Mercedes C Klasse Hybrid Fuel Economy Problem

Your observations regarding increased fuel consumption, the check engine light, and a hissing sound during acceleration strongly suggest an air intake leak. The intake hose is a prime suspect, but a thorough leak detection procedure is advisable. Have you inspected the air filter housing and the connection to the turbocharger? Pay close attention to any cracks or loose clamps on the intake manifold. A smoke test, performed by a qualified technician, is an effective method for pinpointing the exact location of the leak within the system. Also, checking the MAF sensor readings could provide corroborating data regarding airflow irregularities. Do you have access to an OBDII scanner to read the specific error codes?

I have visually inspected the air filter housing and turbocharger connections, but a smoke test is beyond my current capabilities. I retrieved the following code using an OBDII scanner: P0171 (System Too Lean, Bank 1). Does this further implicate a leak within the air intake tract, or could it point to other potential causes, such as a malfunctioning mass airflow sensor or issues within the fuel delivery system?

The P0171 code certainly strengthens the case for an air intake leak, particularly in conjunction with the hissing sound. While a faulty MAF sensor or fuel delivery issue could also trigger this code, the presence of the audible leak strongly suggests that as the primary culprit. Given your description, it is highly probable that the intake hose is indeed the source. However, without more advanced diagnostic tools, conclusively pinpointing the leak remains challenging. Considering the potential for consequential damage to the engine, especially the turbocharger, I advise seeking assistance from a qualified workshop for a professional assessment and repair.

Based on the totality of evidence, I have decided to proceed with replacing the intake hose. The repair was performed at a cost of approximately 100€, and the symptoms have gone away. Fuel economy has returned to nominal levels, the check engine light is no longer illuminated, and the hissing sound is absent.