The Ultimate Guide to Fuel Pumps (2022)

The fuel pump could be considered the unsung hero of the fuel system. Unlike all the glory the carburetor or fuel injectors take, the fuel pump is still in the background pumping making sure they have their time in the spotlight.

Enough with all the talk, lets get started with what you came here to learn.

How does a fuel pump work?

Simply put a fuel pump works by taking fuel provided by the fuel tank and pressurizing it for the fuel system. After which this the fuel injection system or carburetor take over. Don’t worry many people have asked this question over the years and with over 20 years of experience as a technician I have tried to explain it to them the best that I could.

Problem lies in the sometimes very complicated technical articles that are out there do not explain the concepts in simple details. We will try to clear this up for you in this article, so you know the difference between your fuel strainer and your fuel pump.

Let’s get started. First, we must understand there are many different types of fuel pumps that have been equipped on vehicles over the years. The image at the top of this article shows a mechanical fuel pump which you will find on vintage vehicles as well as and electric fuel pump that you will find on todays modern vehicles.

There are also high-pressure fuel pumps on diesel vehicles as well as gasoline direct injected vehicles, but we will not go in depth with the operation of these style pumps.

How do Mechanical fuel pumps work?

There are a few different types of mechanical fuel pumps, diaphragm, or positive displacement pump as well as the plunger type pump.

Why do they call it a mechanical fuel pump? The drive system of the mechanical fuel pump has an actuator lever, this actuator lever rides on a cam lobe of the camshaft and converts the rotary (circular movement, think clock) motion into reciprocating (up and down movement) motion. This movement is than transferred to a chamber where the fuel is pressurized.

Imagine if you could a sealed area with 2 check valves, lets call them the inlet and outlet check valve.  A check valve is a mechanical device that will only allow fuel to flow in one direction.

When the actuator lever moves downward against return spring pressure, the diaphragm that is in the sealed area will start to draw down created a vacuum (suction). This will allow the fuel to travel from the fuel tank past the inlet check valve and into the diaphragm area.  Atmospheric pressure as well as vacuum are what causes the fuel to travel into the fuel pump, but I did promise to keep it simple.

As the camshaft continues to rotate the tension on the lever will decrease to the point where the return spring will cause pressure on the diaphragm in the upward direction. This spring pressure is what creates and regulates the fuel pressure applied to the carburetor. On a mechanical fuel pump, fuel pressure is from 3-10 psi depending on the manufacturer.

The carburetor will generally be equipped with a float bowl and a needle and seat. The float bowl is basically a fuel reservoir that contains fuel that is immediately available to the engine. Once the fuel in the float bowl reaches a specific height the needle and seat seal, causing the fuel to stop flowing into the carburetor.

Once the carburetor begins to inject fuel into the intake the fuel in the float bowl will decrease. This decrease in fuel will cause the fuel that is under spring pressure in the fuel pump to flow into the fuel bowl until it is full again. If you can imagine this balancing act continues as your engine runs, float blow drains, fuel pump supplies.

How do Mechanical fuel pumps fail?

The above description shows you how many moving parts there are in a mechanical fuel pump system. Being a mechanical components parts can wear due to normal wear and tear as well as mechanical failures.

The most common problem for a mechanical fuel pump failure is the internal diaphragm. This internal diaphragm is made of rubber and is a wear component. The constant up and down movement, poor fuel quality, and improper fuel additives can cause the diaphragm to fail prematurely.

The other mechanical components that can fail are the actuator lever, return spring, inlet or outlet check valve, or even damage to the external fuel lines for the inlet and outlet of the pump.

How does an electric fuel pump work?

Thankfully engineers work tirelessly on improving the automotive experience. With the advent of modern fuel injection systems and higher fuel pressure requirements the electric fuel pump was created.

The fuel pump is a small electric motor that has a few important components to ensure that it can function properly.

One of the most noticeable differences of the two types is where they are located. The mechanical fuel pump needs to be located on the side of the engine where the electric fuel pump can really be located anywhere. Most pumps today reside in the vehicles fuel tank contained in what is called a fuel tank sending unit.

Being immersed in fuel helps with having a ready supply of fuel for the pump as well as cooling the pump.

Electric motor in my fuel tank? One of the other reasons that the fuel pump can be located in the fuel tank is that there is no oxygen to support combustion. The fuel tank is sealed from the atmosphere, so the inside of the tank consists of liquid fuel and fuel vapors. These displace any other gas from entering the tank.

The electric fuel pump is commanded on by the PCM whenever it determines the vehicle is going to be started or is running. By supplying a power feed as well as a ground the electric motor in the pump can begin to function.

The electric motor rotates at all times to ensure it is lubricated and cooled at all times. When the pump rotates it moves a turbine which is very similar to a paddle wheel. This paddle wheel draws fuel in through the fuel strainer (fuel filter) and out through past the outlet check ball and into the fuel lines.

