Fuel delivery system

I want to understand how the fuel system in my 2004 Mercury Sable works. So far my understanding is the following, please correct anything that’s wrong or needs to be expanded on:

The fuel pump is inside the gas tank, it is part of an assembly that also has the fuel gauge sending unit. The electric pump pushes the fuel to the engine and keeps about 30psi pressure in the fuel line. This is generally more fuel volume than the engine needs, so a pressure relief valve returns the excess fuel back to the tank by another fuel line. The fuel pump gets 12v power that comes thru a fuse then to a relay in the engine compartment. Also connected in this power line to the fuel pump is an “inertia” switch (emergencey shut-off switch) mounted in the trunk that is normally closed but can open to remove power to the fuel pump if the car is in an accident. To replace the fuel pump the gas tank has to be removed, since it is mounted on the top of the tank extending inside. When operating correctly the fuel pump gets 12v power as soon as the key is rotated to the “ON” position, just prior to “START”. The fuel pump runs all the time when driving, it does not cycle on & off. If I am not getting fuel to the engine the fuel pump relay and inertia switch are not likely to be bad, it is generally a bad fuel pump. If laying under the car & whacking on the gas tank induces the car to start then the fuel pump is confirmed to be bad.

I’m also curious how a 12v electric motor can be operating inside the gas tank without sparking & blowing me up. Is it a brushless motor?

Thank you.

It will not spark or burn because the entire inner workings of the pump are submerged in gasoline that is pumped on through and up to the engine. There is no air inside the pump so therefore no fire.

The gasoline flowing through the pump also acts as a lubricant for the pump armature and bushings that the armature rides on.

When the ignition switch moves to run position but the engine isn’t started, the fuel pump only runs for a second or two and then shuts off. This builds the fuel pressure. Once the engine is started the computer then see’s the crankshaft rotational signal and turns the fuel pump on where it runs continuiously.

If banging on the bottom of the gas tank gets the engine to start, it points to a worn out fuel pump.


Well how about when the gas cap is left loose and all that oxygen gets mixed up in the fuel, it ought to blow sky high then:) (notice the emotion sign)

When you really,really want to make sure that the formula for a fire is not completed inside a fuel tank you pump an inert gas into the tank (which is what the military does and some feel should be required with civilian aircraft). One good example where things did go wrong was TWA 800 but that spark was suposedly from wiring and not a pump.

You could always repair it.

I tried to attach a funny photo…unsuccessfully. Now I can’t figur eout how to delete th whole thing. Doggoned newfangled technology!

Thanks Tester, that’s what I’m afraid is wrong. Got a quote for $450-$500 from a garage to drop the tank and replace the fuel pump assy (includes sending unit I guess). The part itself costs from about $200-$350 depending on mfr.

Just curious, have you ever seen the enertia switch (emergency fuel shutoff switch mounted in the trunk) go bad? Also, does this car have an external inline fuel filter (I was told the fuel pump has a couple of filter screen bags on it inside the tank).

Not sure about this. The fuel pump is suspended from the top part of the tank, extending downward. But when the tank is nearly empty of gas there is plenty of air inside it. And when someone does run the car out of gas there would still be gas fumes inside the tank. In this situation the fuel pump’s electric motor would not be submerged. That means if it did spark there could be an explosion. To me that means the design has to prevent a spark in the first place, either the electric motor is completely encased and sealed, or they use a type of motor that doesn’t use brushes. No way the lawyers would allow an iffy design.

I think you did the attachment ok, I was able to download & look at the picture. That looks like my brother-in-law under that truck!

My condolences.

In all the years Ford has used the inertia switch, I have yet to see one fail.

Yes. There is an external fuel filter. It’s located at the gas tank.