Drop...grenade! The turbine...
Drop...grenade! The turbine wheel on the left was severely over-boosted and suffered a failure on a molecular level. To the right is what the wheel should look like.
As the bearings deteriorate, the wheel will develop a wobble which, at some point, will cause the blades to contact the housing's inner wall. This friction prevents proper spool up and hinders full-boost performance. Additionally, with the scraping there's a transfer of material between housing and wheel, which leads to more imbalance. This type of ailment is also a precursor of turbo failure, which could have catastrophic consequences to items well beyond the turbo.
Any evidence of scraping means the turbo must be rebuilt. The compressor wheel can be spun by the nut and any resistance or rubbing should be easily noticeable; you can expect some kind of audible evidence by this point as well. Bearing clearances can be more precisely checked with a dial indicator and this can be done with the turbo on the engine. Place the dial indicator against the shaft hub. Shaft movement should be no more than .003 to .006 inches (up and down) while end play should be no more than .001 to .003 inches (in and out).
The story here is FOD and...
The story here is FOD and a failing bearing that allowed oil to get around the bearing and be compressed with the charge air. As this assembly deteriorated, oil residue, which accounts for the discoloration of this wheel, would've been evident in the compressor housing, in the outlet pipe, in the intercooler and finally in the form of black smoke emanating from the tailpipe.
Turbo wheel balance can also be affected when the wheel shows signs of chips, or eroded, bent or missing blades. This change in dynamics is easily enough to put the entire assembly at risk. The cause is typically foreign matter entering the compressor, known as Foreign Object Damage or FOD. Generally, smaller debris causes sandblasting erosion or chipping damage to the inducer portion of the compressor wheel, while bigger debris and possible blade-to-housing contact causes bent or broken blades. Never try to straighten a bent wheel; this will fatigue the metal and if the piece breaks off, problems could arise with the valvetrain or cylinders, which means a big repair bill.
Smoking problems can, in some cases, be traced back to the bearing shaft oil seals. A leak here enables the pressure/vacuum generated by the compressor to pull the oil around a failing bearing. Oil residue in the compressor housing and intake pipes will be a warning sign of this ailment.
When approaching the repair of an injured turbo, always remember the the downside. Any wheel with any damage should be replaced expeditiously. The cost of a wheel is $60 to $120. If needed, a wheel and shaft can be replaced at a cost of around $200. Research this portion of the process carefully. It may make more financial sense (as well as getting more peace of mind) to buy a new cartridge. This way you get new compressor and turbine wheels and new bearing shafts; all pre-assembled and balanced.
The key is identifying the symptoms and catching a failing turbo before something "catastrophic" happens. Vigilance will save money and downtime and keep engine damage from occurring. Once a failing turbo has been diagnosed, it's a matter of researching the best remedy.
Bent blade damage
Oil Starvation bearing da...
Oil Starvation bearing damage