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Methods and systems are provided for using a crankcase vent tube pressure or flow sensor for diagnosing a location and nature of crankcase system integrity breach. The same sensor can also be used for diagnosing air filter plugging and PCV valve degradation. Use of an existing sensor to diagnose mul

Methods and systems are provided for using a crankcase vent tube pressure or flow sensor for diagnosing a location and nature of crankcase system integrity breach. The same sensor can also be used for diagnosing air filter plugging and PCV valve degradation. Use of an existing sensor to diagnose multiple engine components provides cost reduction and sensor compaction benefits.

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1. A method for an engine, comprising: boosting intake air with a compressor;during engine cranking, while manifold airflow is lower than a threshold, increasing intake throttle opening of a throttle positioned downstream of the compressor; andindicating crankcase ventilation system degradation base

1. A method for an engine, comprising: boosting intake air with a compressor;during engine cranking, while manifold airflow is lower than a threshold, increasing intake throttle opening of a throttle positioned downstream of the compressor; andindicating crankcase ventilation system degradation based on a transient dip in crankcase vent tube pressure estimated by a sensor positioned within a crankcase vent tube following the intake throttle opening, the crankcase vent tube coupled upstream of the compressor. 2. The method of claim 1, further comprising: identifying a location of crankcase ventilation system breach based on each of an amplitude of the transient dip in crankcase vent tube pressure during cranking and a change in crankcase vent tube pressure during steady-state engine airflow. 3. The method of claim 2, wherein identifying the location of the crankcase ventilation system breach includes indicating whether the breach is at a first side or a second side and setting a different diagnostic code based on whether the breach is detected at the first side or the second side of the crankcase vent tube. 4. The method of claim 3, further comprising sending a message to notify a vehicle operator about the location of the crankcase system breach, and taking mitigating action including adjusting engine operating parameters to limit engine power responsive to the identified location. 5. The method of claim 1, wherein indicating based on the transient dip in crankcase vent tube pressure includes indicating based on the transient dip in crankcase vent tube pressure, following intake throttle opening, having an amplitude that is smaller than a threshold amplitude. 6. The method of claim 1, wherein increasing intake throttle opening includes increasing intake throttle opening to provide a threshold engine manifold vacuum. 7. The method of claim 1, further comprising, indicating degradation of a valve coupled between the engine and a crankcase based on a crankcase vent tube pressure profile following the intake throttle opening, wherein the indicating includes, indicating degradation of the valve based on an estimated crankcase vent tube pressure profile deviating from an expected crankcase vent tube pressure profile during the cranking. 8. The method of claim 7, wherein the indicating further includes, indicating that the valve is stuck in a high flow position based on the estimated crankcase vent tube pressure profile being greater than the expected vent tube pressure profile during the cranking, and indicating that the valve is stuck in a low flow position based on the estimated crankcase vent tube pressure profile being smaller than the expected vent tube pressure profile during the cranking, the method further comprising, indicating that the valve is stuck in the high flow position by setting a first diagnostic code, and indicating that the valve is stuck in the low flow position by setting a second, different diagnostic code. 9. The method of claim 8, further comprising, indicating crankcase ventilation system degradation by setting a third diagnostic code, different from the first and second diagnostic codes. 10. The method of claim 1, further comprising, in response to the indication of degradation, limiting engine boost. 11. A method for an engine crankcase ventilation system, comprising: flowing engine intake air through a turbocharger compressor;adjusting an actuator to raise an intake manifold vacuum to a threshold level;while holding the vacuum at the threshold level via the actuator, indicating crankcase ventilation system degradation based on a transient dip in a crankcase vent tube pressure estimated by a pressure sensor positioned in a crankcase vent tube coupled between an intake manifold and a crankcase, the vent tube coupled upstream of the compressor; andlimiting boost based on the indication of crankcase ventilation system degradation. 12. The method of claim 11, wherein the indicating based on the transient dip in crankcase vent tube pressure is during engine cranking, and wherein the actuator is an intake throttle coupled downstream of the compressor. 13. The method of claim 12, wherein adjusting the actuator includes increasing an opening of the intake throttle. 14. The method of claim 12, wherein the actuator is a PCV valve coupled between the intake manifold and the crankcase, and wherein adjusting the actuator includes opening the PCV valve. 15. The method of claim 12, wherein the indicating includes, if an amplitude of the transient dip in crankcase vent tube pressure during engine cranking is lower than a threshold, indicating crankcase ventilation system degradation. 16. The method of claim 15, wherein indicating crankcase ventilation system degradation includes indicating that the crankcase vent tube is disconnected from one of the intake manifold and the crankcase. 17. An engine crankcase ventilation system, comprising: an engine including an intake passage and a crankcase;a turbocharger with a compressor coupled in the intake passage;a crankcase vent tube mechanically connected to the intake passage upstream of the compressor, the tube also mechanically connected to the crankcase via an oil separator, the vent tube located external to the engine;an actuator coupled downstream of the compressor;a sensor coupled in the crankcase vent tube for estimating a vent tube air flow; anda control system with computer readable instructions for, during engine cranking, when manifold airflow is lower than a threshold flow, adjusting the actuator to raise an intake manifold vacuum to a threshold level; andwhile holding the vacuum at the threshold level, indicating crankcase ventilation system degradation based on differences between an estimated flow rate and an expected flow rate through the crankcase vent tube. 18. The system of claim 17, wherein the actuator is one of an intake throttle and a PCV valve coupled between an intake manifold and the crankcase, and wherein adjusting the actuator includes increasing an opening of the throttle or the PCV valve. 19. The system of claim 18, wherein the expected flow rate through the crankcase vent tube is based on an engine speed and an intake manifold vacuum level, and wherein the estimated flow rate is based on an output of the sensor coupled in the crankcase vent tube, the sensor including one of a pressure sensor, a flow meter, and a venturi. 20. The system of claim 19, wherein the control system includes further instructions for: setting a diagnostic code to indicate crankcase ventilation system degradation; andlimiting an engine speed and load in response to the indication.