The day-to-day demands on today's heavy-duty turbocharger are extreme: temperatures exceeding 1200° F, severe pressure differentials, clearances measured in ten-thousandths an inch and wheel rotating speeds approaching 100,000 rpm. The turbocharger is designed to perform flawlessly in these extreme operating conditions. However, there are several factors affecting turbocharger service life that, if adhered to, will assure thousands of hours of successful turbo operation.
A quick refresher course in turbocharger basics is required prior to beginning a discussion on servicing the turbocharger. The internal combustion engine produces exhaust that is extremely hot and full of energy. Turbochargers harness that heat and energy by returning it to the engine's combustion cycle.
Turbos turn waste into work. The engine exhaust drives a turbine wheel, which is connected via a common shaft to a compressor wheel, which compresses air and pumps it directly into the engine's intake system. The result is a denser charge of air resulting in more efficient combustion, which creates a cleaner exhaust (reduced emissions) and more useable power.
The turbine wheel shaft rotates in a housing located between the compressor and turbine sides which contains the turbo's vital bearing system which, along with the engine's oil supply, lubricates and cools the rapidly rotating wheel and shaft assembly. There are several different types of bearing systems used by turbo manufacturers today, but most are free-floating journal bearings which rotate approximately one-third the speed of the shaft on a thin film of oil.
Approximately 40% of all turbocharger failures are oil-related. Contaminated or dirty oil leads to bearing scratching and scoring which cause excessive bearing wear and premature bearing failure. Another oil-related turbocharger failure is lack of lubrication. As mentioned earlier, the turbo runs at very high speeds. Without full oil pressure to the turbocharger bearings, even a momentary loss of oil pressure can quickly cause overheating and destroy the bearing system.
Lack of lubrication results not only from low oil pressure, but also from kinks and/or clogs in the oil inlet line. Occasionally, gasket sealant used between the oil inlet hose and the bearing housing seeps and clogs the oil passages.
Another cause of turbocharger failures is inlet restrictions from plugged air cleaners, collapsing hose connections or undersized air pipes. These restrictions reduce the air supply to the turbo and ultimately to the engine resulting in excessive exhaust temperatures leading to turbine housing cracking and scaling or even turbine wheel failures.
Inlet restrictions also can produce a vacuum inside the compressor. This can cause over speed conditions in the turbo which can lead to premature bearing failure or even make a compressor wheel burst. This vacuum inside the compressor also can cause oil leakage into the compressor.
The turbo's oil seals depend upon a positive air pressure inside the compressor and turbine to "push" the oil inside the bearing housing and keep oil from seeping into the outer housings. In an inlet restriction situation, the vacuum wants to "pull" the oil past the oil seals. Prolonged oil leakage into the compressor can lead to oil seal damage and excessive engine smoking.
Prolonged engine idling also can cause turbocharger oil seal failure, this time on the turbine side. Continued idling causes the turbo to rotate without producing boost. Consequently, a vacuum condition on the turbine side tries to "pull" oil past the turbine-side oil seal and into the turbine housing.
Over-fueling also can lead to premature turbo failure by producing excessive exhaust temperatures which can cause turbine housing scaling and cracking. As the turbine housing continues to deteriorate from the excessive heat, pieces of the housing can crack off and cause turbine wheel failure.
Hot engine shutdown also can lead to turbocharger oil leakage by causing the oil to coke up inside the oil drain and forcing the oil out the turbine and compressor seals. A clogged or collapsed oil outlet hose also can cause oil to leak.
Another common cause of turbo failures is foreign object damage to either the compressor or turbine wheels. A rapidly rotating wheel quickly disintegrates when a foreign body tries to pass through the wheel's blades. This type of turbine wheel damage is the result of pieces of burned or broken valves and combustion cups passing through the exhaust system. Other turbine damage is due to casting flash that may break out of the manifolds and ports.
Occasionally improperly installed gaskets will allow pieces of the gasket to overhang a port and break off into the exhaust system. Damage caused by nuts and washers that are dropped into the exhaust system is also very frequent. Scuffed and broken pistons often find their way out of the engine and into the turbocharger turbine wheel.
Compressor wheel breakage also can occur from foreign object material although not as frequently as turbine wheel damage. Sometimes pieces of the air cleaner will break loose and go through the compressor. There also have been instances where hose connections fail and pieces of rubber or wire reinforcing from the hose get into the compressor wheel. Again, carelessness in allowing nuts, bolts, washers, rocks, rags and even screwdrivers to get into the intake systems will cause compressor wheel failures.
After examining the various causes of turbocharger failures, common-sense can prevent such failures in the future. Keep the engine full of clean oil to the engine manufacturer's specifications. Also keep the air filter clean and unrestricted. The duct work from the air cleaner to the turbocharger compressor should be free from holes and all connections should be tight to prevent leaks which could allow dirt and debris to enter the turbocharger.
Warm up the engine for two to five minutes prior to throttling up the engine. This procedure assures proper oil pressure to the turbocharger prior to operation under load conditions. Let the engine idle for approximately two minutes prior to engine shut down. This cool-down period prevents oil coking and oil varnishing on the turbine wheel and shaft. Varnishing is a build up of oil on the shaft which increases clearances and decreases the flow of oil to cool and lubricate the shaft.
While this may sound obvious, make sure that the turbo used is correct for the engine application. Turbochargers are precisely matched to the application and misapplication can lead to turbo failure and/or severe engine damage.
Inspect the intake and exhaust systems leading to and from the turbocharger to ensure absence of all foreign material including burrs, nuts, bolts, washers, rags and loose lining fragments. Be thorough-even small particles can cause severe compressor or turbine damage if inducted during high speed operation.
Use new and approved gaskets at the various air, oil and exhaust connections to the turbocharger. Do not use any type of sealing or jointing compounds at oil inlet or outlet flange connectors. Check manifold studs and nuts and check for obvious signs of manifold deterioration (cracking, scaling, etc.). Use a high temperature antiseize compound (such as Fel-Pro C5A) on all threaded fasteners connected to the turbocharger.
Limit drain port tilt to 20 degrees from bottom center in either direction. Tilting in excess of this amount can create a Low idle leakage tendency at both the turbine and compressor seals.
Fill the oil inlet port to overflowing with clean engine oil before connecting the oil feed hose to the turbocharger. Then, prior to mounting the turbo on the engine, spin the compressor wheel by hand to make sure it spins freely.
If the clamp tabs or V-bands are loosened for angular orientation of the compressor cover or turbine housing, be certain that the mating flanges are tightly reseated, and that the fasteners are retightened to the correct torque levels. Complete the orientation of the cover and housing before making any rigid connections to the compressor inlet and outlet, or to the turbine outlet. Then, make certain that all ducting aligns closely with the turbocharger; this will minimize the external stresses acting on the unit. Before connecting the oil drain hose, crank the engine without firing until a steady stream of oil flows from the drain port. Make sure the oil drain hose is not blocked.
Operate the engine at low idle for at least three minutes after completing the installation of any turbocharger. This will prevent oil starvation damage to the bearing systems, and will tend to purge any residual contaminants from the bearing housing prior to unit acceleration.