Turbocharger failure analysis is a science unto itself. Performing a failure analysis on a turbocharger is a valuable endeavor regardless of the application. Turbos are applied to everything from commercial diesels to street performance vehicles and professional competition vehicles. One of the values of this section for the turbo enthusiast or turbo car owner, even if you do not intend to actually perform turbo failure analysis, is to understand what conditions will fail a turbocharger. This will allow you to own, operate, and maintain your turbocharged vehicle with greater success.
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If your engine is turbocharged, the turbo itself becomes the heart of the engine. All of the engine’s intake air enters through the compressor, while the turbine end sees all of the exhaust, and the engine’s lubricating oil runs through the bearing system. A properly performed turbocharger autopsy can reveal a great many things about the turbo system, the engine’s overall condition, its maintenance routine, and even issues regarding the quality of the turbo and supporting turbo system dynamics.
Many who sell and service turbochargers cannot perform an accurate failure analysis. This includes most auto parts stores and heavy truck dealers. However, many specialist independent turbocharger distributors will employ at least one senior technician trained in failure analysis. To many people, the very concept of telling which part of the catastrophic failure was the cause is extremely puzzling. However, just as medical experts can perform an autopsy on a cadaver to determine cause of death, a trained technician can analyze the turbocharger’s components and discover the likely cause of failure. In this way, failure analysis can be invaluable to help correct the conditions and therefore avoid a repeat failure.
While there are many types of turbocharger failures and reasons for these failures, the law of 80/20 applies as it does to most statistical situations. About 20 percent of the failure reasons cause 80 percent or more of the failures that occur. While there are certainly some very confusing failures that can be puzzling for even the most experienced turbo technician, most reasons for failure can be conclusively determined so that corrective measures can be taken.
This chapter is dedicated to the diagnosis of turbocharger failure, and more importantly to the interpretation of these failures, and how to apply the findings and rectify the situation. While most commercial failure analysis reference manuals apply themselves to commercial diesel applications, this section uniquely takes performance applications into account as well, forming a reference for both the performance enthusiast and turbo service industry professional.
Understanding Turbocharger Failure Analysis
The key to understanding turbocharger failures and making accurate diagnoses involves understanding basic engine operation theory, turbocharger operation theory (including how the components interact with each other and to their environment), and a firm grounding in common causes of what can and does fail turbochargers under various operating conditions.
Similar to a medical diagnosis, the turbocharger has two primary ways to potentially determine its cause for failure. In medicine, a practitioner attempts to use observed symptoms to look for a specific bug that can be isolated. Once the problem is found, the correct remedy can be applied. When a condition exists in a patient where no specific bug can be found, diagnosis is done by exclusion. In other words, you use the process of elimination. You eliminate everything that did not cause the failure, until you’re left with what did.
Turbochargers can be similarly treated. The typical way to perform a turbo failure analysis is to begin by examining the complete unit before it’s disassembled to look for obvious signs of trouble, and then systematically analyze the components as it’s disassembled. Frequently, an obvious series of clues will reveal themselves and certain telltale signs will be evident, similar to isolating a bug. But in a fewer cases, there may not be obvious signs of exactly what happened. Therefore certain modes of failure must be assumed and the rest of the parts reviewed to either confirm or deny each assumption, until one can be selected. Several failure causes could be assumed in this manner until the most logical reason is determined. For this reason, all of the information surrounding the application, use of the engine, the type of turbo system, and a description of what was happening at the time of failure are all of great importance in helping to determine the probable cause.
For the most part, there are four basic and common reasons for turbocharger failure. These causes are responsible for 80 percent of all failures. They include:
1) Foreign object damage (FOD)
2) Contaminated lube oil
3) Lack of lubrication (including coking and sludge buildup)
4) High exhaust temperatures
While these four reasons account for most all failures, there are many more reasons to discuss. It’s possible to make the mistake of looking at one failed component and draw an early conclusion incorrectly. This often happens when a failure begins in one place, moves on to the next part and the next, until there is significant damage throughout the turbocharger. Then determining which came first can be the difference between a correct and an incorrect determination of cause. The objective for this chapter is to help teach the fundamental approach to correctly determine the cause for failure so that corrective measures can be taken to avoid unnecessary repeats. While there will undoubtedly be failure modes not addressed in this chapter, we’ll get to upwards of 90 percent, making this as comprehensive a guide as presently exists on the open market.
