Highlander intake hose leak

I have a Highlander too, and your description sounds eerily familiar to what I experienced a while back. The hissing sound is a big clue. In my case, it turned out to be a cracked intake hose. It was a small split, almost invisible, but enough to throw off the air-fuel mixture. The engine management system is like a finely tuned orchestra; if one instrument is out of tune, the whole performance suffers. That's why you see the increased fuel consumption and the check engine light. Did you check the intake hose that connects the air filter box to the throttle body?

Thank you for your very helpful advice! Yes, I did inspect that specific hose, but perhaps not thoroughly enough. Was the crack easily visible, or did you have to flex the hose to reveal it? Also, did the hissing sound only occur under acceleration, or was it present at idle as well? I'd like to rule out other possible causes. Any other hoses I should focus on?

You're very welcome! The crack wasn't readily apparent; I only found it by chance when I squeezed the hose. Under pressure, the split opened up enough to become noticeable. The hissing was more pronounced under acceleration, as the engine pulled in more air. At idle, it was very faint. Given the age of our vehicles and the mileage, there's a good chance the rubber is simply deteriorating. I would strongly suggest you take it to a trusted mechanic. I replaced the hose myself, but the intake manifold is difficult to reach and requires special tools. A smoke test could quickly pinpoint the leak and save you considerable time and frustration.

I took my Highlander to a mechanic as suggested, and they confirmed the intake hose leak. They replaced it for 100€ and the hissing is gone, the check engine light is off and I'm averaging much better fuel efficiency. Thanks so much for the help, responder! I really appreciate it.