So i drove with it off, well on but not screwed on and it didn't have the vacuum effect but there is still diesel vapor escaping out the filler neck:argh:

Driving with the cap lose dose nothin but waste fuel. The cap is supposed to be sealed to prevent evaporation. The problem is the roll over protection valve on the top of the fuel tank. Takes 5 minutes to change.

ok, where can i pick one of those up? and how do i change it

When I was doing this it was a dealer only item but that may have changed in this time frame. Just stick you head up over the fuel tank and you will see it sticking up out of the top of it, just grab it and twist it out. It's held in by a rubber grommet. Once you pull the old one just shove the new one in place the same way.

You may even be able to clean the old one out with some spray cleaner. It's just a simple one way check valve that let's your tank vent in the normal position but seals off in the event of an accident with a rollover to prevent fuel spillage.

http://www.dieseltruckresource.com/p...servoirTop.jpg
:tu:

Thanks guys

Do you know a part number for the rubber cap that goes over the nipple located next to the fuel supply outlet and fuel return inlet? Or another suitable replacement? Mine is torn and leaks when the tank is full. Any help is much appreciated.