Beginning the Analysis: The External Examination and Notation
Once the turbocharger is removed or received from a customer, take a careful look at it from all angles. Don’t be too quick to disassemble the turbocharger before you have reviewed all of the external clues that may exist. Look to see if there are any obvious contaminants or obstructions in the oil inlet. Has either end housing been saturated in oil, showing an obvious excess of engine oil leakage? Use a probe such as a small screwdriver with a clean white rag to wipe into the oil inlet and oil drain cavities to inspect for any obvious dirt or abrasives present.
Look into the compressor inlet searching for signs of foreign objects that may have been ingested. The leading edge of the compressor blades, or “inducer”, will show impact traumas if the compressor swallowed a foreign object. Further, there may also be obvious pockmarks around the compressor cover inducer area leading into the compressor wheel where an object bounced around for awhile before finally entering the wheel and failing the turbocharger.
Does the turbine wheel spin freely, or is it locked up or broken? Has either the compressor wheel or turbine wheel broken apart or broken loose from its shaft connection? Are all the parts there and intact, but the compressor wheel nut loose and backed off? If the turbine wheel and shaft assembly spin freely, and if thrusting each wheel back and forth do not produce excessive end play (typically 0.002–0.004 inch) then the turbo may or may not be failed. If the cause for removal was due to an oil leak, remember that most turbos do not have positive oil seals, but rather use dynamic seal rings that seal the pressurized boost and turbine gases from entering the engine’s crankcase. If either end housing is wet with oil, or the rotor spins freely and appears to have proper end thrust signifying an intact bearing system, the turbo may not be failed at all. Refer to the troubleshooting guide at the end of this chapter where it discusses excessive crankcase pressure and its causes. Treat the turbo carefully as it may be capable of returning to service. Turbo removal due to oil leakage is very common, but oil leakage is very often a symptom, not the cause.
Begin by removing both end housings. This can be done by removing either the bolts that tighten the clamp tabs that hold the turbo together, or by loosening the V-band clamps. Once the housings are removed, the remaining assembly is called the CHRA or cartridge. If FOD (foreign object damage) is the cause of failure, it will be obvious at this point. Foreign object damage is a conclusive diagnosis at this point and is perhaps the easiest and quickest to determine. If that’s the case, no further disassembly is really necessary at this point. However, look at the turbine side carefully if this is the side where damage is seen. There are failure modes that can appear similar in nature to FOD on the turbine end inducer. Become familiar with the characteristic differences between an impact trauma and over speed or over temp failures to turbine wheels. This will be discussed in greater detail.
If the compressor wheel inducer shows damage, the object may or may not be found in the intake system. If the engine is equipped with a charge-air aftercooler, the foreign object, or parts of it, will most likely be located in the cooler. Many an aftercooler has saved an engine from critical damage by acting as a catch net for foreign objects that got ingested into a turbo’s compressor. The small tubes and turbulators present in most aftercoolers will act as a filter of small particles, preventing them from entering the engine to cause even more damage. For this reason, it’s arguably not necessary to remove the cooler at all. However, it is advisable to remove the cooler to attempt a reverse flush to dislodge any remaining parts that may be found. This not only removes the potential for engine ingestion, but also aids in the more conclusive diagnosis as to the cause of failure. Of the many parts that find their way into turbo compressors the following are some of the most common:
- A threaded fastener, nut or bolt, that was inadvertently dropped.
- A mechanic’s shop rag stuffed into the air intake to keep it clean.
- A wrench or other tool left in the intake system.
- Parts of a failed air cleaner (a filter that is too small or extremely dirty may cause this).
- Parts of a previously failed compressor wheel due to a fatigue or wheel burst that were not adequately cleaned out during service.
- Rock(s) and/or other airborne abrasives allowed to enter due to a missing or failed air filter that distorted thus allowing unfiltered air to enter the intake air stream.
In rare cases, and on non-aftercooled engines, a small object like a 1/4-20 nut can be ingested through the compressor, parts of which pass through the engine and impact the turbine wheel as well, but this is rare. In such cases, it’s most probable that some degree of engine damage will also have occurred, the least of which would be a bent valve. Foreign objects will tend to cause somewhat even damage around the wheel and metal will shows signs of impact and tears or obvious lines from the rubbing that took place.
