Test and record the crankcase blowby after approximately 250 hours of operation. Continue to periodically test and record the blowby. Comparing the data over time provides information about the condition of the engine.
Crankcase blowby is one of the three factors that help to determine the major overhaul interval. For more information, refer to this Operation and Maintenance Manual, "Overhaul (Major)".
After a new engine is used for a short time, the blowby can decrease as the piston rings are seated. The blowby will gradually increase as the piston rings and cylinder liners show wear. More fumes are exhausted through the crankcase breather. The crankcase breather will become clogged more rapidly. Excessive crankcase pressure can also cause oil leakage at the gaskets and the seals.
The blowby of a worn engine may exceed the blowby of a new engine by two times or more.
Note: A problem with the piston rings causes the oil to deteriorate rapidly. Information regarding the condition of the piston rings can be obtained from the measurement of the blowby and the results of oil analysis.
A sudden increase in blowby could indicate a broken piston ring. This condition will cause the engine to run rough. The following conditions are other potential sources of blowby:
- Worn valve guides
- A turbocharger seal that leaks
A rebuilt engine can have a high blowby due to the following factors:
- The piston rings are not seated properly.
- Worn parts such as valve guides were not replaced.
By keeping a record of the results, a gradual increase in the amount of the blowby will be noted until the amount has become excessive.
To measure the blowby, use the 348-5430 Multi Tool. Refer to Special Instruction, NEHS1087, " 348-5430 Multi Tool Operating Manual". The instruction is provided with the tool.