Most engine failures happen because of maintenance oversights or driver abuse. Couple your understanding of how components are supposed to function with a careful look at telltale damage, and you can prevent recurrences.
Lube/Overheating Failures
Lack of lubrication and overheating are very much interrelated, and make for highly destructive failure modes. In the end, lack of lube causes an insufficient oil film between the crankshaft journals and bearings, resulting in rapid damage progression.
In Cummins’ Analysis and Prevention of Bearing Failures, the phenomenon is explained: “When lack of lubrication is prolonged, bearing surface temperatures rise dramatically (due to increased friction). Expansions and distortions of the bearing shells and journals occur, and the clearances are closed up. The increased severity of the metal-to-metal contact, along with the rising temperature, burns away what lubricant may be left in the journal, so temperatures are further elevated on a compounding basis.”
Smearing-displacement of a bearing’s overlay-is followed by scuffing, or deeper displacement of metal. The final stage, seizure, occurs when bearing metal melts and seizes to the crankshaft.
The bearing in Figure 1 shows heat discoloration and smearing, with significant material loss. “This was removed from an engine that lost oil pressure,” said Robert Dominick, shop foreman, Southeastern Freight Lines. “Also notice how the locating tabs have been worn away.” That, along with the telltale circumferential scoring on the outside of the bearing, indicate that it was beginning to seize to the crankshaft, and was rotating in its bore.
Lack of bearing lubrication often is caused by low oil level. However, a plugged oil passage, inadequate bearing clearance, oil dilution by fuel or a fast, dry start after a prolonged period of disuse also can be responsible. If oil dilution is the cause, rod and main bearings likely will show similar amounts of accelerated wear.
In the case of low oil level, inadequate clearance or dry start, rod bearings usually will be more heavily damaged, since main bearings are lubricated first.