If the compressor wheel inducer shows no sign of foreign object damage, but the turbine side inducer has impact traumas on the blade tips, it can be a sign of internal engine damage that resulted in turbo failure, but not always. Valve or piston fragments are possible turbine wheel eaters. These small fragments will commonly not be found because they pass through the wheel as they cause damage and end up downstream of the turbine housing and lodge somewhere in the exhaust system. In such a case as a failed valve, the driver will likely have noticed poor performance from the failed valve causing a dead cylinder prior to even worse performance once the turbo failed. In the case of a performance vehicle such as a drag car or tractor puller, these incidents happen so close together that they cannot be differentiated.
Foreign objects that have not passed through the engine can enter the turbine. Just as in the compressor, finding parts of what passed through the turbine also aids in a more conclusive diagnosis and helps in figuring out exactly how the part got there to begin with. In some instances, the In this case, it’s important to inspect the exhaust manifold or header system for objects that may still be inside. The following are some of the parts that can cause turbine wheel inducer failure.
- Valve fragments
- Piston fragments
- A shop rag left in exhaust manifold during service
- Threaded fasteners that inadvertently dropped into the exhaust manifold
- Heat baffles or other fragments that were a part of the exhaust manifold
Other Forms of Failure that Cause Turbine Damage
At this point it is critical to differentiate between true turbine-side FOD and other forms of turbine wheel failure that could be mistaken for FOD. These failures include overheating and overspeed, which accelerate cyclic overstress.
Overspeeding of the turbine is not typically found in most factory applications, but can be. Modifications to the fuel pump that increase fuel flow and change governor settings can cause this condition, as can setting the wastegate such that overboosting occurs for a prolonged period. Additional reasons for excessive exhaust temperatures include plugged air filters restricting inlet air, exhaust restrictions, leaks in the inlet manifold or boost tubes and hoses, or a cracked charge-air cooler (a very common problem in heavy commercial vehicles).
Compressor Wheel Overspeed & LCF
Typically, the turbine wheel will fail first in an overspeed condition. This is due to the extreme heat and the higher mass creating higher centrifugal force. However, compressor wheels do have a fatigue limit and the higher their rotating speed, the fewer speed cycles they will withstand.
When only a portion of the compressor wheel is broken off cleanly, it may be caused by compressor wheel overspeed. In rare cases a whole blade can be thrown off, separated at the root of the blade where it connects to the compressor wheel hub. If this condition is seen after approximately 40,000 to 60,000 miles, low cycle fatigue (LCF) can be suspected and is a design problem within that compressor wheel. This is a rare case because most turbochargers are cycle tested prior to production release, thereby minimizing such in-service failures.
As stated earlier, the higher the speed at which the turbo operates, indicated by higher boost pressures, the shorter its cycle life. Every time a turbocharger speeds up to build boost then slows down for even a gearshift represents one cycle. Turbochargers are designed to live in an environment where they’re tested to last at least 100,000 cycles. A fatigue failure below 100,000 cycles is considered low cycle fatigue. Cycle failures above 100,000 cycles are considered high cycle fatigue (HCF). A turbocharger in a commercial application could see this type of condition if the compressor wheel has been reused in a rebuilt turbocharger and the application uses a fairly high boost.
Normally in performance and competition vehicles such as drag cars or even Formula 1 racing, the turbos don’t last long enough to see such cycle fatigue failures. However, with the advent of ball bearings, some race teams are seeing a single turbo last well beyond a full season. The extreme speeds seen in competition vehicles could begin to cycle fatigue compressor wheels and replacement may be advisable under these circumstances.
Turbochargers, as applied to the engines of today, are nearing the limits of what the materials are capable of withstanding, especially in the area of compressor wheels. This is why many compressor wheels are no longer reused in the rebuilding process on applications known to impart high stresses on them. In order to deal with this problem, turbo manufacturers have incorporated several material and processing methods to cost effectively deal with the stresses of today’s applications.
