To determine if a defective air compressor is causing excessive oil consumption, perform the following:
1. Perform a crankcase pressure test and record the test results.
2. Disconnect the air outlet line from the air compressor;
3. Repeat step and record the test results.
4. Compare the results of test one with test two.
If the engine crankcase pressure remained the same, check the turbocharger;
If the engine crankcase pressure decreased, replace the air compressor