It doesn't... If you get it all. Did you forget to rock the wings and wait for water to run down to the drain first? Was there enough water there that your fuel test tube filled completely, and you mistook that for a full tube of gas? There are lots of reasons you *don't* get all the water out of the tanks even when sumping them first.
Les keeps using blanket statements like "all low wing aircraft" and talking about PA-28's which don't have EFI, so not everyone is talking only about EFI.
Okay my bad - I'll be specific. I'll stick with low wing aircraft like Van's design, with fuel injection.
In my opinion, a firewall mounted gascolator is not only useless but dangerous. If it leaks you risk a FWF fire. Also, in the case where you don't get all the water out of your tanks, it will not detect the remaining water unless you draw water from the tanks to the gascolator and then out the gascolator. Water will not get to the gascolator unless fuel is moving.
Therefore, a gascolator is not a substitute for properly sumping tanks.
Further, if the water is only in one tank and the selector is on the other tank, even drawing fuel from the tank to the gascolator and out the gascolator doesn't help as the gascolator only "sees" the uncontaminated tank.
A gascolator will not tell me if I have introduced water into my tank
before flight. If the gascolator tells me there is water in my fuel
after a flight then it is telling me the water was not sufficient to be hazardous. Of course if the amount of water was hazardous during a flight, that might be of interest to the NTSNB / TSB accident investigators.
Gascolators use drains sealed using O-Rings. IF I get a leaky O-Ring I will get air in my fuel lines (it is on suction side) or spray fuel (it is on the pressure side). Why risk either scenario when a gascolator provides no value.
Lastly, when I bench tested a gascolator - a closed loop test with an electric fuel pump moving 50 GPH, I found the percentage of water / fuel in the gascolator matched the percentage in the fuel. The gascolator did not strip water out of the fuel.