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Know-How Notes: Fuel Pump Basics

fuel pump

There are numerous components inside your car that are rarely given a single thought, parts like the brake booster, radiator thermostat, or the fuel pump. These parts are hidden away, doing their job until the day comes that they don’t. That is when those parts quickly become a burden. The fuel pump is one of those critical components that is forgotten right up until it dies, and of course, when a fuel pump goes bad, it just goes, there is rarely a warning. Fuel pumps are typically not rebuildable (some aftermarket pumps are, but stock ones are not), so replacement is the only option. Most newer vehicles will get a factory replacement pump and that is the end of it, but for older vehicles and modified cars, there are some options. Fuel pumps come in three main flavors; diaphragm, rotary vane, and gerotor. Each has pros and cons to their design.

Diaphragm Pumps

The classic mechanical diaphragm pump is perfect for carbureted engines, but it can't provide the pressure for EFI. This is very reliable for carb applications.
The classic mechanical diaphragm pump is perfect for carbureted engines, but it can’t provide the pressure for EFI. This is very reliable for carb applications.

Diaphragm pumps use a composite membrane, which flexes up-down over a fuel bowl. The bowl has an input and an output, each with a check valve in place to ensure a one-way motion of fluid. As the diaphragm moves up, it creates a vacuum, pulling fuel from fuel tank into the fuel bowl. The actuating lever then pushes the membrane down, pushing the fuel out at pressure, which for carbureted engines is about 6 psi. Older carbureted engines use mechanical diaphragm pumps almost exclusively, and some of the cheaper aftermarket electric pumps use this style as well. Diaphragm pumps are simple in form and function, they last for years and years. Even when left to sit for 20 years, a diaphragm pump will usually still work once it gets a fresh hit of fuel. 

The biggest benefit of a diaphragm pump is that they draw fuel from the tank. Unlike the other types of pumps, this style creates a vacuum in the fuel bowl, so if you run out of gas, the diaphragm pump will quickly pull new fuel into the pump and into the engine without priming, they are in fact, self-priming. Dirty fuel is not an issue either, as the pump can’t really be damaged by dirty fuel (the engine can, but not the pump) as the fuel does not flow through any of the moving parts. Mechanical diaphragm pumps are typically easy to change as well, they are bolted directly to the engine (which drives the pump). Diaphragm pumps are not as efficient as other designs, which limits their use to carbureted applications. There are a few high-performance mechanical pumps, but those are piston-driven. 

Rotary Vane PumpsHolley Red rotary vane pump

Rotary vane pumps, such as the popular aftermarket Holley Red and Blue pumps, employ a paddle-wheel design inside a round base. The paddle wheel rotates off-center of the, creating a crescent-shaped cavity. As the wheel spins, sliding paddles move in and out of the wheel as they rotate against the walls of the pump, drawing fuel into the pump as the crescent opens, and then compressing it as it closes. This is what pressurizes the fuel. This sliding action generates quite a bit of heat through friction. 

Rotary vane pumps typically are used for low pressure applications and are almost in the T-style configuration (motor on top, inlet/outlet on the bottom), while a few are inline sliding vane pumps. High-performance and high-pressure applications, need a roller vane design. While sliding vane pumps have high friction on the vane edges, the roller design uses a rolling bar, eliminating the friction. This style of vane is more suited for high pressure, and are more efficient. Dirty fuel is a big issue for all rotary vane pumps. Dirt increases wear on the paddles and pump housing, reducing pressure output. 

Gerotor Pumpsgerotor fuel pump

Most modern vehicles use the gerotor design, which are the best for electric high-pressure, high-volume applications. A gerotor pump functions through spinning a spur gear that operates an internal ring gear with teeth on the inside of the ring. The ring gear rotates inside the cavity, creating a suction on the inlet and generates a lot of pressure on the outlet. Gerotor pumps are very efficient, quiet and can build very high pressures. The biggest drawback of a gerotor pump is that dirty fuel can wreck a pump quickly, as well as overheating. Lack of fuel kills these pumps faster than anything. When fuel is reduced to the pump, cavitation occurs, which destroys the pump in a matter of minutes. This the reason you want to maintain a minimum of 1/4 tank of fuel in your vehicle’s tank at all times. 

