Over-speeding is a cause of turbocharger failure that is fairly difficult to diagnose. The compressor speed, or boost pressure, is determined in most cases by a waste gate actuator that simply regulates the speed of the turbine by allowing exhaust to bypass the turbine wheel once a desired manifold pressure is achieved. If there is a leak between the compressor and the engine, proper pressure will be harder for the turbocharger to obtain; the waste gate will remain in the closed position until the turbocharger overcomes the leak and provides pressure within the intake manifold. Even a small leak can cause the turbo to work 20% harder than normal to reach its desired pressure, so great care should be given to the plumbing system on the pressure side of the turbocharger. Inspect all hoses and fittings for fitment and tightness, looking closely for piping that could rub and wear holes that could cause leaks. Also inspect the intercooler core and compressor, bypass valve, if equipped for proper function.
The exhaust system after the turbocharger needs regular inspection. Increased back pressure due to clogged or damaged catalytic converters, resonators or mufflers will decrease the flow out of the engine and also raise the temperature inside the turbochargers exhaust housing, which contributes to the oil coking and also raises the risk of gasket failures or cracks in the exhaust manifold and turbine housing area. Usually this only occurs in older cars, or with higher mileage cars. 10 years or 100,000 miles is an average life span of most modern exhaust systems.