Sucking Through A Fuel Pressure Line: Is It Possible?

The ability to suck fuel through a fuel pressure line depends on several factors, including the type of fuel pump, the pressure and volume of fuel, and the presence of restrictions or leaks in the fuel system. Electric fuel pumps, commonly found in modern cars, are typically located inside the fuel tank and create positive pressure to push fuel towards the engine. This design has advantages such as using fuel as a coolant for the pump and reducing the chance of fuel boiling. However, electric pumps may struggle to aspirate or suck fuel through a dry line, especially if there are restrictions on the suction side, such as a spring-loaded check valve. Mechanical fuel pumps, on the other hand, use a diaphragm to create suction and can be more effective at sucking fuel. Factors like head pressure, the height of the fuel tank relative to the engine, and the quality of the pump can also influence its ability to suck fuel through the line. Ultimately, while some pumps may be better at pushing fuel than pulling, proper plumbing, venting, and ensuring a restriction-free system can improve the chances of successfully sucking fuel through a fuel pressure line.

Electric fuel pumps can suck gas through a dry line

Electric fuel pumps can sometimes suck gas through a dry line. However, this depends on a few factors. If the fuel supply system has very little head loss built into it on the suction side of the pump, or if the fuel level in the tank is above the engine, an electric fuel pump might be able to prime itself and successfully draw fuel.

On the other hand, if there are any restrictions on the suction side of the pump, such as a spring-loaded check valve, an electric pump will often struggle to prime itself and will require manual assistance to start operating. Mechanical pumps are generally more forgiving in these situations, but they can also encounter difficulties when trying to move air through and out of the fuel system.

It is worth noting that electric fuel pumps are not effective air compressors, and they may struggle to overcome the check valve in the carburetor, potentially leading to fuel being pumped into the bilge. This could be a safety concern, and it is recommended to consider a manual fuel shut-off valve instead of a spring-loaded anti-siphon valve to mitigate this issue.

Additionally, electric fuel pumps should be properly wired to an oil pressure safety switch to prevent the continuous supply of fuel in the event of a crash or engine stoppage.

Mechanical fuel pumps can fail if you're able to suck through them

Mechanical fuel pumps are used on older engines that have carburetors. They are typically driven by the camshaft or a special shaft driven by the crankshaft. The pump siphons fuel from the gas tank and pushes it to the carburetor when the engine is cranking or running. Inside the pump, a lever moves a rubber diaphragm up and down, creating suction that draws fuel into the pump and then pushes it along.

Mechanical fuel pumps can fail if you are able to suck through them. This indicates a leak in the diaphragm or a one-way valve inside the pump, leading to a loss of fuel pressure and starving the carburetor for fuel. As a result, the engine may run lean, misfire, hesitate, or stall. If the pump fails entirely, no fuel will be delivered to the carburetor, causing the engine not to start or run.

To diagnose a faulty mechanical fuel pump, you can perform the following checks:

• Remove the air cleaner and inspect the carburetor. If you don't see fuel squirting into the carburetor, the pump may have failed or there could be a blockage in the fuel line or fuel filter.
• Visually inspect the pump for any fuel dripping, which indicates a failed diaphragm.
• Disconnect the fuel line at the carburetor and crank the engine to check if the pump is pushing fuel through the line. Strong, steady spurts of fuel indicate a functioning pump, while no fuel or a weak stream suggests a faulty pump or blockage.
• Check the fuel pump pressure by connecting a fuel pressure gauge to the pump outlet or tee into the fuel line at the carburetor. If there is no pressure or it is below specifications, the pump needs to be replaced.

It is important to address fuel pump issues promptly to avoid engine problems and ensure the safe operation of your vehicle.

In-tank fuel pumps are more efficient at pushing fuel than external pumps

Another benefit of in-tank pumps is that they reduce the chance of fuel boiling. This is because the higher pressure raises the boiling point of the fuel. By submerging the pump in the tank, it is also kept away from the engine, which reduces the risk of fires. The pump is also less likely to cavitate, or suck air, since it has a larger body of fuel to supply it.

In-tank pumps also have the advantage of head pressure. This means that the pump always has pressure assisting it in pushing the fuel, making it more efficient. External pumps, on the other hand, have to work harder to pull the fuel uphill before pushing it down the fuel line.

Overall, in-tank fuel pumps are more efficient and cost-effective than external pumps. They are also safer and reduce the chance of fuel boiling, making them a better choice for modern cars.

Fuel pumps are simple – carbureted engines want 8-12 psi, EFI engines want 40-70 psi

Fuel pumps are simple, but they play a crucial role in delivering fuel to your engine. The type of fuel pump you need depends on the engine you have – carbureted or EFI (electronic fuel injection).

Carbureted engines typically require lower fuel pressure, around 8-12 psi. This means that carbureted engines can operate with a simpler fuel delivery system, often using a mechanical fuel pump. Mechanical fuel pumps are driven by the engine and use a diaphragm to draw fuel from the tank and deliver it to the carburetor. They are simple, effective, and don't require a regulator since they don't produce high fuel pressure.

On the other hand, EFI engines require higher fuel pressure, typically in the range of 40-70 psi. To achieve this higher pressure, EFI engines usually employ electric fuel pumps. Electric fuel pumps are mounted near or inside the fuel tank and use electricity to generate the pressure needed to deliver fuel to the engine. Due to the higher pressure and flow requirements of EFI systems, a fuel pressure regulator is usually needed to control the fuel pressure and return excess fuel to the tank.

When choosing a fuel pump, it's important to consider the horsepower requirements of your engine. EFI engines generally require higher flow rates due to the higher fuel pressure. Additionally, the type of fuel used can also impact the choice of fuel pump as some pumps are designed specifically for gasoline, E85, or methanol fuels.

In summary, carbureted engines typically operate at lower fuel pressures (8-12 psi) and can use simpler mechanical fuel pumps, while EFI engines require higher fuel pressures (40-70 psi) and usually need electric fuel pumps along with a fuel pressure regulator to control the fuel delivery system.

Fuel pumps don't technically suck – atmospheric pressure forces fuel into the space created by mechanical action

Fuel pumps don't technically suck—they use atmospheric pressure to force fuel into the space created by mechanical action. This is true for both mechanical and electric fuel pumps. Mechanical pumps, usually found in older cars, are powered by the engine and use a lever that moves up and down as the camshaft spins, creating a pump that pulls fuel through the line by way of suction.

Electric fuel pumps, on the other hand, are powered by a DC motor in the pump assembly. They draw fuel from the tank and send it up the fuel line and into the fuel rail, where it is injected into a cylinder. Electric pumps are typically located inside the fuel tank to keep them cool and reduce the risk of fire.

Both types of pumps rely on the creation of low pressure in the fuel line to draw fuel from the tank. This is not technically "sucking," but rather utilising the higher atmospheric pressure outside the fuel line to force fuel into the low-pressure space inside the line.

In addition, the specific design of the pump mechanism also plays a role in fuel delivery. Mechanical pumps often use a diaphragm and valve arrangement, where the movement of a camshaft or crankshaft forces a pivoted lever up and down, creating suction that draws fuel into the pump. Electric pumps, meanwhile, may use a solenoid (an electromagnetic switch) to attract an iron rod, which pulls a diaphragm down and draws fuel into the chamber.

While the specific mechanisms may vary, the underlying principle remains: fuel pumps use mechanical action to create low pressure in the fuel line, which is then filled by the higher-pressure fuel from the tank. This process ensures a consistent supply of fuel to the engine, where it is mixed with air and ignited to power the vehicle.

Frequently asked questions