Compression Check; Cylinder Leakage Test; Analysis - Mercury 15 EFI FourStroke Service Manual

General Troubleshooting
• Generally increases speed and engine RPM
• Causes bow to bounce in choppy water
• Increases danger of following wave splashing into the boat when coming off plane
• At extremes, can cause the boat to porpoise
Shifting weight to front (bow):
• Improves ease of planing
• Improves rough water ride
• At extremes, can cause the boat to veer back and forth (bow steer)

Compression Check

1. Remove spark plugs.
2. Install compression gauge in spark plug hole.
3. Hold throttle plate at WOT.
4. Pull on the recoil rope or crank the engine over until the compression reading peaks on the gauge. Record the reading.
5. Check and record compression of each cylinder. The highest and lowest reading recorded should not differ by more than
15%. A reading below 827.4 kPa (120 psi) might indicate a total engine wear problem. The following chart, is not a
representation of compression values specific to the engine that is actually tested. It is only an example.
Condition
Good
Bad
6. To find the maximum allowable minimum compression reading difference, use this formula; highest compression reading x
0.85 = the lowest allowable difference. 1241 kPa x 0.85 = 1054.85 kPa (180 x 0.85 = 153 psi).
7. Compression check is important because an engine with low or uneven compression cannot be tuned successfully to give
peak performance. It is essential, therefore, that improper compression be corrected before proceeding with an engine
tune‑up.
8. Cylinder scoring: If powerhead shows any indication of overheating, such as discolored or scorched paint, visually inspect
cylinders for scoring or other damage as outlined in Section 4 ‑ Powerhead.

Cylinder Leakage Test

Testing for the amount of cylinder leakage, along with compression testing, can help the mechanic pinpoint the source of a
mechanical failure by gauging the amount of leakage in an engine cylinder.
NOTE: Refer to the manufacturer's instruction procedures included with the tester.
NOTE: The spark plug hole is 12 mm in diameter. Use Snap‑On adapter with valve core removed.
Cylinder Leakage Tester

Analysis

Due to standard engine tolerances and engine wear, no cylinder will maintain 0% leakage. It is important the cylinders have a
somewhat consistent reading between them. Differences of 15 to 30% indicate excessive leakage. Larger engines tend to have
a greater percentage of cylinder leakage than smaller engines.
If excessive leakage is present, first check if the piston is at top dead center of its compression stroke. Leakage will naturally
occur if the exhaust or intake valve is open.
To determine the cause of high percentage leak, locate the source of the air leak. Listen for air escaping through the throttle
body, adjacent spark plug holes, exhaust pipe, or the crankcase oil fill plug. Use the following table to aid in locating the source
of cylinder leakage.
Air Escaping From:
Throttle body
Exhaust system
Crankcase oil fill plug
Adjacent cylinders
Page 1E-4
Compression Test Differences
Maximum Reading
1241 kPa (180 psi)
1172 kPa (170 psi)
© 2018 Mercury Marine
Minimum Reading
1062 kPa (154 psi)
972.2 kPa (141 psi)
Snap‑On EEPV309A
Indicates Possible Defective:
Intake valve
Exhaust valve
Piston or rings
Head gasket
90-8M0125265 eng NOVEMBER 2017