Compression Check; Cylinder Leakage Testing - Mercury 25 EFI Service Manual

General Information
• Corrosion of metal parts
• Deterioration of rubber or plastic parts
• Fuel permeation through the rubber fuel lines
• Likelihood of phase separation (water and alcohol separating from the gasoline in the fuel tank)
Fuel leakage is a fire or explosion hazard, which can cause serious injury or death. Periodically inspect all fuel system
components for leaks, softening, hardening, swelling, or corrosion, particularly after storage. Any sign of leakage or
deterioration requires replacement before further engine operation.
IMPORTANT: If you use gasoline that contains or might contain methanol or ethanol, you must increase the frequency of
inspection for leaks and abnormalities.
IMPORTANT: When operating a Mercury Marine engine on gasoline containing methanol or ethanol, do not store the gasoline
in the fuel tank for long periods. Cars normally consume these blended fuels before they can absorb enough moisture to cause
trouble; boats often sit idle long enough for phase separation to take place. Internal corrosion may occur during storage if
alcohol has washed protective oil films from internal components.

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 Testing

Engine Firing Order
Cylinder sequence
NOTE: Cylinder leakage testing, 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. Refer to the manufacturer tester instructions for proper testing
procedures.
NOTE: Spark plug hole is a 14 mm diameter. Use Snap‑On Tool MT26J‑200 adapter (or equivalent) with valve core removed.
Cylinder Leakage Tester
1. Remove the spark plugs.
2. Rotate the engine clockwise until resistance is felt.
3. Continue to rotate flywheel so the timing marks on the flywheel and cylinder block are in alignment. This will be the
compression stroke for cylinder #1.
4. Remove the spark plug from #1 cylinder.
5. Complete the cylinder leak down test on the #1 spark plug hole. Refer to the manufactures tester instructions for proper
testing procedures.
6. After testing cylinder #1, install a dial indicator on the next firing order sequence cylinder.
Page 1C-14
WARNING
!
Compression Test Differences
Maximum Reading
1241 kPa (180 psi)
1172 kPa (170 psi)
© 2016 Mercury Marine
Minimum Reading
1062 kPa (154 psi)
972.2 kPa (141 psi)
Snap‑On EEPV309A
90-8M0105568 eng MARCH 2016
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