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.
A plugged oil passage, on the other hand, will produce localized damage, and may be mistaken for a manufacturing or installation defect.
Like bearings, pistons are affected in varying degree, depending on how long they’ve been subjected to inadequate lubrication. However, since they do not operate in as oil-rich an environment, pistons are more temperature-sensitive than bearings. So, even if normal lubrication is provided, overheating can quickly cook away the oil film between the piston and the liner. The result is similar to that produced by lack of lube-heat discoloration or scuffing, as shown in Figure 2.
According to Dave Strauss, service training instructor, Volvo Trucks North America, “Piston,
ring and liner scuffing can occur in varying degrees or progress to a complete piston seizure. This may be the result of inadequate piston cooling, oil flow or, most commonly, cooling system overheating.”
One more place to look for evidence of no-lube operation is the turbocharger. Heat discoloration, accompanied by damage to the shaft, especially at the turbine end of the bearing journal, is a good indicator of this condition.
Another is when turbo bearing oil holes have a smaller diameter on the inside of the bearing than on the outside. That’s because, lacking lubrication, they are smeared partially shut from direct contact with the turbine shaft.
Cooling System Failures
A common cooling system failure is cavitation erosion of cylinder liners. It begins as mild-looking corrosion, “usually in the areas of highest coolant flow against the liner,” said Strauss. Here, air in the cooling system is the culprit, aggravated by the vibration of the liner caused by the movement of the piston. The bubbles nesting on the liner implode violently, removing a little metal as they do so, as illustrated in Figure 3. Eventually, this can bore through the liner and cause severe engine damage.
Figure 4 shows a liner well on its way to total failure as a result of cavitation erosion. Good cooling system maintenance-using the right antifreeze at the right concentration, and keeping up proper levels of recommended additives-prevents this process.
In an improperly maintained cooling system, corrosion can strike virtually anywhere. The radiator section in Figure 5, for example, shows the results of a direct attack on internal metal, including solder.
This corrosion can open seams, causing coolant loss and overheating. Also, corrosion byproducts, often in conjunction with old, unstable corrosion inhibitors, can plug radiator and cooling system passages, again resulting in overheating.
Contamination
Contamination, either in engine oil or in the intake air, can cause failures that wear disguises. For example, consider coolant leaking into a crankcase through a blown gasket or O-ring, or a cracked block, head, liner or oil cooler.
Once in the oil, glycol in the coolant quickly coats sliding surfaces with a varnish that reduces heat transfer and increases friction. If not discovered in time, the varnish is baked away by increased friction and temperature, and the result often is a bearing or piston seizure that looks like it was induced by lack of lubrication.
This makes glycol detection one of the best arguments in favor of regular oil analysis.
Abrasives constitute the most common oil contaminants. These can be wear particles from the engine-tiny pieces of iron, aluminum, etc., or core sand. They also can be globs of particulate matter from combustion. While it’s intended that these particles be caught by the oil filter(s), plenty of them are small enough to pass through the filter, or are included in bypass oil during a cold start or in the event of a plugged filter.
Look for evidence of abrasion on any wearing surface. In bottom-end bearings, look for circumferential scratches or gouges, depending on particle size. This grooving reduces bearing area and film thickness, increasing the likelihood of metal-to-metal contact. (See Figure 6.)
Here again, it’s possible for evidence of this abrasion to be overshadowed by a more obvious failure, such as a thrown connecting rod. That can happen when the abrasion eats away the close tolerances between the rod and crankshaft. Then, what should be controlled loading becomes thousands of impacts per minute. The parts hammer away at each other until something breaks.
Contamination in intake air leaves its own, unique footprints. Most telling are straight, axial, cylinder liner abrasion and stepped wear beginning at the top of the top ring’s travel. This, coupled with wide, flattened rings in relatively young engines, points to inadequate air filtration. (See Figures 7 and 8.)
According to Strauss, it could be the result of improper or too frequent air filter replacement-remember, an air filter gets more efficient with use-or loose air hose clamps, allowing the engine to suck unfiltered air. In this case, the first noticeable symptoms could be excessive oil consumption and low compression, with attendant loss of power.
The Driver
Easily the most common form of driver abuse is excessive idling. Not only does it waste fuel and create unnecessary engine wear, but it has the potential to be downright catastrophic. When an engine idles for a long time, it cools down, and combustion becomes less complete.
“Unburned fuel gets deposited on exhaust valves,” according to Strauss, “and migrates up between the valve stem and guide, leaving a sticky varnish on both.” The exhaust valves in Figures 9 and 10 are the victims of fuel varnishing caused by excessive idling.
“It’s more likely to happen in cold climates and on older, mechanical engines,” Strauss said. “But it can happen on any engine that’s idled too long. I’ve seen it get so bad that the valve sticks in the open position, and a piston hits it. That’s when you’ve really got trouble.”
In summary, having an engine failure is never a pleasant experience. But if it can teach a valuable lesson in maintenance or driver training-and help prevent future failures-the loss will not have been in vain.
