Like all automotive components, turbos wear. Also like all automotive components, proper maintenance can go a long way toward extending the life of a turbo. Regular oil changes with synthetic oil, ensuring there are no kinks in turbo oil lines, using a turbo timer and running a properly sized downpipe and blow-off valve are a few of the most popular turbo life extension techniques.
Too often the sound of a blow-off valve overshadows its true value. The blow-off vavle relieves pressure when the throttle is closed, which protects against compressor surge. This surge occurs when boost dead-ends against the throttle plate and backtracks into the compressor housing where it contacts the wheel and pushes against the flow of air. This phenomenon can damage bearings, torque the shaft, or cause oscillation of the rotating assembly. Unchecked, surge leads to thrust failure at the bearings and possible imbalance, as the nose nut is backed off the quill on right-hand-threaded wheels. As the nut loosens, there may well be wheel/housing contact, which is bad news.
Sometimes a turbo can fail because of a cataclysmic event or it can merely suffer through a long, slow death. The question is, how can you identify and address a compromised turbo and what are the consequences of inaction?
Here we see a classic bearing...
Here we see a classic bearing failure. Note the etching around the base of the wheel. Since the shaft snapped in two, this was a catastrophic failure to the Nth degree.
Looking at a cross section of the symptoms you'll most likely experience reveals a loss of max boost pressure, noises from the turbo, an increase in oil consumption, fouled spark plugs and the tell-tale excessive exhaust smoke.
To diagnose a failing turbo as the cause of the power/boost loss, other possibilities must be eliminated.
Start with checking full-throttle boost to ensure the turbo is generating the usual, expected max boost. If there's a discrepancy, don't tear off the turbo just yet.
Excessive backpressure can keep a turbo from attaining full boost. The most likely culprit is a clogged catalytic converter. Check that. If it's in working order, move to the wastegate.
The FOD Squad is out in force....
The FOD Squad is out in force. FOD, or Foreign Object Damage, is the leading cause of wheel failure. When damage occurs, bits of the wheel can be blown through the intercooler and into the valvetrain or all the way into the cylinders. That would be bad.
The wastegate may not be closing all the way under aggressive throttle conditions, which allows exhaust gases to vent around the valve, robbing the turbine wheel of vital flow and keeping it from realizing full-boost potential. To confirm this scenario, the wastegate will need to be examined.
For an internal gate, the linkage can be manipulated to check for proper closure and to detect any resistance or binding in its articulation. If you're unsure about the results, you may need to test with the turbo off the engine.
External wastegates will have to be removed. In their static state, external wastegates are in the closed position, so flip the unit over and see if the wastegate/flapper valve is fully sealed. The actuation of the wastegate can be observed by pressurizing the unit with compressed air. But beware: These units can't take the 100 psi plus generated by conventional shop air compressors. It would be wise to limit pressure to about 10 to 15 psi; this should be enough to observe the unit's operation without damaging the diaphragm.
Here we see a severely damaged...
Here we see a severely damaged compressor wheel. FOD doesn't get much uglier than this. What often happens is FOD causes an imbalance, which worsens with time. Eventually the bearings allow wheel-to-housing contact, which results in a transfer of metal between the wheel and housing. This accelerates the rate of imbalance and speeds the wheel toward its ultimate demise, unless you see the warning signs and catch it before failure.
Vacuum leaks are another possibility. This line of thought rests on the assumption the turbo is making the boost but it's being lost somewhere in the intake tract. This can be easily determined with a boost gauge that also reads vacuum. Typically, an engine should produce 16 to 22 inches of vacuum at idle. If significantly less is observed, there's a backpressure problem or a leak in the intake system, either beyond the throttle plate or by way of loose vacuum lines tapped into the manifold, etc.
If these efforts don't net any results, then the problem is most likely with the turbocharger. The first item to check is the shaft bearings. Remember, these wheels can spin in excess of 120,000 rpm so any imbalance or play can have grave consequences. A majority of wheel assemblies are balanced to .001 ounces; that's a high tolerance.