Every driver pumps fuel into the tank or has a gas station attendant do it. But most drivers never really think about how gasoline or diesel finds its way to the engine to be burnt until they encounter bad fuel pump symptoms.
What is a fuel pump and how does it work? What do fuel pump issues look like? How do you tell if a fuel pump is bad? Here’s what you need to know about its purpose and function, and bad fuel pump symptoms to watch out for.
What is a fuel pump?
A fuel pump is a device that pumps fuel from a car’s tank into the engine’s combustion chamber. When your car’s engine is running, it needs a steady supply of fuel to achieve a perfect air-fuel mixture at all times.
The fuel pump constantly supplies far more fuel to the engine than it requires under normal operation. There must always be more fuel coursing through the fuel lines than needed, otherwise your engine will run lean. That is, the air-fuel mixture will be too much air and not enough fuel. That’s not healthy for any engine in the long term.
Different types of fuel pumps
The type of fuel pump you find in your car will vary depending on your car’s age, its make, the type of engine under the hood and the fuel it runs on. Let’s break it down further.
Mechanical fuel pumps
On some vehicles, particularly older-model carbureted cars and trucks, a mechanical fuel pump is used to draw fuel to the engine. It uses the engine’s rotational motion from either the camshaft or the crankshaft to drive the pump, depending on the design. As a shaft is turned, a cam moves one end of a lever inside, which pulls on a rubber diaphragm causing fuel to be sucked toward the engine. When the cam releases the lever, the diaphragm closes, holding the fuel from draining back.
How fast the fuel flows toward the engine depends on the pump size. The design is straightforward and reliable, although it’s only good for engines with low fuel pressure requirements.
Electric fuel pumps
An electric fuel pump serves a similar purpose as a mechanical pump but in a very different way. Rather than being directly operated off of other engine systems, it’s powered by the car’s 12-volt electrical system. Typically it’s powered up when the ignition is turned to the “on” position, and the pump will prime the fuel pressure before the engine is fired up.
A range of electric fuel pumps exist, each serving a purpose for the cars that use them.
An in-tank fuel pump is arguably the most common type for fuel-injected vehicles today. This pump is submerged in the fuel inside the tank along with the fuel level sensor, keeping it cool while it operates. It uses an impeller-style system to create pressure and send fuel through the lines. Since they’re pushing fuel to the engine rather than pulling it from the tank, it can create higher fuel pressure and flow rates.
An inline fuel pump is installed in the fuel line between the fuel tank and the engine’s fuel rail. A traditional electric motor with a rotating armature and brushes pulls fuel up from the tank, then pressurizes fuel that’s delivered to the engine. Only when the pump’s desired pressure has been achieved does a pressure valve inside open to deliver it.
One intriguing fuel pump design is the rotary vane fuel pump made famous by Holley. It has a cylindrical chamber with an offset wheel against one side. Fuel is drawn into the chamber as a sliding paddle on the internal wheel opens up to the cavity inside. At the same time, the sliding paddle closes against the side of the chamber and compresses fuel through a second opening in the housing, sending it on its way to the engine.
A gerotor electric fuel pump operates like a mechanical oil pump in an engine. A spur gear rotates inside a ring gear, meshing on one side. An electric motor spins the spur gear which causes the ring gear to rotate, drawing fuel through the pump and pressurizing it on its way to the engine.
Bad fuel pump symptoms to watch out for
Like most systems in a passenger vehicle, the fuel pump may not last forever. That said, the fuel pump is usually not readily accessible when you suspect there’s a problem, and it’s definitely not easy to see what’s going on inside. Unfortunately, a Google search alone won’t provide clear answers. Rather, a relatively easy way of determining if you may have a bad fuel pump is by looking for symptoms.
When you accelerate is when your car requires the most fuel. It’s what’s known as being “under load.” If your car doesn’t perform like it once did as you hit the gas, there’s a chance that not enough gas is hitting the engine. When the engine is starved of fuel, it can’t produce the same horsepower, and the performance is affected.
Sputtering at high RPMs
If you’re driving at high RPMs, fuel consumption is vastly higher than at idle or on the highway. It makes the fuel pump work hard to push or pull fuel to the injectors, and the natural result of high RPMs is lower fuel pressure. If the fuel pressure decreases, there isn’t enough fuel being delivered to the injectors to maintain the air-fuel mixture. Then the spark plugs won’t ignite the mix properly, resulting in uneven burn and sputtering.
