PROCEDURE FOR COMPRESSION TESTING
When diagnosing an engine to see what is wrong with it, or checking an engine to see if it is in good shape, often the first step to take is to perform a compression test on it. A compression test is relatively quick and simple to do, and does not require any tools except for a socket wrench and a compression gauge test kit. Doing a compression test provides a good picture of how well the engine is performing, and if there is something wrong with it. Of course, it cannot diagnose every possible engine fault, but for a test that only takes a few minutes of time, it provides a remarkable amount of information about the state of an engine, for instance, whether it is mechanically sound and tuneable.
What to look for in a compression gauge:
A compression gauge with threads on it, which screws into the hole, is preferable (rather than the kind that you simply hold in the hole with your hand) for testing with. Most compression kits come with different hose attachments that have one end threaded (See Fig.1), make sure to match the correct hose to your spark plug.
Figure 1 (weisstool.com)
Procedure:
 Run the engine until it is warm (if it will run)
 Disconnect all drives to the engine
 Open choke and throttle valves wide
 Remove the air cleaner
 Remove the spark plug and insert compression gauge with correct hose attachment (matches threads of spark plug)
 Crank the engine over (min. 3 cranks, max 8 cranks, choose a number and do same for all cylinders) as fast as possible to obtain an accurate test
Note: Engines equipped with compression release camshafts may have to be cranked in reverse rotation to obtain an accurate test. Most can be cranked forward.

 Release pressure and repeat test to ensure accuracy
 Record measured pressure (See Table 1)

Interpreting the results:
Now you have compression readings for your engine. As a rule of thumb an engine with 100psi or more is tuneable, but you should still compare the results against the "normal" reading for your engine in the repair manual (You do have a repair manual for your model of engine, don't you?) and go from there. Although your readings should be at or close to the readings in the manual, it is actually more important that your readings for each cylinder be close to each other than that they match the manual exactly. If there is a significant variance in the readings (more than 10 or 20 psi), it usually indicates that the engine has one or more of the following problems:
• Leaking cylinder head gasket
• Warped cylinder head
• Worn or seized piston rings
• Worn cylinder bore
• Damaged piston
• Burned or warped valves
• Improper valve clearance
• Broken or weak valve springs
To determine whether the valves or rings are at fault, pour a tablespoonful of SAE30 oil into the spark plug hole. Crank the engine several times to spread the oil and repeat the compression test. The heavy oil will temporarily seal leakage at the rings. If the compression does not improve, the rings are satisfactory and leakage is due to the valves, cylinder head, or a damaged piston. If the compression is much higher than the original test, the leakage is due to defective piston rings.
Table 1: COMPRESSION TEST RESULTS
CYLINDER # 1 2 3 4 5 6 7 8
DRY PRESSURE PSI
WET PRESSURE PSI


PROCEDURE FOR LEAKAGE TEST
We already used a compression test to determine an engines condition. The problem with this test is that there are many variables. It can only be used to check engine condition by comparing cylinders to each other or a past norm. Variables such as cam profile, engine cranking speed etc, will affect the readings.
A leak down tester uses air from a compressor and measures the rate at which it leaks through the engine. This is done with the engine not running and the beauty of this is that the source of the leakage can be found by listening for the escaped air. Tools needed are a leak down tester kit (See Fig.2) and an air compressor of at least 2hp and delivers a minimum of 90psi of air pressure. Figure 2 (snapondiag.com)
Procedure:
 Run the engine until it is warm (if it will run)
 Remove spark plugs
 Set engine to top dead center (TDC) #1 of the compression stroke. Both valves must be closed
CAUTION: On smaller engines crankshaft needs to be secured or else pressure from the airline will cause shaft to rotate
 Connect hose to spark plug hole
 Connect pressurized gauge to hose connection
 Slowly adjust the regulated pressure gauge with the regulator **** to read 80psi
NOTE: Due to standard engine clearances and normal wear, no cylinder is expected to maintain a perfect 80psi. It is important only that all cylinders on a multi-cylinder engine have a somewhat consistent reading. Always consult the manufacturer’s recommendations.
 Do same test on rest of cylinders on a multi-cylinder engine. Remember to set TDC of each piston for compression stroke.


Interpretation of results:
With gauge connected, you can rock crankshaft to see if leakage changes. If so, this is a sign that the ring lands are wearing, new engines will also do this until the rings are seated.
By listening for escaping air at the carburetor intake, the exhaust system, and the crankcase breather you can determine the cause of low-pressure readings (See Table 2).

NOTE: If the test indicates a valve problem, first check valve clearance to make sure the valve is not being held open due to inadequate clearance.

Table 2: Location of air leak and the indicated defect
Air Escaping From: Indication
Carburetor intake Defective intake valve
Exhaust system Defective exhaust valve
Crankcase breather Defective piston rings


COMPRESSION AND LEAKAGE TEST
WORK SHEET

Make & Year of Vehicle:_______________________ Date:_____
Engine Type:_____________ Number of Cylinders:_______
Engine Displacement:______________________________

Compression Test Procedure:

 Run the engine until it is warm (if it will run)
 Disconnect all drives to the engine
 Open choke and throttle valves wide
 Remove the air cleaner
 Remove the spark plug and insert compression gauge with correct hose attachment (matches threads of spark plug)
 Crank the engine 3 to 8 times as fast as possible to obtain an accurate test
 Release pressure and repeat test to ensure accuracy
 Record measured pressure
Note: Engines equipped with compression release camshafts may have to be cranked in reverse rotation to obtain an accurate test. Most can be cranked forward.

Results:
CYLINDER # 1 2 3 4 5 6 7 8
DRY PRESSURE PSI
WET PRESSURE PSI




Leakage Test Procedure:
 Run the engine until it is warm (if it will run)
 Remove spark plugs
 Set engine to top dead center (TDC) #1 of the compression stroke. Both valves must be closed
CAUTION: On smaller engines crankshaft needs to be secured or else pressure from the airline will cause shaft to rotate
 Connect hose to spark plug hole
 Connect pressurized gauge to hose connection
 Slowly adjust the regulated pressure gauge with the regulator **** to read 80psi
NOTE: Due to standard engine clearances and normal wear, no cylinder is expected to maintain a perfect 80psi. It is important only that all cylinders on a multi-cylinder engine have a somewhat consistent reading. Always consult the manufacturer’s recommendations.
 Do same test on rest of cylinders on a multi-cylinder engine. Remember to set TDC of each piston for compression stroke.


Bibliography:

Erjavec, J. and Scharff, R. (1996). Automotive Technology: A systems approach. Second edition. Delmar Publications Inc. Albany, NY.
Roth, A.C. (2004). Small Gas Engines: fundamentals, service, trouble-shooting, repair, applications. The Goodheart-Willcox Company, INC. Tinley Park, Illinois.


Diagrams:

Figure 1: http://weisstool.com/store/products/STR-TU30.jpg

Figure 2: http://www.snapondiag.com/snapon-gauges.asp