Mercury MERCRUISER, Service Manual, Page: 284 / 566

Compression Gauge Testing
IMPORTANT: Use a fully charged battery during this testing procedure.
IMPORTANT: Compression test failures generally indicate a serious engine malfunction has occurred. The engine should be
repaired prior to being returned to service.
IMPORTANT: Follow the instructions provided by the compression test gauge manufacturer. Use the correct threaded adapter.
1. With the engine cold, verify that the battery is fully charged.
IMPORTANT: Ground the ignition coil to distributor cap wire directly to engine ground to prevent spark from the
disconnected spark plug wires. A remote starting tool may energize the ignition coil through the "R" terminal of the starter
solenoid even though the key and lanyard stop switches are in the OFF position.
2. Disable the ignition system.
3. Remove all of the spark plugs.
4. Temporarily stop the throttle lever in the WOT position to hold the throttle plate completely open.
5. Install the compression gauge to the cylinder being tested.
6. Zero the compression gauge.
7. Crank the engine, cycling the test cylinder through four compression strokes.
8. Record the compression test results for each cylinder.
Compression Test Diagnosis
Minimum compression
690 kPa (100 psi)
Compression difference
(The lowest compression cylinder should be no less than 70% of the highest compression cylinder.)
70%
NOTE: Example: If the highest compression cylinder has 150 psi (1035 kPa) of compression, the lowest allowable
pressure for any other cylinder would be 105 psi (725 kPa). (150 × 70% = 105 psi [1035 × 70% = 725 kPa]).
9. Repeat until all the cylinders have been tested.
10. The interpretation of low cylinder compression test results can be aided by injecting three squirts of oil from a pump style
oil can into the test cylinder combustion chamber and rechecking compression.
Possible Cause Test Result Further Testing
Normal
Compression builds quickly and evenly in
each cylinder.
None.
Leaking piston rings
Compression is low on the first stroke,
increases on the following strokes but does
not reach normal compression.
Inject three squirts of oil into the cylinder and retest.
Compression should improve significantly.
Leaking valves Compression is low throughout the test cycle. Inject three squirts of oil into the cylinder and retest.
No effect.
Leaking head gasket Two adjacent cylinders have low
compression.
Inject three squirts of oil into both cylinders and
retest. No effect.
Leak‑Down Testing
Additional diagnosis of low compression cylinders can be performed using pressurized air to identify the source of combustion
chamber leaks.
1. Rotate the engine by hand until the number 1 cylinder is at top dead center (TDC) of the compression stroke.
2. Identify and note the engine firing order.
3. Rotate the crankshaft 90° to position the next cylinder in the firing order at TDC. Repeat until the first targeted test cylinder
is positioned at TDC.
NOTE: Example with a firing order of 1‑8‑4‑3‑6‑5‑7‑2: To test cylinder number 3, rotate the engine an additional 270° (3 ×
90°) to position the fourth cylinder of the firing order, cylinder number 3, at TDC of its compression stroke. To continue the
test and check cylinder number 6, rotate the engine an additional 90°.
4. Thread the air supply adapter into the spark plug hole.
IMPORTANT: The air supply must be regulated to provide 138–206 kPa (20–30 psi) of air pressure. Use the appropriate
compression‑gauge style threaded adapter to introduce the pressurized air into the test cylinder.
IMPORTANT: Do not rotate the engine by turning the crankshaft balancer bolt.
5. Hold the crankshaft balancer bolt with a large wrench to prevent the crankshaft from rotating when applying air pressure to
the cylinder.
IMPORTANT: The engine will try to rotate when air pressure is applied. Maintain a firm grip on the wrench to prevent
unexpected motion.
Engine Troubleshooting
Page 3C-12 © 2016 Mercury Marine 90-8M0099748 eng DECEMBER 2015