Diagnosing A Mechanical Fuel Pump: Losing Pressure When Hot

Mechanical fuel pumps are generally very reliable, but they can lose pressure when hot. This can be caused by a variety of factors, such as a faulty pressure gauge, vapor lock, or a leak in the fuel system. Other potential issues include a worn fuel pump lobe on the cam, a clogged fuel tank pickup, or insufficient tank venting. It is also possible that the fuel is boiling due to high underhood heat or that there is an issue with the fuel pump itself, such as a faulty diaphragm or a problem with the mounting bolts.

Fuel bowl draining

To drain the fuel bowl, follow these steps:

• Locate the yellow lever on the fuel bowl.
• Place a bucket under the engine to catch the fuel.
• Turn the yellow lever to drain the fuel from the bowl. You will hear a gurgling sound coming from the bowl as it drains.
• Keep the engine off while draining the fuel.
• Optionally, attach a vacuum hose to the steel tubing near the cross member to direct the drained fuel.
• If you have a fuel filter, clean it and the fuel bowl of any residue.

Regarding the issue of a mechanical fuel pump losing pressure when hot, there are a few potential causes:

• Leaks in the fuel line or connections: Check for any signs of leaks, as this can lead to a loss of pressure.
• Fuel evaporation: Modern fuel has a high concentration of light hydrocarbons, which can cause it to evaporate more easily, especially when the engine is hot.
• Fuel pump check valve issues: The check valves in the fuel pump may not be functioning properly, allowing fuel to drain back into the tank or carburetor.
• Choke issues: If the choke is closing when it shouldn't, it can lead to flooding and starting issues.

Fuel cap venting

Fuel tank venting is essential to allow your tank to breathe. Without a vent, you wouldn't be able to fill your tank with fuel or withdraw fuel from it. As the temperature changes, the volume of fuel in your tank changes: it expands as it warms up and shrinks as it cools down. This means the fuel level in your tank changes throughout the day, even if your vehicle is stationary.

Up until the 1960s, most vehicles used vented gas caps. These are gas caps with a hole in them that allows air to enter the tank. However, this design has a drawback: the hole can also allow fuel to splash out when accelerating or turning corners. Modern vented gas caps have a one-way release valve that is pressure-activated. When the pressure reaches a certain point, the valve opens slightly to relieve the pressure. This prevents a vacuum from forming inside the tank as the fuel level decreases.

There are a variety of vented fuel caps available on the market, such as those designed for transfer tanks, auxiliary fuel tanks, and diesel fuel gas transfer tanks. When choosing a vented fuel cap, it is important to ensure that the vent line is higher than the highest point on the tank, including the fuel filler neck. Additionally, the vent line should not have any dips that could trap fuel or condensation.

Fuel hose material

The material used for fuel hoses has evolved over the years, with different materials being favoured at different times. Here is a rundown of the most common fuel hose materials:

Rubber

Rubber was the first material used for fuel hoses and was standard for many years. It is flexible and can be cut to the required length, making it easy to work with. However, rubber hoses tend to perish over time and can rub through if not secured properly. They are also susceptible to degradation from certain fuel additives and blends, particularly those containing ethanol, which can cause them to dry out and pose a fire hazard.

Braided Stainless Steel

Developed in the 1950s for military aircraft, braided stainless steel hoses with a chlorinated polyethylene (CPE) core became popular in the 1960s for performance and racing applications. This type of hose is more resistant to degradation by fuel and oil and can handle higher temperatures and pressures than rubber hoses. However, changes in fuel blending have caught up with this type of hose, and they are now susceptible to degradation from certain fuel additives, particularly ethanol.

Nylon

More modern vehicles may use nylon fuel lines, which are typically found in plastic fuel line systems. Nylon fuel lines do not perish and are lighter than metal tubing, but they melt at lower temperatures and cannot be easily repaired.

Steel

Many vehicles with rear-mounted fuel tanks use rigid steel fuel pipes that run from the tank to the engine bay. Steel pipes are strong and inexpensive, but they can corrode over time, leading to fuel leaks.

Copper

Older vehicles may use copper fuel pipes. These are easy to fit and repair, but copper is heavy and expensive compared to other options.

PTFE (Teflon)

Polytetrafluoroethylene (PTFE), more commonly known by the brand name Teflon, is a synthetic fluoropolymer that is chemically inert to all common fuels except acid. It has a very high-temperature stability and a tight bend radius, making it ideal for fuel hose applications. PTFE hoses are so durable that, with proper installation, they should last the life of the vehicle. While PTFE hoses are more expensive upfront, their long lifespan makes them a cost-effective choice in the long run.

Fuel pump gasket

A fuel pump gasket is a crucial component in the fuel delivery system of an automobile. It acts as a seal between the fuel pump and the engine, ensuring a tight fit and preventing fuel leaks. Gaskets are typically made of materials such as nitrile rubber, fluoroelastomer, or aluminium composite.

There are various fuel pump gaskets available in the market, designed to fit different vehicle models. For instance, the FEL-PRO brand offers a wide range of fuel pump gaskets compatible with vehicles like the Ford F150, Chevrolet Silverado 1500, Toyota Camry, and Ford Focus.

When choosing a fuel pump gasket, it is essential to ensure compatibility with your vehicle's fuel pump and engine. A defective or worn-out gasket can lead to fuel leaks, which pose safety hazards and negatively impact engine performance.

Additionally, some gaskets come with warranties, such as those offered by Felpro and Motorcraft, providing added assurance and peace of mind. It is always recommended to refer to your vehicle's manual or seek professional advice when selecting and installing a fuel pump gasket.

Fuel pump mounting bolts

ARP, for example, offers fuel pump bolt kits made of chromoly with a black oxide finish or stainless steel with a polished finish. These kits are designed for specific engine types, such as Chevy big block and small block engines or Pontiac V8 engines. The bolts feature either a hex or 12-point head design and are known for their high tensile strength, rated at 170,000 psi.

In addition to ARP, other brands like Gardner-Westcott Company, Brian Tooley Racing, and Mopar offer fuel pump bolts for various vehicle applications. When choosing fuel pump mounting bolts, it is important to select the correct type for your vehicle to ensure a secure and proper fit.

Amazon also offers a wide range of options for fuel pump mounting brackets and clamps, which are essential for securely holding the fuel pump in place. These brackets and clamps are designed to fit specific vehicle makes and models, ensuring a precise installation. Some of the brands available on Amazon include EVIL ENERGY, Spectre Performance, X AUTOHAUX, and QWORK.

Frequently asked questions