Fuel Delivery System


Fuel Delivery System

The components of a typical gasoline delivery system are fuel tanks, fuel lines, fuel filters, and fuel pumps.

Fuel Tanks

Modern fuel tanks include devices that prevent vapors from leaving the tank. For example, to contain vapors and allow for expansion, contraction, and overflow that results from changes in the temperature, the fuel tank has a separate air chamber dome at the top. Another way to contain vapors is to use a separate internal expansion tank within the main tank. All fuel tank designs provide some control of fuel height when the tank is filled. Frequently, this is achieved by using vent lines within the filler tube or tank. With tank designs such as this, only 90 percent of the tank is over filled, leaving 10 percent for expansion in hot weather. Some vehicles have an overfill limiting valve to prevent overfilling of the tank. If a tank is filled to capacity, it overflows whenever the temperature of the fuel increases.

Fuel tanks can be constructed of pressed corrosion-resistant steel, aluminum, or molded reinforced polyethylene plastic. Aluminum and molded plastic fuel tanks are becoming more common as manufacturers attempt to reduce the overall weight of the vehicle. Metal tanks are usually ribbed to provide added strength. Seams are welded, and heavier gauge steel is often used on exposed sections for added strength.

Most tanks have slosh baffles or surge plates to prevent the fuel from splashing around inside the unit. In addition to slowing down fuel movement, the plates tend to keep the fuel pick-up or sending assembly immersed in the fuel during hard braking and acceleration. The plates or baffles also have holes or slots in them to permit the fuel to move from one end of the tank to the other. Except for rear engine vehicles, the fuel tank in a passenger car is located in the rear of the vehicle for improved safety.

The fuel tank is provided with an inlet filler tube and cap. The location of the fuel inlet filler tube depends on the tank design and tube placement. It is usually positioned behind the filler cap or a hinged door in the center of the rear panel or in the outer side of either rear fender panel. Vehicles designed for unleaded fuel use have a restrictor in the filler tube that prevents the entry of the larger leaded fuel delivery nozzle at the gas pumps. The filler pipe can be a rigid one-piece tube soldered to the tank or a three-piece unit. The three-piece unit has a lower neck soldered to the tank and an upper neck fastened to the inside of the body sheet metal panel.

Filler tube caps are nonventing and usually have some type of pressure-vacuum relief valve arrangement. Under normal operating conditions the valve is closed, but whenever pressure or vacuum is more than the calibration of the cap, the valve opens. Once the pressure or vacuum has been relieved, the valve closes. Most pressure caps have four antisurge tangs that lock onto the filler neck to prevent the delivery system's pressure from pushing fuel out of the tank. By turning such a cap one-half turn, the pressure in the tank will not be released all at once. Then, with another quarter turn, the cap can be removed.

Starting with the 1976 model year, a Federal Motor Vehicle Safety Standard (FMVSS 301) required a control on gasoline leakage from passenger cars and certain light trucks and buses, after they were subjected to barrier impacts and rolled over. Tests conducted under these severe conditions showed the most common gasoline leak path was the gasoline supply line from the fuel tank to the carburetor.

Most rollover leakage protection devices used on carburetor-equipped engines are variations of a basic one-way check valve. These protective check valves are usually installed in the fuel vapor vent line between the tank and the vapor canister and at the carburetor fuel feed or return line fitting. In some systems the check valve is part of the carburetor inlet fuel filter.

Under normal operation, the mechanical fuel pump pressure is sufficient to open the check valve and supply fuel to the engine. However, if the vehicle is involved in a rollover accident, fuel spills out of the carburetor, the engine stalls, and the fuel pump ceases to operate. This decreases fuel system pressure to the point where the check valve closes. This prevents fuel from reaching the carburetor where it would leak out.

A check valve might also be fitted in the fuel tank filler cap, and most caps' pressure-vacuum relief valve settings have been increased so fuel pressure cannot open them in a rollover.

Many electric fuel pumps found on vehicles with fuel injection systems have an inertia switch that shuts off the jump if the vehicle is involved in a collision or rolls over. The Ford inertia switch consists of a permanent magnet, a steel ball inside a conical ramp, a target plate, and a set of electrical contacts. The magnet holds the steel ball in the bottom of the conical ramp. In the event of a collision, the inertia of the ball causes it to break away from the magnet, roll up the conical ramp, and strike the target plate. The force of the ball striking the plate causes the electrical contacts in the inertia switch to open, cutting off power to the fuel pump. The switch has a reset button that must be depressed to close the contacts before the pump operates again.

Some fuel line systems contain a fuel return arrangement that aids in keeping the gasoline cool, thus reducing chances of vapor lock. The return system consists of a special fuel pump equipped with an extra outlet fitting and necessary fuel line. The fuel return line generally runs next to the conventional fuel line, except that flow is in the opposite direction. The fuel return system allows a metered amount of cool fuel to circulate through the tank and fuel pump, thus reducing vapor bubbles caused by overheated fuel.

