Where does air in the fuel come from

There are a number of places air can get into the fuel system on the IDI. Most often air is allowed into the system after you shut down. The return system has "O" rings and the plastic caps can get distorted over time. When you shut the engine off, if there is any breach in the itegrity of the return system, fuel will "drain back" into the tank and the void will be filled with air. Air can GET IN WHERE FUEL CAN'T get out. Another spot for this is the supply line from the filter header to the back of the IP. Fuel will drain onto the valley pan and you might not see it, and once again the void is filled with air.

While your running, air can be sucked into the fuel system from a failed "O" ring on the tank connection or the selector valve. In rare cases the flex line from the frame to the inlet side of the lift pump can get cracked.

erik49051 said:

I have a rough idle, if a leak is comming from "While your running, air can be sucked into the fuel system from a failed "O" ring on the tank connection or the selector valve", would this be visible in a clear line running from the return rail up to the filter head?

Thanks

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The clear line test will positivley identify air in the system. If you make a loop, then yeah you can tell which direction it's coming from. I don't have the specifics handy but I know it's been posted a few times under "clear line test".