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ASE P2: Fuel System

Most late-model vehicles have a tank-mounted high-pressure electric fuel pump.


Some fuel system parts, such as the fuel filter, may need to be changed for preventive maintenance, but most parts (fuel pump, hoses, injectors and pressure regulator) are only replaced if they have failed.


Most late-model vehicles have a tank-mounted high-pressure electric fuel pump. Older carbureted vehicles mostly use an engine-mounted low-pressure mechanical pump. Electric pumps come in various designs (single or double roller vane, turbine or gerotor), though more-efficient turbine style pumps may be used to replace some older-style roller or gerotor style pumps.

Electric fuel pumps often are replaced unnecessarily due to misdiagnosis. The fuel pump may not be working because of a bad electrical connection or ground, a bad fuel pump relay, a plugged fuel line or fuel filter, or even an anti-theft system issue.

A tank-mounted fuel pump is part of the fuel pump module, which includes the fuel level sending unit. On some late-model vehicles, a “lifetime” fuel filter is also part of the fuel pump module assembly. The fuel pump can usually be replaced separately if it has failed, but on some applications the entire fuel pump module must be replaced. Installing a whole new module assembly is easier and reduces the risk of future problems. Either way, a new fuel filter and fuel pump inlet strainer sock should also be installed when changing a fuel pump.


The fuel filter protects the fuel system against dirt and debris. Inline filters on many vehicles have spring lock fittings that require a special tool to release. The fuel filter should be replaced according to recommended service intervals, or every 30,000 to 50,000 miles. Same for the engine air filter.
Fuel injected engines use fuel hose that is rated to withstand high pressure, unlike fuel hose for older carbureted engines that is low-pressure hose. Hose should be replaced if leaking, cracked, hard, brittle or damaged. New clamps (if used) are recommended.

Dirty fuel injectors is a common complaint, and can often be remedied by adding fuel injection cleaner to the fuel tank, or cleaning the injectors on or off the car. If an injector has failed or does not respond well to cleaning, it can be replaced with a new or remanufactured injector. A whole new set of injectors is usually recommended if more than one high-mileage injector has failed or is clogged.


The fuel pressure regulator controls pressure within the fuel injection system and routes excess fuel back to the fuel tank. Usually mounted on the fuel rail on the engine, the regulator has a spring-loaded diaphragm attached to a vacuum hose. A bad regulator can cause a loss of fuel pressure, stalling and hard starting. The fuel pressure regulator on some newer vehicles with “returnless” EFI systems is located in the fuel tank with the pump.

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