Peugeot Hissing Sound Fuel Consumption Issue

The symptoms you describe, particularly the increased fuel consumption, check engine light, and hissing sound during acceleration, strongly suggest a leak in your air intake system. This could potentially be located anywhere from the air filter housing to the throttle body. Have you had a look at all the vacuum hoses associated with the intake? These are often a culprit. An engine misfire could also cause similar issues. I would recommend carefully inspecting the intake hoses for cracks or loose connections.

Thanks for the info! Vacuum hoses... I'll have to look into that. Where exactly would I find these? Are they easy to spot, or are they hidden away somewhere? Also, if it is a leak in one of the hoses, is that something I could potentially fix myself, or would it be better to take it to a workshop?

The vacuum hoses are connected to the intake manifold and other components of the engine. Look for small rubber or plastic hoses running between the various parts. Depending on your mechanical skills, it may be better to visit a workshop. An intake hose leaking is a common problem. Considering the symptoms and the build year of your Peugeot 4008, it's quite possible that the rubber has degraded over time, and the cost to fix it would be around 100€. This is of course an estimate.

Thanks for the advice! I took it to a local workshop, and you were right, it was an intake hose leaking. They replaced it, and everything seems to be running smoothly now. Fuel consumption is back to normal, and the hissing sound is gone. I appreciate your help!