Hiace Hissing Sound and Fuel Consumption

I had a similar issue with my Hiace. That hissing sound is a telltale sign! Have you checked the intake manifold or any vacuum hoses? Maybe even the throttle body? Could be a split or loose connection somewhere. Is the hissing constant, or only under acceleration?

The hissing is definitely louder when I accelerate. I haven't specifically checked the intake manifold, but I visually inspected the main hose from the air filter. Are there any specific spots on the manifold or vacuum hoses that are prone to leaks on these engines?

On mine, it was a small vacuum hose near the throttle body that had cracked. Unless you're super confident, it might be worth getting a smoke test done at a workshop to pinpoint the leak quickly. With those symptoms, and that mileage it probably is one of the hoses. A new hose shouldn't be too expensive.

I took it to the workshop, and you were spot on it was a cracked intake hose. They replaced it for around 100€. Fuel consumption is back to normal, and the hissing is gone!