See Figure 4-4. The fuel pick up, commonly known as the “ sock” has three functions:
1. Strain out large solids.
2. Act as a strainer to prevent entry of water.
3. Act as a wick to drain fuel down to the bottom of the tank since all pickup pipes do not reach the very bottom of the tank.
The tank filter is a Saran (Polyvinylidene Chloride) sock and is fastened to the fuel inlet line of the in-tank fuel filter and fuel pick-up assembly.
The fuel tank filter sock has a bypass valve which opens when the filter is covered with wax allowing fuel to flow to the fuel heater.
Without this sock fuel line heater would be ineffective because the fuel would be trapped in the tank. Since the bypass valve is located at the upper end of the sock, fuel will only be drawn into the waxed sock if the tank contains more than approximately 4 gallons of fuel. Therefore, it is important to maintain a minimum of 1/4 tank of fuel when temperatures are below 20 degrees F.
The Saran sock material has a nominal pore size of 130 microns. In addition to acting as a particle filter for the mechanical lift pump, the Saran tank filter acts as a wick to pick up fuel from the bottom of the tank and as a water filter; water is excluded on the basis of the difference in surface tension between the water and the sock material on the one hand and the fuel and the sock material on the other.
By law in many states, water in fuel should be no more than 1/2 of 1%. That quantity of water will be absorbed by the fuel. Periodically, station operators check for water by putting a special gel on the dip stick. If it turns color, then water is present and it can be pumped out. Unfortunately, not all station operators are responsible and this prompted the use of the Saran sock.
The fuel pickup tube doesn’t reach the bottom of the tank. However, since the sock acts as a “wick” the fuel level can actually be lower than the level of the tube and fuel will be drawn out right down to empty. Also, with this design, the level of water in the tank can be much higher before water enters the fuel system. This is about five gallons. Water that gets into the tank will eventually be absorbed by good fuel and will pass harmlessly through the fuel system. Water will be absorbed at a rate of about one gallon per 1000 miles.