Logging Your Outboard Diagnostics

Compression and Leak-Down Tests: What do they mean and when should you have them done?

Every annual service is the standard time to do a combustion chamber compression test but, there is another time and it could be the most critical.

Cylinder compression is the measurement of your engine's ability to produce pressure in the combustion chambers of each cylinder.

Leak-Down is the engines ability to maintain that pressure after it has built up.

This measurement of compression/pressure is the piston, piston rings and cylinder's ability to build a specific amount of pressure on each stroke; essential in developing full power as the fuel charge is compressed prior to ignition.

The Leak-Down test is a measure of each cylinder's sealing efficiency and ability to maintain pressure once it has been created.

On a four stroke engine, the "strokes" of the pistons occur in this order: intake, compression, power and exhaust. Every other downward stroke of the piston is a power stroke. Four stroke engines use intake and exhaust valves in the cylinder head to allow fuel/air charges to enter and exhaust gases to exit, and the sealing efficiency of these valves are also part of the equation.

On a two stroke it is the same basic process, the fuel/air charge and exhaust gases are conducted through ports or openings in the cylinder walls as the piston travels up and down the cylinder bore. Fuel charges are metered via reed valves.

Keeping good records and a history of these readings is critical when and if you have to file a warranty, extended warranty, or insurance claim for either lower-unit gearcase or powerhead failure.

Just because you impacted the lower unit and had it replaced is not a guarantee you did not also damage the top end components and these tests are not always performed unless you request them.

Problems that occur in the near future after a major component replacement are not uncommon and being thorough in recordkeeping is your best defense while your boat is in the shop, not after it leaves.

These tests and comparing them to your prior records could be the difference between winning and losing your claim with your manufacturer or insurance carrier.

Have a great Thanksgiving!

Chris Mapp
Coastal Bend Marine
Port O'Connor TX
361 983 4841