove the spark plug. (Note: Clean the spark plug hole prior to removal.)


Remove Spark Plug

Step 6 - Once the spark plug has been removed, use a compression gauge set to measure the cylinder compression.


Compression Gauge Set

Step 7 - After choosing the proper gauge extension, insert the hose into the spark plug hole and tighten by hand.


Insert Compression Gauge

Step 8 - Then, connect the gauge to the hose fitting.


Connect Gauge

Step 9 - While supporting the gauge, have a helper crank the engine over for about 5 seconds.


Cranks Engine Over

Step 10 - The gauge will start to climb on the first rotation and continue until max pressure is achieved, at which time the reading is noted. Once the reading is obtained, press the pressure release on the side of the gauge which will return the gauge to zero. Individual cylinder compression is useful information and can inform on a collective wear point of an engine (cylinder wear dictates the engine's life expectancy) and inform of an individual cylinder problem which can then be pinpointed for repair. Compare cylinder compression readings, typical compression readings are between 125 p.s.i. and 160 p.s.i., cylinders should be within about 5% of each other. If poor compression exists in one or more cylinders, it can cause a rough idle and low power. Possible causes for a low compression condition are: burned intake or exhaust valves, broken piston or piston rings, broken valve spring or a blown head gasket. The most common reason for an engine to lose complete compression is a timing belt or timing chain failure, to check this, remove the oil fill cap and observe camshaft rotation when the engine is cranked.


Checking Engine Compression

Step 11 - Once complete, remove the gauge from the spark plug hole.


Remove Gauge

Step 12 - After the gauge has been removed, reinstall the spark plug and tighten.


Reinstall Spark Plug

Step 13 - Next, reinstall the ignition coil and mounting bolt, then reconnect wiring connector.


Reinstall Coil

Step 14 - After testing is complete, reinstall the fuel pump fuse and close the PDC (power distribution center.)


Reinstall Fuse

A dropped valve seat will cause low or no compression, this problem is prone to aluminum and high mileage cylinder heads.


Dropped Valve Seat

A camshaft lobe is used to open the intake and exhaust valves, occasional these lobes will fail causing low compression.


Flat Camshaft Lobe

Helpful Information

An engine depends on equal compression readings to run smoothly. A compression test of all cylinders is needed to check the engine for wear and internal damage. Occasionally, a timing belt or chain will jump "out of time" causing the camshaft to lose correlation with the crankshaft, and therefore causing low compression across all cylinders. The best method for testing this condition is to remove the timing belt/chain cover and inspect timing marks. These problems if left un-repaired can cause more extensive engine damage and even catastrophic failure.

There are two types of gauge styles, one threads into the spark plug hole which is more accurate. The other style of gauge is constructed with a rubber plug that is designed to be pressed against the spark plug hole, this style of gauge is difficult to use and may not be accurate.

Optional compression testing methods include an amperage load monitor for the starter. This test measures the battery amperage draw to each cylinder while the engine is cranking, showing which cylinders have good compression and which have the least amount compression.

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2015-04-25)