SupraIV hissing noise and fuel consumption

I had something similar happen with my SupraIV. Those symptoms sound familiar. Have you checked the intake hoses, especially the vacuum hose near the intake manifold? A crack there could cause those issues. Is the hissing pretty loud when the engine is cold?

Thanks. Intake hoses? I haven't checked that yet. Is this a common issue with the SupraIV? Should I be looking for anything specific when inspecting the hoses?

I wouldn't say it's a super common issue, but I've seen it before. A split or tear in a vacuum hose can throw off the engine's air-fuel mixture, triggering the check engine light and impacting fuel economy. With that hissing noise, I'm leaning toward a vacuum leak. Perhaps best to visit a workshop where they can do a smoke test.

Thanks for your input. It was indeed a split intake hose, as you suggested. Replacement cost me around 100€.