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.