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I know the high pressure line running down the left side of the car carries fuel, but what about the other slightly smaller diameter line (slightly bigger than the brake lines) that connects to the front and back with rubber hoses? Is this line simply a vapour return line?

Just took a look under the car, there are actually 3 fuel lines, 1 high pressure with the fuel filter, 1 low pressure smaller diameter, and one the same diameter as the high pressure, not sure what each is for other than the main w/filter line.

One is the main fuel supply, the other is for the fuel return from the pressure regulator bleeding off excess pressure and the third is from the purge solenoid to purge the charcoal canister of vapors. This is part of the emissions system to collect fuel vapors. In the old days when gas was cheap, the vapors were just vented to the atmosphere thus hurting gas mileage.

high pressure = 3/8"
return = 5/16"
vapor ~ 1/4"?

vapor should be smallest

So the small vapor line runs back into the tank? Can it just be cut short at the front with a breather filter/cloth cover or capped then or how was it vented pre emissions? I can't see it helping much with increasing gas mileage realistically.

As for emisions, this car already is amazingly at a super low level every year its smog tested which is amazing considering its an older car.

The fuel vapors are routed to a charcoal canister where they collect and become a liquid again. The canister is purged and the fuel is sent back to be burnt.

http://auto.howstuffworks.com/evaporati ... ystem1.htm

Good info, thanks.

To replace the high pressure and return fuel lines do you have to drop the tank? One of the hoses is tucked between the frame and tank at one end and theres not alot of room to work with. Also it appears to connect to the tank with 2 metal lines - are these replacable or do you need to change the gas tank out?

I believe you will have to drop it to change the line. You will need to get to the fittings connecting the sending unit to lines.