Some form of liquid vapor separator is incorporated into most modern vehicles to stop liquid fuel or bubbles from reaching the vapor storage canister or the engine crankcase. It can be located inside the tank, on the tank, in fuel vent lines, or near the fuel pump. Check the service manual for the exact location of the liquid vapor separator and line routing.

Inside the fuel tank there is also a sending unit that includes a pick-up tube and float-operated fuel gauge. The fuel tank pick-up tube is connected to the fuel pump by the fuel line. Some electric fuel pumps are combined with the sending unit. The pick-up tube extends nearly, but not completely, all the way to the bottom of the tank. Rust, dirt, sediment, water cannot be drawn up into the fuel tank filter, which can cause clogging. The ground wire is often attached to the fuel tank unit.

Inspecting the Fuel Tank A fuel tank should be inspected for leaks, road damage, corrosion, and rust on metal tanks, loose, damaged, or defective seam, loose mounting bolts, and damaged mounting straps. Leaks in the fuel tank, lines, or filter may cause gasoline odor in and around the vehicle, especially during low-speed driving and idling. In most cases, the fuel tank  must be removed for servicing.

Fuel Lines

Fuel lines can be made of either metal tubing, flexible nylon, or synthetic rubber hose. The latter must be able to resist gasoline. It must also be nonpermeable, so gas and gas vapors cannot evaporate through the hose. Ordinary rubber hose, such as that used for vacuum lines, deteriorates when exposed to gasoline. Only hoses made for fuel systems should be used for replacement. Similarly, vapor vent lines must be made of material that resists attack by fuel vapors. Replacement vent hoses are usually marked with the designation EVAP to indicate their intedned use. The inside diameter of a fuel deliver hose is generally larger than that of a fuel return hose.

Many fuel tanks have vent hoses to allow air in the fuel tank to escape when the tank is being filled with fuel. Vent hoses are usually installed alongside the filler neck.

The fuel lines carry fuel from the fuel tank to the fuel pump, fuel filter, and carburetor or fuel injection assembly. These lines are usually made of rigid metal, although some sections are constructed of rubber hose to allow for car vibrations. This fuel line, unlike filler neck or vent hoses, must work under pressure or vacuum. Because of this, the flexible synthetic hoses must be stronger. This is especially true for the hoses on fuel injection systems, where pressures reach 50 psi or more. For this reason, flexible fuel line hose must also have special resistance properties. Many auto manufacturers recommend that flexible hose be used only as a delivery hose to the fuel metering unit in a fuel injection system. It should not be used on the pressure side of the injector systems. This application requires a special high-pressure hose.

All fuel lines should occasionally be inspected for holes, cracks, leaks, kinks, or dents. Many fuel system troubles can occur in the lines are blamed on the fuel pump or carburetor. For instance, a small hole in the fuel line admits air but does not necessarily show any drip marks under the car. Air can then enter the fuel line, allowing the fuel to gravitate back into the tank. Then, instead of drawing fuel from the tank, the fuel pump sucks only air through the hole in the fuel line. When this condition exists, the fuel pump is frequently tested, and if there is insufficient fuel, it is considered faulty, when in fact there is nothing wrong with it. If a hole is suspected, remove the coupling at the tank and the pump and pressurize the line with air. The leaking air is easily spotted.

Since the fuel is under pressure, leaks in the line between the pump and carburetor or injectors are relatively easy to recognize. When a damaged fuel line is found, replace it with one of similar construction - steel with steel, and the flexible with nylon or synthetic rubber. When installing flexible tubing, always use new clamps. The old ones lose some of their tension when they are removed and do not provide an effective seal when used on the new line.

Fuel supply lines from the tank to the carburetor or injectors are routed to follow the frame along the underchassis of vehicles. Generally, rigid lines are used extending from near the tank to a point near the fuel pump. To absorb engine vibrations, the gaps between the frame and tank or fuel pump are joined by short lengths of flexible hose.

Steel tubing should inspected for leaks, kinks, and deformation. This tubing should also be checked for loose connections and proper clamping to the chassis. If the fuel tubing threaded connections are loose, they must be tightened to the specified torque.

Any damaged or leaking fuel line - either a portion or the entire length - must be replaced. To fabricate a new fuel line, select the correct tube and fitting dimension and start with a length that is slightly longer than the old line. With the old line as a reference, use a tubing bender to form the same bends in the new line as those that exist in the old. Although steel tubing can be bent by hand to obtain a gentle curve, any attempt to bend a tight curve by hand usually kinks the tubing. To avoid kinking, always use a bending tool.

