B5.9G, B5.9LPG – Troubleshooting and Repair Manual – 001-043   Piston

Overheating of the engine from a loss of coolant will cause the cylinder to overheat, resulting in seizure of the piston. Loss of piston cooling lubricating oil through blocked cooling nozzles can also lead to piston seizure.

Compression Gauge and Adapter

It is time consuming and expensive to begin removing internal engine components to diagnose failures. A compression gauge can be used as an aid to detect failures.

Compression Check

If the air and fuel system are functioning correctly, perform a compression check to determine whether the problem is:

• Piston ring sealing
• Valve sealing
• Cylinder head gasket sealing or a crack in the cylinder head.

NOTE: Due to variables such as starter and battery conditions that affect engine cranking speed, it is difficult to establish an absolute value for compression pressure; however, the following values can be used as guidelines:

Natural Gas Engines

• New engine (cranking speed at 250 rpm) 1931 kPa [280 psi]
• Used engine (cranking speed at 250 rpm) 1586 kPa [230 psi].

Liquefied Petroleum Gas Engines

• New engine (cranking speed at 250 rpm) 1655 kPa [240 psi]
• Used engine (cranking speed at 250 rpm) 1310 kPa [190 psi].

It is recommended that the compression pressure be checked and compared on all cylinders. All cylinders should be within 207 kPa [30 psi] of each other.

• Piston Ring Sealing

If the compression is low but can be increased significantly by squirting lubricating oil into the cylinder, the cause of low compression is inadequate sealing between the piston rings and the cylinder walls.

• Valve Sealing

Refer to Procedure 002-020.

• Head Gasket Sealing

Refer to Procedure 002-021.

The cause of piston ring wear can range from wear over a long period of service to a dust-out in a short period because of poor maintenance of the air intake system.

During repair, it is essential that the cylinder wall be deglazed so that new rings will seat against the cylinder wall. Failure of the rings to seat can result in high blowby and excessive oil consumption.

WARNING  Coolant is toxic. Keep away from children and pets. If not reused, dispose of in accordance with local environmental regulations.

WARNING  Do not remove the pressure cap from a hot engine. Wait until the coolant temperature is below 50°C [120°F] before removing the pressure cap. Heated coolant spray or steam can cause personal injury.

• Drain the coolant. Refer to Procedure 008-018.
• Remove the cylinder head. Refer to Procedure 002-004.

WARNING  Some state and federal agencies have determined that used engine oil can be carcinogenic and cause reproductive toxicity. Avoid inhalation of vapors, ingestion, and prolonged contact with used engine oil. If not reused, dispose of in accordance with local environmental regulations.

WARNING  To reduce the possibility of personal injury, avoid direct contact of hot oil with your skin.

Drain the lubricating oil. Refer to Procedure 007-025.

After lubricating oil is drained, install the drain plug and a new sealing washer.

Remove the dipstick bayonet.

Remove the lubricating oil pan and gasket. Refer to Procedure 007-025.

NOTE: The piston and connecting rods must be removed as an assembly.

Remove and disassemble the piston and connecting rod assembly. Refer to Procedure 001-054.

Disassemble the piston and connecting rod assembly. Refer to Procedure
001-054.

NOTE: Refer to Procedure 001-047 for piston ring inspection.

CAUTION  Do not use the bead-blast method to clean the piston. The piston can be damaged by blast material embedded in the aluminum.

Soak the pistons in cold parts cleaner.

Soaking the pistons overnight will usually loosen the carbon deposits.

CAUTION  Do not clean the pistons and rods in an acid tank. The piston skirts will be damaged if acid is used to clean them.

Wash the pistons and rods in a hot, soapy solution.

CAUTION  Do not use a ring groove cleaner and be sure not to scratch the ring sealing surface in the piston groove.

Clean the remaining deposits from the ring grooves with the square end of a broken ring.

WARNING  When using solvents, acids, or alkaline materials for cleaning, follow the manufacturer’s recommendations for use. Wear goggles and protective clothing to reduce the possibility of personal injury.

WARNING  Wear appropriate eye and face protection when using compressed air. Flying debris and dirt can cause personal injury.

Wash the pistons again in a detergent solution or solvent.

Dry with compressed air.

Inspect the piston for damage and excessive wear. Check the top, ring grooves, skirt, and pin bore.

NOTE: If severe piston damage has occurred, check the turbocharger and other exhaust components for damage from debris.

Measure the piston skirt diameter as illustrated.

Measure the pin bore.

Piston Pin – Inspection

Inspect the piston pin for nicks, gouges, and excessive wear.

Measure the pin diameter.

The piston and connecting rod must be installed as an assembly. Refer to Procedure 001-054.