How To Check Your Engine’s Compression

September 11, 2009 | DIY Tech | Andrew Wojteczko | Comments (0)

Required Tools:

Compression Check gauges are available from a wide array of manufacturers. You can expect to spend between $10 and $50 for a traditional analog gauge depending which model you choose, and as high as $100 for some digital models. The traditional analog style is what we have used here, and works very well. If testing an engine out of car, you will need a 12V automotive type battery, a pair of jumper leads, and a small jumper wire or inline switch.

Step-by-step Compression Check Instructions:

Step 1

Remove all necessary items to gain access to your spark plugs. This may include a spark plug wire cover like seen here on our Honda S2000’s engine.

Step 2

Disconnect and remove coils (coil on plug type) or remove plug wires (Distributor type).

Step 3

Disable power to ignition by removing ignition fuse/relay depending on vehicle application.

Step 4

Disable fueling to prevent flooding of cylinders while testing. We simply disconnected the injector connectors. Other options include removing the fuel control relay, or with a programmable EMS disable your injectors.

Step 5

Loosen spark plugs approximately one quarter turn.

Step 6

Using compressed air, blow into each plug chamber to remove any debris that has accumulated or come free after loosening the plugs.

Step 7

Remove spark plugs. This is a great opportunity to further inspect the health of the engine. Look for any shiny white particles or “pimples” on the spark plug electrode, which could indicate that detonation is occurring (a potential engine damaging condition where unburnt gases left in the combustion chamber ignite from excess heat and pressure). Read more information on how to read a spark plug.

Step 8

Your compression tester should include multiple adapters for the different plug sizes available. Compare the spark plug you removed with the adapters provided with your tester. Select and install the corresponding adapter.

Step 9

Thread the compression gauge into the #1 cylinder and then have a friend jump in the driver seat while you monitor the compression gauge.

Step 10

Request your friend to depress the throttle fully, and then crank the engine a minimum of 3 full cycles or 6 revolutions of the crankshaft. Use the same amount of cranking duration for each cylinder. (Note: the compression gauge has a one way valve to prevent the stored pressure from being relieved each time the engine is off its compression stroke).

Step 11

Once cranking is complete, record the value on the compression gauge and depress the release valve to reset the gauge to zero.

Step 12

Carry out steps 8-10 for the remaining cylinders.

If you’d like to test an engine that is not installed in a car, first make sure there is oil in it. Using jumper cables and a car battery, attach battery positive to the large positive terminal on the starter and battery negative to a clean spot on the transmission bellhousing. Using the small jumper or switched wire connect from the large positive terminal on the starter to the solenoid activation lead on the starter, it will begin to crank when the switch is depressed or as soon as the jumper makes contact. Now hold the throttle body full open (for electronic throttle remove the throttle body) and crank for 6 revolutions. Release the switch or remove the small jumper wire. It’s that easy.

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What to do if your compression values are below target?

If compression values are low, there is an issue with the engine. One or more of the following issues can result in a low compresion value reading:

• Scoring on the cylinder walls preventing the compression rings from properly sealing.
• Damaged/improperly sized compression rings not properly sealing.
• Damaged pistons not allowing the engine to build compression.
• Intake valves not sealing properly to their seats.
• Exhaust valves not sealing properly to their seats.
• Compression check gauge defective/out of calibration or not tightened correctly.

If you are looking at purchasing a used engine and the test results appear to be out of spec, especially if one cylinder in particular is abnormally low, you might be best to look at some alternatives unless you are prepared to do some serious repairs/rebuild.

If you are diagnosing an engine and want to further identify where the problem lies, the next step is to perform a leak down test.

Stay tuned for DIY Tech #2, Performing a leakdown check.

September 11, 2009 | DIY Tech | Andrew Wojteczko | Comments (0)