The first method used was called HIPing, or hot isostatic pressure. This process takes the cast compressor wheels and heats them to a near molten state, then subjects the casting to high pressure to squeeze out all of the casting porosity, making the wheel stronger. Honeywell patented a process called a boreless design where the wheel bore that is drilled right through the area of highest stress concentration is eliminated (see Chapter 2). In this design, the wheel literally threads onto the turbine shaft. These wheels are HIPed as well, and are made from both castings and billet T354 aluminum. But even this design process is insufficient for some applications and the use of cast titanium is now seen in selected compressor wheels depending upon the application.
Contaminated Lube Oil
If neither wheel shows signs of impact trauma in the inducer area or other form of wheel failure, continue the disassembly. Carefully disassemble the remaining components and be cautious not to clean off the parts as you dismantle the CHRA portion of the turbo.
Lay the parts out and analyze them closely, looking at all forms of wear and heat discoloration. Remember that the number-one failure of turbochargers is contaminated lube oil. This is caused most frequently by either severe environments where there is a large amount of dirt in the air or by infrequent oil changes. In either case, the engine’s lubrication system goes into filter bypass mode where the filter assembly is so dirty that oil pressure builds up and bypasses the filter to avoid rapid and catastrophic engine failure.
These photos illustrate examples of bearing system components that have failed due to contaminated lube oil. Note that in such cases it’s likely that damage will be seen on either or both wheels, but mainly in the contour areas where the bearings have worn to such a severe degree that allows the wheels to come in contact with their respective housing. This is what caused total turbo failure and the need to remove the turbo from the engine.
The typical symptoms of contaminated lube oil include:
- Journal bearing OD & ID wear
- Thrust bearing surface wear
- Turbine shaft scoring
- Turbine and compressor wheel rub against housing
- Shaft fracture
- The causes of contaminated lube oil include:
- Poor oil filter maintenance
- Oil filter bypass systems (during cold start and when filters are clogged)
- Contaminates in oil galleries and oil lines that have aged and cracked internally
- Dirt left in engine or turbocharger after overhaul
Some of the more typical particles found in engine oil that cause this failure include:
- Cleaning shot, grit, or glass beads
- Brazing flux or globs of brazing alloy
- Sand or other airborne dirt that entered the crankcase by way of inlet air
- Metallic debris separating from other engine components beginning to fail
This type of failure is important to understand and, if possible, an oil analysis should be performed to determine whether the engine itself should be inspected to prevent severe engine damage.
Lack of Lubrication
At first, the idea of lack of lubrication can sound rather confusing. How can a turbocharger have everything connected properly and get plenty of engine oil one minute, and then all of a sudden change? Well it’s very possible due to a number of reasons. This is better understood when you consider that typically the lack of lubrication comes from a condition where the normal oil supply is interrupted by wear, foreign contaminates, or improper operation such as hot shutdown.
The following photos depict the typical conditions found in the internal parts of the turbocharger when a lack of lubrication is the root cause of failure. Just as in the case of contaminated lube oil, catastrophic turbo failure is commonly seen where one or both wheels have come in contact with their housing. The thrust bearing will show the heat build up from a lack of lubrication. If this is the case, yet the journal bearings show little sign of discoloration, then it’s quite possible that debris such as Teflon tape used to seal the oil inlet connection has come loose and lodged in the passage way that feeds oil to the thrust bearing. For this very reason, it’s a common rule that Teflon tape should never be used in oil lines that supply turbochargers. The typical symptoms of a lack of lubrication failure include:
- Severe shaft discoloration
- Journal bearing discoloration
- Journal bearings seized to the turbine shaft
- Journal bearing hammered out
- Thrust bearing wear and discoloration
- Shaft fracture, typically at the turbine end The causes of lack of lubrication can include:
- Initial run without first priming the turbo with oil, known as “oil lag”
- Poor oil filter maintenance
- Damaged or collapsed oil supply line
- Insufficient oil in the sump
- Sealing compounds such as Teflon tape blocking the oil inlet or feed line
- Sludge or coke buildup in bearing housing from hot shutdowns
- Heavy debris buildup left from overhaul clogging oil passages
- Oil passages not completely machined or broken drills lodged in oil passages from manufacturing
If you find this type of failure, use a small inspection light and a small wire as a probe to determine if all of the oil passages in the bearing housing are clear. If they are, care should be taken to make sure that engine-mounted support systems are not the cause (a broken or collapsed oil line or a leaking oil inlet gasket).
