Jim Grant's Tech Tips
‘94 Mercury Villager, Fuel Pump Fuse
As soon as I try to start my ‘94 Mercury Villager the fuel pump fuse blows. I replaced the fuel pump and I have the same problem. When I disconnect the electrical connector and try to start the vehicle the fuel pump fuse still blows. This tells me the wiring from the disconnect to the pump and the pump are good. Also, when I take the fuel pump relay out I try to start the vehicle and the fuel pump fuse will not blow. I replaced the fuel pump relay no help. This is telling me I have a short to ground or could some other component be bad?
You’re likely right that you have a short to ground and that it is another component. That other component is called a bypass air valve. The bypass air valve is powered by the same fuse and relay as the fuel pump. That explains why when you disconnected the relay the problem went away. The bypass air valve will be found on your motor. This valve meters air flow into the engine based on temperature. Inside the bypass valve is a heating element wrapped around a bi-metal strip. As we know, when metal is heated it expands. Different metals expand at different rates. If you bond 2 different pieces of metal together you create a bi-metal strip that bends when it is heated. Inside the bypass air valve is a door that this bi-metal strip is connected to. When cold the door is open. When power goes from the fuel pump relay to the heating element that is wrapped around the bi-metal strip, the heat from the element causes the strip to bend. As the strip bends the door is closed. Now what you want to know is just what this all does for your engine? The bypass air valve is your cold high idle valve. Without it working, the engine will have a tendency to race at higher RPMs. A quick test of this valve will be to unplug it and start the engine. If the fuse doesn’t blow, then your bypass air valve needs to be replaced. If the fuse does fail there is a wiring problem. That will be another story in itself.