Cylinder compression testing is a valuable tool that can reveal a great deal about the internal condition of the engine. This is especially true of older engines, or engines with a questionable maintenance history. If components such as piston rings and valves are doing their job, compression readings will be up to specification and consistent across all cylinders. A cylinder that is more than 25% below the highest compression reading indicates a problem.
A number of variables affect the results of a compression test, they are:
1. Engine Temperature
2. Barometric Pressure
3. Ambient Temperature
5. Cranking Speed Of Motor (typically affected by starting battery voltage)
| During a compression test all spark plugs are removed to allow the engine to crank over at a speed well above normal cranking speed. This increased cranking speed ensures that all cylinders reach their maximum pressure and also reveals how many compression strokes it takes to get there. For a cylinder that takes two strokes to reach maximum pressure, leakage past valves and rings is likely to be low. However a cylinder that requires five or more compression strokes is probably leaking. |
In the marine environment, cranking speed of the engine is usually the most problematic in terms of conducting a compression test. A marginal cranking battery can be quickly run down; this is especially true if the vessel is designed with a dedicated starting bank and twin engines.
In addition to a run-down battery, other factors will determine if a compression test is feasible for your vessel. Inboard engines are sometimes installed so close to hull structures or to another engine (in the case of twins) that removal of the spark plugs is extremely difficult or impossible without some disassembly.
Finally, since the test involves open pistons and the potential for escaped fuel, proper ventilation is a must to conduct a safe test.
Because of the variables involved with conducting this test, I will be unable to advise you if it feasible until I have actually seen the vessel.
The fee for this test is $150.00 per engine tested. For a complete breakdown of pricing please view the Pricing Schedule page for more information.