The following illustrations contain several additional photos and descriptions of failures and causes that are less typical than the previously mentioned variations of the four primary types of turbocharger failures. While this is not meant to be an all-inclusive list, these amount to most all of the remaining failure types.
Turbine Wheel LCF
Oil in Housings
Most turbos do not have positive oil seals, but instead use seal rings that primarily seal the pressurized gases from the compressor and turbine ends from entering the crankcase. Seal rings do, however, perform a secondary function of minimizing oil leaking into either housing.
Hardened sludge caused by a closed breather system (PCV valve) can create buildup that can reduce turbocharger performance. This condition is not typically seen on engines that vent the crankcase to atmosphere.
Possible Causes of Oil Leakage into Either End Housing
This is not an absolute rule, but when oil is found leaking out both ends of the turbocharger, the cause may not be found in the turbo. If the oil drain line is damaged or crimped, oil flow, which is gravity fed, may back-up into the bearing housing and build up thereby flooding the seal ring areas and make the oil deflection systems designed into the turbocharger non functional.
Engines with high crankcase pressures will pressurize the bearing cavity of the turbo. This can be caused by worn out piston rings in the engine, broken piston rings, or a hole in a piston causing what tractor pullers call the feared “death breath.” If severe engine damage is found to have caused this type of oil leakage, the turbo can normally be cleaned up and returned to service without any problems.
An overfilled crankcase will inhibit proper oil drain. Similarly, an improper oil drain location that enters the oil pan below the normal oil level can cause this same problem. This is why it is critical to be sure to locate the oil drain above the normal oil level in the crankcase.
Thrust Bearing Failure
While not particularly common, thrust-bearing failure does happen. However, it’s usually caused by some other problem. In a thrust bearing failure, the wheels will come in contact with their housings because the thrust bearings limits end play in the turbocharger. As previously discussed, contaminated lube oil with foreign matter can clog the very small oil passages that feed the thrust bearing, causing oil starvation and a lack of lube failure evident by severe heat buildup on the thrust bearing, but no particular heat discoloration on either journal bearing or turbine shaft.
If you choose a turbo where the turbine end is too small and the turbine pressure far exceeds the compressor boost pressure, a negative pressure differential can exist that will fail the turbocharger. The typical three-piece bronze bearing systems are the most susceptible to failure due to adverse pressure differentials across the turbocharger. Anyone running a smaller turbine but trying to make big boost could be in for trouble. Turbos like a power balance across both ends to function properly, as does the engine. The pumping losses from too tight of a turbine match not only rob engine power, but can also fail the turbo.
Sometimes a broken thrust bearing is paired with broken thrust rings. If the turbo match is not good or the turbo operates often in a surge condition, this can hammer the thrust bearing and break it. While a turbocharger can typically stand up to occasional brushes with surge and normal pressure differentials, frequent problems will indeed cause a failure. Typical three-piece bronze journal bearing systems will tolerate between 20 to 30 lbs max pressure differential. Ball bearings, however will tolerate much more, up to 10 times as much, which is one reason they’re so popular on drag racing applications where pressure differentials can be high.
In some drag and road racing applications, the use of anti-lag systems discussed in Chapter 8 will cause extremely severe pressure differentials on the turbine side due to the turbine acting as an expansion chamber for combustion. Drag race teams that utilize traditional three-piece bronze bearing systems in conjunction with anti-lag systems will find high consumption of turbochargers as they hammer the thrust out of the turbo in the name of lower ETs. Ball bearings tend to eliminate this problem. If this is your problem, the failure will be seen on the negative side of the thrust bearing, which is commonly misunderstood.
Failure Analysis Summary
The turbocharger is central to engine performance. Since it sees all of the engine’s operating systems, accurately diagnosing a failure becomes critical to accurate engine troubleshooting. In many cases, such as automotive competition, problems develop so quickly and consequential damage can occur faster than the driver can react. But in other instances, early troubleshooting can save a great deal of time and money.
The following chart is designed to help direct turbo owners toward areas of potential problems before they become bigger problems. The first step in keeping your turbocharged engine healthy is knowledge and understanding of how it operates and what types of situations and conditions can cause problems.
Turbocharger On-Engine Troubleshooting Guide
Written by Jay K. Miller and Posted with Permission of CarTechBooks