Turbos (short for turbocharger) are attached to gasoline or diesel engines to improve performance. The turbo is a compressor used to squeeze more air into each piston than could be drawn in with only atmospheric pressure. The turbo has two sets of spinning blades connected with a shaft. One set of blades is set in the engine's exhaust and the other in the air intake manifold. The exhaust gases spin the blades to power the compressing action produced with the second set of blades. Turbos spin at several thousand revolutions per minute and can fail for several reasons.
When an engine starts producing blue exhaust (actually a blue tinge in white exhaust smoke) it means the engine is burning excessive lubricating oil. The turbo shaft is well lubricated and if the turbo fails, the oil will enter the air intake. The oil will then enter the cylinder where it will burn along with the fuel and change the exhaust colour and also the smell.
Turbos fail when the bearing seat for the shaft seizes or simply wears out with time and use. The failed seat will then allow oil to leak and can indicate serious turbo problems.
Many machines (cars, trucks, heavy equipment, propeller aircraft, etc.) equipped with turbocharged engines will have a gauge indicating turbo boost. If the boost begins to wane then the turbo is malfunctioning. Whether the turbo shaft, compressor or drive blades are damaged will not show on the gauge but when turbo boost pressure is reduced then airflow through the turbo is restricted.
The turbo boost indicator could also reveal an obstruction or leak in the airflow manifold. Whichever the cause, however, the turbo is not performing at peak efficiency and will require service.
Turbos operate off of the engine's exhaust gases. For the turbo to compress the air there must first be an exhaust stream to actuate the compressor blades. This is a common occurrence known as turbo lag: waiting for the compressor to inject compressed air into the cylinders and then experiencing the resulting performance boost.
If the acceleration lag grows longer and longer, however, it can indicate turbo problems. If the blades are not turning at all then it can actually lead to engine interference: instead of boosting the performance, the turbo is inhibiting airflow to the cylinders. The acceleration curve will lengthen as the turbo blade assembly rotates slower and slower. As the blades slow, the compression will lessen and result in noticeable diminishing performance.