Once in the fuel lines the fuel injectors complete the remainder of the job. Check out our The Ultimate Guide to Fuel Injectors, to learn more information on how that system works.

Inside of the fuel pump there is pressure relief valve, if pressure becomes too high due to the engine not requiring all of the fuel that the pump is supplying it opens at a set point and causes the fuel to be recirculated back into the tank.

Once the engine is shut down the outlet check ball plays an important role as well. To prevent airlocks when restarting the engine, the outlet check ball prevents the fuel that is in the fuel rail as well as fuel lines from draining back into the tank. If this fuel would drain back into the tank, it could cause an extended crank first time you start the vehicle.

What is a fuel sending unit?

Photo by: densoautoparts.com

A fuel sending unit is a term for a combination of items that are located in the fuel tank. These items generally consist of an electric fuel pump as well as a fuel level sensor.

The fuel level sensor provides a varying resistance to the instrument panel to let drivers know how much fuel is remaining in the fuel tank.

Along with these components there are fuel lines that travel out of the tank, a fuel tank pressure sensor (primarily used in the Evaporative emissions system) and a method of sealing to prevent fuel vapors from escaping into the atmosphere.

Because of all these components the fuel sending unit is a complex component and can have a high price tag when the time comes to replace this unit.

How do electric fuel pumps fail?

I have seen many electric fuel pumps fail over the years as a technician, but the most common failure is internal fuel pump failure. The fuel pump as it rotates over time causes the brushes to wear. This wear eventually leads to failure of the pump itself.

Accelerating internal fuel pump failure is the use of improper fuel. Using fuel that is improperly filtered can cause sediment to find its way into the pump and cause damage. Using fuel that is contaminated with water can also cause premature damage.

Water can cause corrosion on internal components inside of the fuel pump and also damage other components located in the fuel tank as well. With ethanol content ever increasing in our fuels this is a hazard that we need to keep in mind.

Using a high-quality fuel injector cleaner or upper end lubricant is important to ensure that our fuel quality remains at the highest possible level.

Symptoms of a faulty fuel pump

A failing fuel pump can up in many different manners for vehicle owners. The most obvious in this case is the common no start. A failed fuel pump does not provide fuel to the fuel rail and injectors and the vehicle will crank, crank, crank and never fire.

Hesitation under acceleration or lack of power is another complaint that could be caused by a faulty fuel pump. As the internal components of a fuel pump begin to wear and start to fail, they will begin to produce less and less fuel pressure/volume. When you start the vehicle and drive with very light acceleration this does not cause any issues.

A CEL (check engine lamp) may also become illuminated if you have this type of failure with your vehicle. The PCM has the ability to run many different types of ongoing self diagnosis and this type of failure could set a misfire code or a lean code.

 However, when the engine requires the most amount of fuel, when hard under acceleration for example it is not able to provide enough fuel. This causes the fuel injectors to be starved for fuel and the engine to misfire.

Noise coming from the tank can inform drivers of a fuel pump that is about to fail. It is always important to know what noises are normal for your vehicle. If you hear anything out of the ordinary, a loud screeching noise for example that is coming from the fuel tank area you may have a fuel pump that is about to fail.

How much does it cost to replace a fuel pump?

An electric fuel pump or more commonly a fuel sending unit can cost from $200-$1200 to replace. The replacement procedure is not a straightforward repair either.

Depending on the vehicle, the fuel tank must be removed, and a special lock ring replaced before removing the fuel sending unit from the vehicle.

A mechanical fuel pump will be lower than the cost of an electric fuel pump. Prices can range from $30-$300 depending on manufacturer.

No matter what type of fuel pump your vehicle is equipped with ensure that you spend the extra money to purchase a quality known brand supplier when purchasing your replacement fuel pump. The labour that is involved in doing this type of repair can be costly, so we need to ensure that the pump will last as long as possible to ensure you get the best possible lifespan out of your new pump.

Does fuel quality matter?

As we always say, yes fuel quality always matters when it comes to overall fuel health. We talked about the advantages of using top tier fuel in other articles, but I will say it again. Make sure you always use top tier fuel when filling up your vehicle.

Here is a mechanic secret that we will let you in on. When filling up your vehicle take a look around the fuel station before filling up. Is there a fuel truck that is filling up the fuel stations tanks, if so keep driving and stop at the next fuel station.

The fuel tanks that are below ground have sediment and water that will build up inside of the tanks. Normally the sediment will sink to the bottom of the tanks where they are completely harmless. When the tanks are being filled however this sediment can become stirred up and suspended in the fuel.

If you happen to be the poor person that fills up with fuel next, you will pump all the sediment into your fuel tank on your vehicle. Possibly causing many fuels related headaches in the near future.

What next?

Hopefully you learned much more about the fuel pump and its importance in the fuel system. Also the importance of good quality fuel and the proper use of fuel additives. If you would like to learn more about other components of the fuel system. Again, check out our great The Ultimate Guide to Fuel Injectors.

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