Based on the condition of my engine compartment fuel filter, it would probably be a good idea for me to drop the tank and replace/clean the pick up screens so I may as well do the mod. My question is this. where do the air bubbles come from in the return fuel line? It's a c;losed system and if you are sucking air at the pick up your running on fumes to begin with and will be coasting to a stop very soon. Just one of those questions that keep me up at night

 

I have thought about this question myself and i believe the thinking is when it drains into the tank it can create bubbles but i believe what you say is right. I mean you shouldn`t let the tank get that low. i think the main reason is to remove the in-tank filters( dumb place for filters) and put one on the frame rail. Another question to ponder is, Wouldn`t the the air bubbles just rise to the surface of the fuel and not be an issue unless you really are scraping the bottom?

 

There are several possibilities for air intrusion into the fuel.

1. Broken fuel pickup foot in the tank. Due to decay or tearing
2. Flapper valve just above the fuel pickup foot. Old or weak valves
3. Fuel mixing valve and screens in the tank. Keeps air in the fuel stream after it is already there.
4. Connections outside of the tank. Metal to rubber connections are made for pressure, not vacuum, but are used in vacuum arrangement. Poor design.
5. Any loose fitting in the fuel supply system. Poor maintenance.

1&2 allow air in when the fuel level is low. The hutch mod eliminates or checks the first 4 if done right. The 5th one is rare and regular checks repair that one.

How does air get into the system?
Low fuel level or poor sealing of the system lets air get to the fuel filter housing on top of the engine. The return fuel line comes from the top of the fuel filter (driver side). Air rises to the top of the filter housing and is forced into the return line. This is also where your fuel pressure spring is. Return fuel is pumped back to the tank into the mixing valve and screens. The return fuel is mixed directly with fresh fuel and is sent back to the fuel filter on top of the engine. Any air in the return line is also mixed and sent back. The only way to get the mixed air out in a stock system is through the injectors, which has an erosive effect on them.

The hutch mod eliminates the mixing valve and screens in the tank, and routes return fuel away from the fuel intake so air can escape instead of being recirculated. Because the screens are removed, you need a filter to replace them before the pump.

 

Yes, air bubbles would rise to the surface if they were not mixed back into the supply with the return fuel. The in-tank filters are also mixing return fuel (and air) back into the supply. Letting air bubbles rise is the reason for the return fuel re-route. Ease of maintenance is the reason for elimination of in-tank screens. The Hutch mod does both.

 

The main sources of air are the line connections on the suction/vacuum side (infeed) of the fuel pump. One at the tank and the other at the pump. The connections are designed for pressure, not suction.

So, #4 from srteach is the answer.