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Fuel Pumps & Assemblies

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Fuel Pumps & Assemblies

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What is a fuel pump and where is it located?

A fuel pump is a device that moves fuel from the fuel / gas tank up to the engine. They come in a huge variety of shapes and sizes for every possible application that you could ever imagine. There are three main types of fuel pumps – mechanical pumps, in-tank electric pumps, and external electric pumps. Each style has a purpose, and some are more installation-friendly than others.

Mechanical Fuel Pumps

Mechanical fuel pumps are the oldest and most simple design. They have very few moving parts, and are often used on carbureted gas engines and diesel engines. These types of fuel pumps are mechanically driven off of the engine’s camshaft or crankshaft. As the engine speed rises up, so does the fuel pump's fuel volume, which then allows the engine to drink more fuel as needed.

On gas engines, most mechanical fuel pumps are low pressure, which is exactly what carburetors need. They are by far the least expensive fuel pump assembly to buy because of their simplicity and interchangeability. Installation of these fuel pumps is often nothing more than removing two bolts and a couple of hoses. The downfall of mechanical fuel pumps on gas engines though, is that when they fail, they often leak fuel into the crankcase, which thins out the engine oil. This is obviously not ideal for the bearings inside the engine.

For diesel engines, the mechanical pumps need to create extremely high fuel pressure to properly spray the diesel fuel through the injectors. While still a mechanical design, the diesel fuel pumps are more complicated than the style on gas engines. As you can expect, this also means that they come at a far higher price. Because of this, diesel fuel pumps are often professionally rebuilt instead of replaced. Some diesel cars and trucks also have a small electric fuel pump in the fuel tank that feeds the main engine driven mechanical pump. This helps prevent fuel starvation.

Electric Fuel Pumps

Electric fuel pumps are “the norm” for the vast majority of vehicles these days. As you can tell from the name, they are powered by the magic of electricity. They come in two different styles, "internal" and "external".

Internal or In-Tank Electric Fuel Pumps

Internal (also called ”in-tank”) fuel pumps are the most common when it comes to modern vehicles. They are submerged in fuel inside the gas tank. Being surrounded in fuel keeps the fuel pump cool, but often makes replacing them a bit of a headache. On many vehicles, the only access to the in-tank fuel pump is by removing the fuel tank all together. When a fuel pump assembly is mounted inside the gas tank, the pump is often fitted with a “strainer” or "sock" on the bottom of it. A fuel pump sock is the first barrier for debris floating around inside the fuel tank. It prevents large particles from entering the fuel pump and fuel system. Smaller particles are then caught by the main inline fuel filter further down the line.

In-tank electric fuel pumps can also come assembled with a fuel sending unit. A fuel sending unit electronically submits information to the fuel gauge or computer by measuring the fuel level in the gas tank with a float. 

External or Inline Electric Fuel Pumps

External (also called “inline”) fuel pumps are located outside of the fuel tank, and usually mounted to the frame of the vehicle. This type of pump can be used in two different ways. Sometimes they are the only fuel pump within the fuel system, but other times they are the “main” high volume fuel pump with a “helper” in-tank fuel pump as a fuel feed for them. This allows high powered engines to maintain the fuel pressure and volume that they require during “spirited” driving conditions. External fuel pumps are obviously easier to replace because the gas tank doesn't need to be touched.

How do I know if my fuel pump needs to be replaced?

In the vast majority of cases when a fuel pump assembly needs to be replaced, the engine itself won’t run because it isn’t receiving any fuel. So, that is when a replacement fuel pump would be needed. Now, why do they actually break in the first place? Well, electric fuel pumps can be damaged from a few different things. Age and wear are the most common reasons, but experience says that clogged up fuel socks / filters, and constantly empty fuel tanks will shorten the life of fuel pumps as well. One way to check to see if you fuel pump is running is to turn the car into the "on" position and listen for a humming sound coming from the fuel tank a few seconds later. If you don't hear anything, there's a chance you may have a faulty fuel pump.

Can I replace the fuel pump myself?

Replacing a fuel pump assembly can be really easy or a giant headache depending on the vehicle that you are working on. Most mechanical pumps on gas engines can be easily replaced by any do-it-yourselfer in their driveway. Electric fuel pump replacement difficulty can be hit or miss depending on the vehicle. Some are a drama free 15 minute procedure, and some can easily take up an entire day. Grabbing a service manual for your vehicle or doing some good old fashion searching online - including here at 1AAuto.com - can help guide you on your fuel pump replacement journey.

Since fuel pump removal varies depending on the type of vehicle, the list of tools can vary, but a pair of pliers, screwdriver, ratchet and sockets, drip pan, hammer, and flare nut wrench are usually a good start. For a mechanical pump, you usually want to start by disconnecting the negative battery cable. The process may involve disconnecting the fuel tank hose and the carburetor outlet. Then the pump can usually be removed after taking off two bolts. It's a good idea to clean off any remaining gasket material. To reinstall, simply apply some new gasket material, insert the bolts, and then the gasket into place, and insert the pump into place. Make sure the rocker arm and push rod are in their correct spots. Attach both the outlet line then the fuel tank hose and tighten any clamps. Then reconnect the negative battery terminal.

Electric pumps have a tendency to be a little more trickier and time consuming.  The process typically requires the disconnection of the fuel lines and fuel pump relay, and in many cases the removal of the gas tank. The first steps are to remove the fuel pump relay and to disconnect the negative battery cable. The next step might require removing the back seat if the pump is accessible without having to remove the entire gas tank, but if you have to remove the gas tank drain as much fuel as possible from the tank. Then disconnect the fuel filler hose and the fuel pump wiring harness. Then disconnect the fuel lines. To keep the fuel tank in place, support the fuel tank with an item such as a block of wood and a jack, or enlist the help of an assistant. Then remove the bolts that hold the straps in place. Once the gas tank is out, if the retaining ring won't budge, hammer the ring very carefully with a screwdriver to get it moving, which will give you access to the fuel pump.

To reinstall, place the fuel pump into the tank. Then tighten the retaining ring, and, depending on the ring, you may have to tighten it by using the hammer and screwdriver method. To insert the fuel tank back into place, an assistant can be extremely useful. Jack up the tank while your assistant holds onto it. If your straps have been severely rusted or were damaged in the removal process, we recommend installing new straps. Once that's complete, reconnect the fuel lines and clamp any hose clamps into place. Then connect the fuel pump relay and, lastly, reconnect the negative battery cable.

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