177. On small diesel engines, a noticeable decrease in the time interval between the replacement of the lube filter cartridge indicates __________.

Increased frequency of lube filter replacement indicates piston ring blow-by. Combustion gases and soot entering the crankcase contaminate the oil, overloading the filter. Piston ring blow-by is leakage past worn rings, introducing contaminants that accelerate filter plugging. Other options—dirty air filter, excessive oil temperature, and excessive oil pressure—do not directly cause rapid filter clogging due to combustion contamination.