The fuel pump on your vehicle supplies fuel to the engine, and depending on the age of the vehicle, may be mechanical or electric. Adequate fuel pressure is necessary to ensure that your engine has the fuel capacity required to run at its best, and a failing fuel pump can limit fuel delivery and hinder performance – even preventing the vehicle from starting in cases of complete failure. Diagnosing fuel pump issues yourself is often simple, and requires only a few basic steps and tools. Read on to learn more about fuel pump failure symptoms and how to diagnose a bad fuel pump.
What Causes Fuel Pumps to Fail?
Fuel pumps can fail in several ways, but the most common reason for fuel pump failure is simply wear and tear over time. Unfortunately, other factors can also result in fuel pump failure, including bad gas or other fuel system contaminants like dirt or rust, or electrical shorting in the fuel pump motor. Though these problems may be difficult to diagnose, it is possible to test the fuel pump to see if it is receiving the power it needs to function. Bad fuel also often has a distinctly different smell than normal fuel, and you may notice an odor of varnish at the filler neck if this was a factor in the fuel pump’s failure.
Often, especially in cases where the fuel pump stops working completely without warning, it may not be a fuel pump failure at all. Check your vehicle’s service manual to determine what feeds power to the fuel pump. If there is a fuse or relay that is responsible for activating or powering the fuel pump, this can be a great place to start your diagnosis. If there is no power to the fuse or relay that turns the fuel pump on when you turn your key, this indicates a deeper electrical system problem that may require professional service. If there is power to the relay or fuse, test or inspect these components to be sure that they are in working order. Sometimes fuel pump failures are as simple as replacing one of these components to restore power to the pump itself. You can learn more about what causes fuses to blow in our How-To Hub.
In some vehicles, a separate fuel pump shutoff relay may also be a factor in fuel pump failure. Fuel pump shutoff relays are designed to turn the fuel pump off in the case of an accident, preventing fuel from continuing to flow and limiting the risk of a fire. If this relay is failing, it may be necessary to replace it to keep the fuel pump working as it should.
Mechanical fuel pumps can also suffer damage that can be difficult to diagnose. These fuel pumps are designed with a lever arm that contacts the fuel pump lobe on the camshaft, and they use an internal diaphragm to create fuel pressure. Over time, this diaphragm can wear out or separate inside the fuel pump, allowing fuel to enter the crankcase. The most obvious sign of a mechanical fuel pump failure is the smell of fuel on your oil dipstick, and it may be necessary to change the oil when you replace the fuel pump to ensure proper oil viscosity.
Signs of a Bad Fuel Pump
When your fuel pump is weak or fails entirely, there are a few obvious signs. The most common symptom of a bad fuel pump is your engine failing to start due to a lack of fuel reaching the cylinders. Fuel pumps can often be heard running from the fuel filler neck and will prime the fuel system before the engine is started, so turning the key to the “Run” position and listening near the filler neck might help determine if the fuel pump is activating at all.
If the fuel pump has not failed completely but is weak and not producing adequate fuel pressure, your vehicle may fail to start, run rough, or can misfire. This often triggers a “Random Misfire” or “Multiple Cylinder Misfire” code on a code reader, because the misfire will not occur consistently in a single cylinder. Of course, this isn't the only cause of a misfire, so further diagnosis may be necessary. Weak fuel pumps can often be tested using a fuel pressure tester and a Schrader valve located on the fuel rail of most vehicles. Check your service manual for appropriate fuel pressure ranges, and if your fuel pump is producing less pressure than needed, it may need to be replaced.
Other factors can produce similar symptoms to a weak or failing fuel pump and should also be considered. The symptoms of a clogged fuel filter or fuel pump pickup filter include misfires or low fuel pressure. These failures occur due to debris in the fuel system preventing the fuel pump from picking up or pressurizing fuel adequately. Other clogs in the fuel system, including injectors, fuel rails, fuel lines, the fuel pressure regulator, and more could also cause similar symptoms. It may also be a good idea to check not only the fuel pump relay and fuse, but also test for the presence of power at the fuel pump connector if it’s easily accessible. All of these tests can be done simply using a multimeter, but it may be necessary to locate a wiring diagram or use your service manual to determine how much power should be present at the connector.
How to Replace a Bad Fuel Pump
Replacing a fuel pump yourself can often be more difficult than diagnosing whether or not it needs to be replaced. In the case of in-tank electric fuel pumps, this repair often involves dropping the fuel tank. Fuel tanks can be extremely heavy when they are filled with fuel, and working with fuel can be dangerous, so it’s usually a good idea to drain the tank as much as possible to ensure you can safely remove it.
Fuel systems also typically use fuel line quick connectors to attach the fuel pump lines to the fuel lines or fuel filter. These can require specialty quick disconnect tools to safely disconnect the lines and free the fuel pump. Once the fuel pump is accessible, disconnect the electrical connector and determine how the fuel pump is attached to the tank. A lock ring is usually used to hold it in place. These lock rings should be removed carefully and may require specialty tools. In most cases, the lock ring and O-ring or gasket that seals the fuel pump should also be replaced. It is also a good idea to replace the fuel pump strainer, since this is the only time it will be accessible.
Some fuel pumps come as a complete assembly that also includes the fuel sending unit, float, and more. In other cases, the fuel pump may need to be removed from the sending unit assembly and a new fuel pump installed in its place before the unit is reassembled into the tank. Having the complete, step-by-step procedure and a service manual for your vehicle can help ensure that you can make this repair and that all steps are followed to perform the job safely.
Diagnosing and Replacing Your Fuel Pump
Now that you know what you’re looking for, you’ll be able to more easily diagnose fuel pump failures on your vehicle. Though we recommend that fuel pump service be performed by a qualified service center, replacing the fuel pump is possible even for DIY mechanics if proper precautions and preparations are observed. If you’re ready to make your repair, shop fuel pumps at O’Reilly® and find the right fuel pump and tools you need to make your repairs. If you’re not confident in your diagnosis or don’t feel comfortable making the repair yourself, we can also provide recommendations for a trusted repair shop in your area, both in-store, or using our Find a Repair Shop tool online.
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