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Properly Diagnose Your Customer’s Engine Problems

Tuesday, October 1, 2013

Engine Diagnosis Methods. Several techniques can be used to properly diagnose your customer’s engine problems. This month’s article will cover the engine diagnosis methods including: compression tests (dry & wet) and the leak down test. These tests are straightforward yet provide valuable information useful for diagnosis.

Compression Test - This test measures the dynamic pumping pressure of the cylinder when the crankshaft is rotated. This test should be performed when the engine is warm – not cold and not hot.

Dry Test – Install the compression gauge into the spark plug hole and crank engine over 5-10 revolutions. For consistency, crank the engine over the same number of revolutions for each cylinder. If one cylinder has a lower reading this indicates the problematic cylinder. The dry test should yield readings within 10% of manufacturer’s specifications. If no compression information is available, use 100 psi as a reference test pressure. However, it does not indicate whether the cylinder or piston rings are the cause. The wet compression test is effective at determining the source of the problem.

Wet Test - With the spark plug removed, squirt about one teaspoon of 30-weight motor oil into the spark plug hole. Take a compression reading and observe the difference between the wet and dry tests.

  • Readings from the wet test should not increase by more than 10 percent, cylinder to cylinder.
  • If the compression increases with the wet test, the results identify the problem as the piston rings and/or cylinder walls. The theory behind this test is that the oil is providing a wet seal for the rings. If they are not sealing on their own they will when the oil creates a seal and an increase in compression will be observed.
  • If the compression stays the same, the results point to the valve train. The theory is that when the rings are sealing, the oil will have no effect on compression and therefore the valves are most likely the cause of the problem.

Leak Down Test - This test pinpoints specific leakage. This test uses a set of pressure gauges with a regulating device and can quantify the percentage of leakage. This is a static test that takes more time to perform compared to a regular
compression test.

The ignition system should be disabled and/or grounded to prevent shock or fire.

The engine must be at top dead center where both intake and exhaust valves are closed (compression stroke). Before pressurizing the cylinder, be sure that you can prevent the piston from moving in the cylinder (engine from turning over). Pressurize the cylinder through the spark plug hole at around 100-115psi. You will be able to measure the percentage of pressure leakage.

  • Leak from crankcase indicates the piston rings are not sealing or a burnt piston.
  • Leak from the air intake indicates a bent or burnt intake valve.
  • Exhaust leaks from the exhaust manifold or muffler indicates a bent or burnt exhaust valve.
  • Leak from the radiator indicates a leaking gasket or cracked cylinder head or block.
  • If air is leaking from the companion cylinder it indicates a blown head gasket.

Properly functioning cylinders show a cylinder to cylinder leakage of less than 8%. A greater drop should prompt you to investigate further.

Each test provides different diagnosis information and takes a different amount of time to perform. By using either method of diagnosis you will be able to provide your customers with accurate information and continue to provide the highest level of quality service available.

Information in this article has been provided from various engine rebuilding handbooks. Always use caution when performing these tests. All Industrial Engine Service does not assume responsibility for any consequences that arise out of its application.

This month’s article was written by John Gelsimino Jr. of All Industrial Engine Service. All Industrial Engine Service (AIES) was started by All Lift Service Company, a Komatsu forklift truck dealership which was established in 1972. AIES was started to end warranty problems associated with other engine suppliers and provide the dealership with the highest quality remanufactured engines available. AIES remanufactures LP and diesel forklift truck engines for companies and dealerships across the country.

If you have any questions regarding this article or any other Tech Talk article please feel free to give me a call @ 877/303-LIFT (5438). As always, stay tuned to future helpful Tech Talk articles!

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