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Colour on shaft indicates overheating due to insufficient cooling, coolant vaporization or excessive exhaust gas temperatures.
Large chips on lower side of the turbine wheel indicate that a foreign object has passed through the exhaust venturi, while the smaller grinding on the top of the turbine wheel indicate contact with the turbine housing.
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The straw and bronze colours indicate heats between 200-260°C
The purple colours indicate heats around 280°C
The blue colours indicate heats from 300-340°C
Ideally we should not see any colour.
Engine oil operating temperature should be around 80-100°C but as seen below, temperatures have been much higher.
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The point where the turbine wheel has been touching the housing due to housing warp.
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Variation of concentrated colours suggest lots of heat on a single point.