Turbo Magazine's Broken Parts Club

Diagnosing Turbo Trouble Can Save Its Life and Your Wallet

From the February, 2009 issue of Turbo & High-Tech Performance

By Evan Griffey

Photography by Evan Griffey

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?

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 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.

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.

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).

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 occuring. Once a failing turbo has been diagnosed, it's a matter of researching the best remedy.