The two most-used tubing fittings are the compression fittings and the double-flare. The double-flare, which is the most common, is made with a special tool that has an anvil and a cone. The double-flaring process is performed in two steps. First, the anvil begins to fold over the end of the tubing. Then, the cone is used to finish the flare by folding the tubing back on itself, doubling the thickness, and creating two sealing surfaces.

The angle and size of the flare are determined by the tool. Careful use of the double-flaring helps to produce strong, leakproof connections.

The flare tool can also be used to make sure nylon and synthetic rubber hoses stay in place. that is, to make sure the connection is secure, put a partial double-lip flare on the end of the tubing over which the hose is installed. This can be done quickly, with the proper flaring tool, by starting out as if it was going to be a double-flare but stopping halfway through the procedure. This provides an excellent sealing ridge that does not cut into the hose. A clamp should be placed directly behind the ridge on the hose caused by the raised section on the metal line.

There are a variety of clamps used on fuel system lines, including the spring and screw types. The crimp clamps are used most for metal tubing, but they require a special tool to install.

To control the rate of vapor flow from the fuel tank to the vapor storage tank, a plastic or metal restrictor may be placed in either the end of the vent pipe or in the vapor-vent hose itself. When the latter hose must be replaced, the restrictor must be removed from the old vent hose and installed in the new one.

Fuel Filters

Automobiles and light trucks usually have an in-tank strainer and a gasoline filter. The strainer, located in the gasoline tank, is made of a finely woven fabric. The purpose of this strainer is to prevent large contaminant particles from entering the fuel system where they could cause excessive fuel pump wear or plug fuel metering devices. It also helps to prevent passage of any water that might be present in the tank. Servicing of the fuel tank strainer is seldom required.

The gasoline filter is usually located in the engine compartment and is the one this section examines because it is replaceable and might require service on a regular basis. The most common types of gasoline filters are in-carburetor filters and in-line filters.

In-Carburetor Filters There are three basic types of in-carburetor gasoline filters. Pleated paper filters use pleated paper as the filtering medium. Paper elements are more efficient than screen-type elements, such as nylon or wire mesh, in removing and trapping small particles, as well as large-size contaminants. Sintered bronze filters are often referred to as a stone or ceramic filter. Screw-in filters are designed to screw into the carburetor fuel inlet. The fuel line attaches to a fitting on the filter. This filter has a magnetic element to remove metallic contamination before it reaches the carburetor.

In-Line Filters In-line gasoline filters are installed in the fuel line. In carbureted engines, the in-line gasoline filter is usually installed between the fuel pump and the carburetor. In vehicles with a fuel injection system, the location of the fuel filter is determined by the manufacturer. Fitted with a pleated paper element, the in-line filter is sometimes installed as an extra protective measure. The optionally installed in-line filter then works in conjunction with the in-tank and in-carburetor gasoline filters. Because of its large capacity, an in-line filter is often the most economical solution to a fuel system's contamination problems. An arrow on the filter shows the direction of fuel flow.

Servicing Filters Fuel filters and elements are serviced by replacement only. Replacing the gasoline filter or element at the intervals recommended by the vehicle or engine manufacturer is the most effective method of minimizing fuel starvation and other carburetor problems. On occasions when the fuel system has been subjected to excessive amounts of contaminants, more frequent filter changes may be required.

Fuel Pumps

The fuel pump is the device that draws the fuel from the fuel tank through the fuel lines to the engine's carburetor or injectors. Basically, there are two types of fuel pumps: mechanical and electrical. The latter is the most commonly used today.

Mechanical Fuel Pump The mechanical fuel pump has a synthetic rubber diaphragm inside the unit that is actuated by an eccentric located on the engine's camshaft. As the camshaft rotates during engine operation, a shaft or rocker arm in the pump is moved up and down or back and forth, depending on the fuel pump's position on the engine. This causes the diaphragm to move back and forth, drawing fuel from the fuel tank.

The mechanical fuel pump is located on the engine block near the front of the engine. Normally, it is a sealed unit that cannot be repaired. If the pump leaks from either the vent hole or from a seam, it must be replaced. If engine performance indicates inadequate fuel, the pump, while mounted on the engine, should be tested for pressure and volume.

Incorrect fuel pump pressure and low volume (capacity of flow rate) are the two most likely fuel pump troubles that affect engine performance. Low pressure causes a lean mixture. Excessive pressure causes high fuel consumption and an overly rich mixture with fuel injection or carburetor flooding. And, low volume causes fuel starvation at high  speeds.

To determine if the fuel pump is in satisfactory operating condition, tests for both fuel pump pressure and fuel pump capacity (volume) should be performed. These tests are performed with the fuel pump installed on the engine and the engine at normal operating temperature and at idle speed.

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