When using a gerotor pump in an aftermarket application, it is important to mount the pump as close to the tank as possible with a good gravity feed from the tank. A common misconception is that gerotor pumps do not produce vacuum on the inlet side, meaning they must be gravity fed by the fuel tank. In reality, gerotor pumps can generate significant vacuum. As the vacuum increases, the boiling rate of the fuel decreases, and the fuel to turns to vapor. This causes cavitation, which is an air bubble imploding. Cavitation is like setting off a bunch of tiny explosions inside the pump, it doesn’t take long, even a few minutes, to destroy the pump. By mounting the pump as close to the tank as possible, with a good gravity feed, you reduce the amount of vacuum generated by the pump, eliminating the vapor issues. 

Direct-Injection and Diesel Fuel Pumps

Diesel and direct-injected gasoline engines use a mechanical pump to generate the higher pressures needed for operation. These piston-actuated pumps are similar to the diaphragm pump, the piston design mounts to the engine like a standard diaphragm pump, but it uses a piston instead of a rubber diaphragm, allowing it to generate much higher pressures. GDI engines need 2,500 psi, while diesel engines use up to 30,000 psi fuel pressures. These are often driven off of a lug on the camshaft. Like a diaphragm pump, the piston pump draws fuel into the pump and forces it out under pressure; the difference here is that there is no diaphragm to rupture and the piston action can create substantially more pressure and flow more volume.

Because the pump is mounted to the block and runs off the camshaft, the pump only produces what the engine needs; there is no need for a return line from the fuel rails. The mechanical pump is fed by an electric lift pump. This is the same for both direct-injection gasoline engines and diesel engines. High-performance GDI (the term used for Gasoline Direct-injection) engines require some alteration when boosting power. Comp Cams now makes tuned cam lobes for the Gen V LT1 series GM engines, so you can boost your fuel output to match that new supercharger, and big-bore mechanical pumps are becoming available as well. 

Choosing The Right Pump

For custom applications, regardless of the style of pump you choose, you need to know what size to get. While the old penchant of bigger is better is often true, there are situations where too much fuel pump can actually cause problems. A fuel pump is designed to move fluid and build pressure. A typical carburetor requires 7-14 psi of fuel pressure to ensure the bowls stay full. EFI systems require wildly different pressure, but typically range from about 26 to 60 psi, more with boost.

Recently we had a 400 horsepower small-block with EFI that required 26 psi of fuel pressure. Because of some fuel delivery issues, we ended up strapping a monster fuel pump that was capable of 100 psi and could feed a 1,500 hp-carbureted engine. We noticed that the fuel pressure gauge would slowly creep up to almost 30 psi, even while the car was driving because the fuel pump was too big, causing the engine to run rich. It was simply pushing fuel past the built-in regulator on the TBI setup. A separate high-performance regulator could solve that problem, however the point is, too big can be a problem in certain situations. Eventually, the pump failed because the feed line from the tank was not large enough to support the needs of the pump, and it cavitated, which destroyed it. Setting up your fuel system requires more forethought than just “any pump will do.”

Keeping Your Fuel Pump Alive

When it comes to life span of a fuel pump, there are a few things you can do to keep your pump alive. Most OEM electric fuel pumps are located inside the fuel tank. This is to provide the pump with adequate fuel and to keep the pump cool and quiet. You can barely hear the pump if you turn your ignition switch to the “on” position and listen. A few seconds of whirring, and then it stops. This is pressurizing the fuel so that the engine can start.

The key here is keeping the pump cool. A hot pump is a dead pump, so the recommendation is to never let your gas tank get below 1/4-tank. Sure, the occasional dip below 1/4 is unavoidable, but consistently driving with less than 1/4-tank is very hard on in-tank fuel pumps.

The other trick is to replace your fuel filter. Most manufacturers advise changing the fuel filter every 20k-40k miles. When the filter gets plugged up, your engine struggles to get the fuel it needs, and the pump has to work harder pushing through the filter, shortening its life.

Check out all the fuel & emission system parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on fuel pumps and other fuel system parts, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Jefferson Bryant View All

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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