Whining noise from fuel tank area
If the fuel pump has overheated or is just plain worn out, you could start hearing some noise. The typical fuel pump noise you’d hear when it’s on the way out is a high-pitched whining noise from wherever it’s located. In most cars, it’s in the fuel tank. Although it might still work for a while—even indefinitely—a whining noise that exceeds the normal low hum indicates that the pump is damaged, and you shouldn’t trust it to be reliable.
Long cranking time
The engine should start within a second or two when you crank it. Extended cranking time often means that fuel has bled back into the tank, and there’s air in the fuel lines. That’s not normal. A fuel pump should have a check valve in it that keeps it primed, or prevents fuel from bleeding back.
Engine stalls out
A car can operate completely fine one moment and stall the next. That happens when the fuel pump has been damaged and is prone to overheating. It’s like flipping a switch—the engine will completely die and you’ll coast along to a stop after driving for a period of time. It may or may not begin working again when the fuel pump cools off.
Surging when you drive or at idle
A weak fuel pump that has moments of normal operation or a fuel pump that periodically pushes too much fuel to the engine can cause surging. When it happens, it feels like someone blipped the throttle for a second, then everything goes back to the way it was. It can happen when you’re driving, or you could see the RPMs surge when you’re sitting at idle as well.
Increased fuel consumption
A fuel pump can also deliver too much fuel to the engine on some vehicles. On cars that don’t have a fuel return line that sends excess fuel back to the tank, it’s possible for a faulty fuel pump to jam too much fuel through the injectors and into the combustion chamber, and you’ll notice it in fuel costs.
When you see black smoke from the tailpipe, it’s an indicator that your engine is burning too rich. That is, there’s too much fuel and not enough air in the mix. You might notice that your spark plugs are fouling often because of the excessive fuel, and it could all be due to a fuel pump that’s delivering too much fuel.
Engine temperature climbs
On the other hand, insufficient fuel supply will cause your engine to run lean. That means there isn’t enough fuel in the air-fuel mixture. If that occurs, engine temperatures can climb higher than normal. In extreme cases, it can even cause the engine to overheat.
Other ways to check for a bad fuel pump
You don’t always notice bad fuel pump symptoms. The opposite might be true—there may be things that aren’t happening that should be. For example:
- If your car doesn’t start and you don’t hear the low hum from the fuel tank with the ignition turned on, it can indicate that the fuel pump isn’t powering up.
- You could notice that the gas gauge isn’t moving with the key on even though you know there’s gas in the tank. Although the fuel level sensor and fuel pump aren’t necessarily connected, the wiring harness likely is and that could be where the problem lies.
- The fuel pump fuse might be blown. Changing the fuse is easy enough, but why is it burnt? It could be a sign that the fuel pump is faulty and drawing more amperage than it should.
Don’t leave a stone unturned while looking at fuel pump problems. Carefully pressing the Schrader valve on the fuel rail should deliver a spurt of fuel if there’s fuel pressure, and using a loaner fuel pressure gauge from a store like AutoZone will let you see if fuel pressure is within normal operating values.
But keep an eye out for weird quirks for your specific car as well, commonly found by searching automotive forums. As one example, Ford installs an inertia switch that cuts your car’s fuel supply if it detects a collision, but a big pothole or bump can also cause it to trigger.
Fuel pump replacement costs
So you’ve come to the conclusion that you have a bad fuel pump. You’ll need to decide if it’s a job you’re willing to tackle or if you’re better off having a pro change it out.
How much a fuel pump costs to replace can vary greatly. For example, a mechanical fuel pump on a 1964 Buick Riviera is probably less than $40 and is a straightforward DIY job for someone with moderate mechanical skills, taking under an hour. But an in-tank fuel pump for a 2009 BMW 5 Series will run you nearly $500 for the fuel pump and could take three hours or more to change since the fuel tank is directly under the rear seat. On other models—like most pickup trucks—you’ll need to lower the fuel tank to change the pump. If you’re paying to have it done, more time always equals higher labor costs.
As a DIY project, a fuel pump is doable. It’s pretty easy to change a mechanical pump or an inline fuel pump using basic hand tools, but an in-tank pump is likely to pose challenges. You may need a fuel pump ring removal tool, a pump to drain the tank, and a hydraulic jack to lower and lift the tank into place. If you aren’t experienced at working on your own cars, you might want to consider handing this job off to a pro.