Here, we discuss differences between mechanical and electric fuel pumps, along with a list of terms you may come across when shopping for a new engine fuel pump.
Engine Fuel Pump History
From the earliest days of the automobile, fuel pumps have existed to draw fuel out of the fuel tank and move it into your engine’s combustion area. (A note for the purists: some very early cars, like the Ford Model T, used “gravity feed”, which required the fuel tank to be mounted higher than the engine. These cars could only climb steep hills by ascending them in reverse!)
Originally, fuel pumps were “mechanical” in nature – meaning they were operated and controlled by manual means (engine rotation and vacuum pressure). When cars became sportier and developed more horsepower, electric pumps were utilized, which could deliver a higher-pressure flow of fuel. The advent of electronic fuel injection and modern emissions systems required more precise metering of fuel pressure and flow rate. The electric fuel pump continued, but now had its output regulated based on electronic commands from a computer.
In the scope of this post, we will look at how mechanical and electric fuel pumps work in order to help you understand the differences between them. We have included a list of definitions below that you are most likely to come across during a search through our product selection. We recommend a look through them, because they will also give you a fuller understanding of the type of fuel pump your own vehicle will have.
Engine Fuel Pump Terms
Diaphragm – A rubber bellow inside a mechanical fuel pump that pulls fuel up the fuel lines from the tank by expanding and creating a vacuum.
Electric Fuel Pump – A fuel pump with its own electric motor that is governed by engine electronics. Typically found on vehicles with fuel injection, electric fuel pumps were sometimes equipped on several older models with carburetors. Standard electric fuel pumps operate at a higher-pressure range than mechanical ones – typically between 50-200 psi. However, low-pressure electric fuel pumps for older vehicles with carburetors will operate in the 5-10 psi range.
Fuel Lift Pump – A secondary low-pressure supply pump that is used to transmit fuel to a high-pressure pump found on some diesel engines.
G.P.H. – Abbreviation for Gallons Per Hour. This is a measurement of the volume of fuel that can be delivered by a fuel pump.
High Pressure Fuel Pump – Found on more modern fuel-injections engines that feature direct fuel injection, where fuel is injected directly into the cylinder instead of a pre-chamber. Fuel for these engine applications must be pressurized at a much higher rate – usually into the thousands of p.s.i.’s.
Inline Fuel Pump – “Inline” is simply a descriptive term for electric fuel pump. An inline pump can be mounted either in the fuel tank, or externally.
Mechanical Fuel Pump – A fuel pump controlled and powered by driven by an arm that comes in contact with the long end of a teardrop-shaped lobe mounted on the engine’s camshaft. Typically found in vehicles with carburetors, most mechanical fuel pumps operate at low pressures of between 5-10 psi.
P.S.I. (psi) – Abbreviation for Pounds per Square Inch. While psi is a general measurement of pressure, when used with fuel pumps it signifies the speed at which fuel is being delivered into the engine. Fuel that is under a higher amount of pressure will flow faster, and vice-versa.
“Puller Style” Fuel Pump – Found mostly on carbureted engines, mechanical fuel pumps mounted up front on the engine block are “puller” pumps, because they pull the fuel from the tank through the fuel lines using suction. Because puller pumps often draw only air for a few seconds when a starter motor begins cranking, they are designed to operate under dry conditions when fuel is not actually flowing through them. Therefore, accidently running out of fuel will not cause great damage to a puller pump.
“Pusher” Style Fuel Pump – Designed for use with fuel-injected engines, pusher fuel pumps are powered by electricity and controlled by engine electronics. Pusher pumps are located either inside the fuel tank, or very near it. These draw fuel out from the bottom of the tank and push it up the fuel lines to the engine. These work under higher pressure and must be immersed in fuel all the time for cooling and lubrication purposes.
Mechanical Fuel Pumps
Mechanical fuel pumps are mounted on the side of the engine block. They are known as “puller” style pumps because they pull fuel out of the bottom of your tank and draw it through fuel lines using suction. This suction is created by a rubber diaphragm that is sandwiched between two check valves inside the pump. An inlet check valve opens to draw fuel into the pump, then closes to prevent backflow once it has filled up with fuel.
At that point, the other outlet check valve opens to send the collected fuel right to the carburetor sitting on top of the engine. This occurs repeatedly, and the fuel pump is driven by an arm that comes in contact with the long end of a teardrop-shaped lobe mounted on the engine’s camshaft. As engine rpms increase, fuel pump activity also increases accordingly.
This diagram allows a better look inside a typical mechanical fuel pump.
Mechanical fuel pumps run at a lower psi. than electric fuel pumps for two reasons. First, this type of pump simply does not need to work as hard in order to keep pressure in the fuel line up because it’s mounted so close to the engine. Secondly, a lower psi is ideal for carburetors because higher pressure can cause damage to delicate needles and seats within a carburetor assembly.
Additionally, high fuel pressure can overflow carburetor fuel bowls and cause the fuel to spill out onto hot engine parts. Most carburetors are designed for 4 to 6 psi but can be adapted for use with a higher-pressure fuel pump by a fuel pressure regulator that contains a separate overflow return line to the fuel tank.
Electric In-Tank Fuel Pumps
In this type of setup, the fuel pump is mounted on a bracket that sits at the bottom of a case inside the fuel tank. These are accessed from the top of the tank and will usually have an access port on the trunk floor or underneath a rear seat cushion. Unlike mechanical fuel pumps, electric fuel pumps do not have diaphragms or mechanisms to compress them. These pumps are designed to always be immersed in fuel.
In-tank fuel pumps are often accessed from the top, through a hole beneath the rear passenger seat or in the trunk floor.
When electric power is supplied to the pump, an electric motor whirs away and pushes fuel all the way up the lines to the front of the vehicle. When engine electronics determine more fuel is needed, the pump speed is increased. Electric fuel pumps are found mostly on vehicles with fuel injection, but some manufacturers did fit low-pressure electric fuel pumps to vehicles with carburetors.
Electric External Fuel Pumps
Like in-tank fuel pumps, these are electric powered – but sit completely outside of the fuel tank, usually nearby on a frame rail or other secure body panel. They draw fuel from the tank via siphoning, then push it up the lines to the engine. Some vehicles with electronic fuel injection will feature a secondary external fuel pump in addition to an in-tank unit.
While it is not a guarantee, you have probably figured out by now that if your car or truck has a carburetor, it’s very likely to be fed by a mechanical fuel pump. Conversely, just about any fuel injection system requires the use of an electric engine fuel pump.
Finally
Essentially your choice of fuel pump depends on your personal needs. Over the years I have used both on a variety of classic cars, and in each case the decision was made based on budget, engine horsepower, ability to locate the engine fuel pump, and fuel system (carburetor vs fuel injection).
