MiTo Diesel: Hissing + Poor MPG Points to Air Intake

Had the same issue on my Alfa Romeo Giulietta 2.0 JTD (2011). The loud hissing sound combined with poor fuel economy pointed to a vacuum leak in the air intake system. The check engine light was triggered by the mass airflow sensor detecting inconsistent readings. The main culprit was a cracked intake hose between the air filter housing and throttle body. When the engine vacuum pulled air through this crack, it caused the hissing noise and threw off the air-fuel mixture. Leak detection testing confirmed multiple small splits in the rubber hose. This kind of issue can lead to engine misfire and long-term damage if not addressed. The repair involved replacing the intake hose and cleaning the throttle body, as unmetered air had caused carbon buildup. After fixing it, fuel economy returned to normal and the check engine light cleared. The symptoms match your situation perfectly. Would recommend having a professional do a smoke test to pinpoint the exact location of the leak. Its worth getting it fixed properly rather than attempting temporary fixes, as these leaks tend to worsen over time.

Thanks for sharing your experience. Your detailed description helped me identify the same problem with my MiTo. Got it fixed two weeks ago with a complete intake manifold replacement since the original showed multiple cracks. The check engine light finally cleared and the engine runs much smoother now. Did you track the repair costs? Also wondering if any related issues popped up since the fix. My mechanic mentioned the throttle body might need attention soon due to carbon buildup from the previous air leak.

Thanks for the follow up. After fixing my air intake system leak at the hose two months ago, everything has been running perfectly. The total repair cost was 100 Euro for replacing the cracked intake hose, which was much cheaper than a full manifold replacement like in your case. Since the repair my fuel consumption is back to normal and that annoying hissing sound is completely gone. No related issues have appeared after the fix. The mechanic performed a thorough leak detection test to ensure there were no other weak spots in the system. Regarding the throttle body concern, carbon buildup is definitely something to monitor after an intake leak. My mechanic cleaned the throttle body during the repair as a preventive measure. Its worth getting this checked during your next service to avoid performance issues down the line.

After reading your reply, I feel better about my repair decision. Had the work done last week, spent 450 Euro for the manifold replacement plus labor. The turbocharger response feels much better now and the engine runs like new. Went to a different mechanic this time who actually took the time to explain everything. They used proper leak detection equipment and showed me the cracked manifold. The hissing noise completely disappeared after the fix and fuel economy improved significantly. The mechanic did a complimentary throttle body cleaning during the repair. Now the engine has much better throttle response and smoother idle. No more check engine light and the car feels solid again. Should have done this repair months ago instead of putting it off.