I was a bit surprised to find that the 2008 HiHy fuel system is unique from every other vehicle I've owned. Now I'm curious as to what it's really doing.

When you press the Fuel Filler Door Release button, the Owner's Manual says that it may take up to 10 seconds to release the door and that the Multi-Function Display alerts you to when the vehicle is ready to be refueled. It also mentions that it is releasing pressure from the system and another section in the manual, about unusual noises from the vehicle, mentions that you may hear a hissing sound about 5 hours (I think this is the correct figure) after shutting off the vehicle. It says this is a leak test for the system. I know that this must be part of the SULEV (Super Ultra Low Emissions Vehicle- Zero evaporation) compliance system, but I'd like to know more about what it's actually doing and how it works.

Does anyone else know more about the fuel system in the 2008 HiHy and what it's acutally up to?

Does the previous model HiHy also work this way?

Does it normally operate under pressure?

Inquiring minds want to know!

The service manual could tell you everything about what is going on.

In simple terms:

The HiHy keeps the fuel system pressurized at all times. Gasoline under pressure doesn't evaporate. This helps the car achieve a PZEV rating. Partial Zero Emissions Vehicle cannot have any evaporative emissions.

The old HiHY and the Camry Hybrid do the same thing. The Prius uses a bladder to keep excess air out of the tank.

Cool. Thanks for the information, n8. Any idea what the normal operating pressure is?