GLC gasoline engine hissing sound and poor mileage

I had similar symptoms on my old Volvo: loud hissing, poor fuel economy, and the check engine light. It was a cracked intake hose, causing unmetered air to enter the engine, throwing off the fuel mixture and triggering the engine misfire. Have you checked the hoses visually? Look for cracks or splits near the clamps. A leak detection test can confirm it.

Thanks. Could you elaborate on the 'leak detection test'? Can I do it myself, or is specialized equipment needed? Also, how much did the repair cost in your case? I'm trying to estimate the expense.

The test often involves a smoke machine to introduce smoke into the intake system to find leaks. Some shops use a spray bottle with soapy water; bubbles will form where there are leaks. Given the symptoms, an intake hose leak is probable. However, diagnosing these issues can be tricky, and further damage could arise from incorrect repairs. I advise taking your vehicle to a reputable workshop. It is a common enough issue.

I appreciate the advice. I took the vehicle to a workshop. It was a cracked intake hose, as suspected. The repair cost about 100€ Thank you for pointing me in the right direction.