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Fuel Systems, Page 1 of 6
Links to topics on page 1 Links to topics on page 2 Links to topics on page 3 Links to topics on page 4 Links to topics on page 5 Links to topics on page 6 The fuel delivery system The fuel delivery system consists of all the components which supply the engine with fuel. This includes the tank itself, all the lines, one or more fuel filters, a fuel pump (mechanical or electric), and the fuel metering components (carburetor or fuel injection system). Fuel tank Fuel tanks are normally located at the rear of the vehicle, although on rear or mid engine vehicles they are usually located at the front. The tank contains a fuel gauge sending unit, a filler tube and on most fuel injected vehicles, a fuel pump. In most tanks, there is also a fine mesh screen "sock" attached to the pickup tube. This is used to filter out large particles which could easily clog the fuel lines, fuel pump and fuel filter. Since the advent of emission controls, tanks are equipped with a control system to prevent fuel vapor from being discharged into the atmosphere. A vent line in the tank is connected to an activated carbon or charcoal filled canister in the engine compartment. Vapors from the tank are stored in this canister, until they can be purged later for combustion in the engine. On many carbureted engines, the float bowl is also vented to this canister. Fuel pumps Mechanical pumps Mechanical pumps are usually found on carbureted engines or on engines that utilize a mechanical fuel injection system. Mechanical fuel pumps on carbureted engines are usually mounted on the side of the engine block or cylinder head and operated by an eccentric on the engine's camshaft. The rocker arm of the pump rests against the camshaft eccentric, and as the camshaft rotates, it actuates the rocker arm. Some engines use a pushrod between the rocker arm and camshaft eccentric. Inside the fuel pump, the rocker arm is connected to a flexible diaphragm. A spring, mounted underneath, maintains pressure on the diaphragm. As the rocker arm is actuated, it pulls the diaphragm down and then releases it. Once the diaphragm is released, the spring pushes it back up. This continual diaphragm motion causes a partial vacuum and pressure in the space above the diaphragm. The vacuum draws the fuel from the tank and the pressure pushes it toward the carburetor or injection pump. A check valve is used in the pump to prevent fuel from being pumped back into the tank.
See Figures 1 and 2
Figure 1 Cutaway view of a common mechanical fuel pump. 
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Figure 2 Some mechanical pumps are mounted to the cylinder head, rather than the engine block. 
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Certain mechanical fuel injection systems also utilize a mechanical fuel pump, typically some diesel engines and early gasoline fuel injection systems. Many of them use a fuel pump essentially identical to the carbureted fuel system's. Some, however, use a vane type fuel pump mounted directly to the injection pump/fuel distributor assembly. The injection pump/fuel distributor assembly is driven by the timing belt, chain or gears which in turn drives the fuel pump. The vanes draw the fuel in through the inlet port then squeeze the fuel into a tight passage. The fuel then exits pressurized through the outlet port.
Electric pumps
See Figures 3 and 4
There are two general types of electric fuel pumps: the impeller type and the bellows type. Electric pumps can be found on all types of fuel systems.
The impeller type pump uses a vane or impeller that is driven by an electric motor. These pumps are often mounted in the fuel tank, though they are sometimes found below or beside the tank. The vanes or impeller draw the fuel in through the inlet port then squeeze the fuel into a tight passage. This pressurizes the fuel. The pressurized fuel then exits through the outlet port.
Figure 3 Some electric fuel pumps are mounted outside of the fuel tank. 
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Figure 4 Most electric fuel pump are mounted inside the fuel tank and are part of the fuel level sending unit. 
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The bellows type pump is rare. This pump is ordinarily mounted in the engine compartment and contains a flexible metal bellows operated by an electromagnet. As the electromagnet is energized, it pulls the metal bellows up. This draws the fuel from the tank into the pump. When the electromagnet is de-energized, the bellows returns to its original position. A check valve closes to prevent the fuel from returning to the tank. The only place for the fuel to go now is through the outlet port.
Fuel filters
In addition to the mesh screen attached to the pickup tube, all fuel systems have at least one other filter located somewhere between the fuel tank and the fuel metering components. On some models, the filter is part of the fuel pump itself, on others, it is located in the fuel line, and still others locate the filter at the carburetor or throttle body inlet.
Inline and spin-on filters
See Figures 5, 6, 7 and 8
Inline and spin-on filters are located between the fuel pump and fuel metering components. They are connected to fuel lines either by clamps, banjo bolts, flare fittings or quick-disconnect fittings. Most are "throw-away" units with a paper element encased in a housing. Some have a clear plastic housing that allows you to view the amount of dirt trapped in the filter. Some filters consist of a replaceable pleated paper cartridge installed in a permanent filter housing. Their use is limited mostly to diesel and heavy-duty gasoline engines.
Figure 5 Some spin-on type filters have a replaceable cartridge. 
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Figure 6 This type of disposable inline filter is secured by clamps.
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Figure 7 Many inline filters are mounted along the frame rail under the vehicle... 
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Figure 8 ... others can be found in the engine compartment. 
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Carburetor/Throttle Body Inlet Filters
See Figures 9, 10 and 11
Fuel filters can also be located in the carburetor or throttle body inlet.
For carburetors, they consist of a small paper or bronze filter that is installed in the inlet housing. They are extremely simple in design and are about as efficient as an inline type. The bronze filter is the least common and must be installed with the small cone section facing out. One type is held in place by a threaded metal cap that attaches to the fuel line and screws into the carburetor fuel inlet. On another type, the fuel filter threads directly into the carburetor.
On throttle body units, these filters are used as a supplement to the primary inline filter. They usually consist of a conical screen, similar in appearance to an air conditioning orifice tube. They can be accessed after removing the fuel line from the throttle body unit.
Figure 9 Some filters are located in the carburetor inlet fitting... 
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Figure 10 ...others screw directly into the carburetor. 
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Figure 11 Some filter elements are made of sintered bronze. 
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Fuel/water separator
See Figures 12 and 13
This is usually found on diesel cars and trucks. It can either be part of the fuel filter housing or it can be a separate remote unit all together. Most operate as a two-stage filter. The lower stage removes dirt particles down to about 1 micron in size and allows the water to form large droplets. In the second stage, fuel freely passes through the filter, but water will not. Water collects in the bottom of the filter housing, and a drain plug on the bottom of the housing is usually provided.
The separate units are usually mounted next to the fuel tank. They collect water as it settles out of the fuel tank. Some may light a warning lamp on the dash when it requires draining.
Figure 12 Some fuel/water separators are part of the filter housing assembly. 
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Figure 13 Remote fuel/water separator units are usually mounted next to the fuel tank. 
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Continue to page 2 of Fuel systems
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