Lexus GS Hissing Sound Air Leak

Sounds like you're on the right track with the air intake leak. The hissing sound is a good indicator. Have you tried visually inspecting the intake hoses for any cracks or disconnections? Also, does the engine seem to be misfiring at all, especially at idle?

I've had a quick look at the hoses, but nothing obvious jumped out at me. Where are the most common spots for leaks on these engines? And yes, now that you mention it, the engine does seem a bit rougher than usual at idle.

Okay, the rough idle strengthens the intake leak theory. On the 2000 GS, check the intake hose that connects to the throttle body and the MAF sensor. Those are common failure points. Given the symptoms, I'd recommend getting a smoke test done, or taking it to a workshop. It's a pretty easy fix, a new intake hose should solve it. The cost should be around 100€.

Thanks for the advice! I had a closer look at the intake hose, and you were right, there was a split on the underside. Replaced it and the hissing is gone, fuel economy is back to normal, and the check engine light is off. The bill was exactly 100€. Appreciate the help!