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/Fuel Pump Diagnostics and Repair Steps.pdf
Fuel Filler Cap Tight seal is critical to proper OBD system operation A loose cap can lead to a DTC that will set the Check Engine light Fuel Tank Pressure Sensor Monitors fuel tank evaporative pressure for emissions control purposes Fuel Pump The heart of the fuel delivery system the fuel pump delivers fuel to the engine Airtex fuel pumps are designed to meet or beat OE specs and are 100 tested to ensure performance and long life Fuel Strainer The pump s first line of defense against con taminated fuel Failure to replace the fuel strainer will void fuel pump warranty Fuel Tank The fuel pump on most modern vehicles is housed in the fuel tank Tanks must be clean and free of contamination before a new fuel pump is installed Contaminated fuel is responsible for 20 of fuel pump product failures Supply and Return Lines Deliver fuel to and from the engine Check for kinks or restrictions Fuel Filter Protects injectors and engine from con tamination not caught by the fuel pump strainer Important to replace as a part of routine maintenance as well as with any fuel system service Oxygen 02 Sensor Checks exhaust gases and sends signal to ECM to adjust fuel mixture for emissions control purposes Fuel Pressure Regulator Controls fuel pressure to fuel injectors Fuel Injectors Deliver fuel to engine combustion chambers 1 2 3 4 5 6 7 8 9 Fuel System Delivery Overview Guide To Successful Fuel System Repairs 1 2 3 10 4 5 6 7 8 9 10 It s important to note that the fuel pump is one part of a complex fuel delivery system Any single component not operating properly can appear to be a fuel pump problem Proper diagnostics are critical Repeat fuel pump failures which can cause major problems with your customers are often caused by minor installation errors One common installation error is incor rectly replacing the pump inlet filters in such a way that it causes an inaccurate fuel gauge reading In other cases the fuel pump inlet filter may be blocked or dislocated by a dent ed or distorted fuel tank These issues can be easily diagnosed by adding several gal lons of fuel to the tank before testing fuel pump performance In a few rare cases worn fuel pump relays can cause erratic or intermittent fuel pump operation The cost of a new relay is minimal compared to the potential problems caused by a worn relay Fuel pump life can also be shortened on some vehicles if the owner habitually drives the vehicle on a nearly empty tank Low fuel levels can cause the fuel pump to overheat by ingesting air when accelerating cornering or stopping Ingesting air can also momentarily reduce fuel pressure and cause fuel related driveability complaints The Importance of Correct Volume Fuel pump volume is equally as important as fuel pump pressure when testing for repeat failures On the high end several tool manu facturers supply testers that simultaneously measure the exact fuel pressure and flow while the engine is running On the low end most fuel pressure testers have a bypass valve that measures the excess volume flow ing through the fuel pressure test Schrader valve In most cases this type of tester will provide a reasonable estimate of the volume of fuel flowing to the engine Under Pressure When testing fuel pumps equipped with exter nal fuel pressure regulators it s important to test fuel pressure regulator performance by momentarily closing or restricting the return flow hose from the regulator to the fuel tank This test will provide an indicator of maximum fuel pressure output that in most cases will be at least 150 higher than regulated pres sure Because crimping can damage braided hose and plastic lines an alternate method should be used to close the return line If maximum pressure doesn t increase by momentarily closing the return line the low fuel pressure might be caused by a worn fuel pump a clogged fuel filter or crimped fuel line If the fuel pressure is too high due to a crimped fuel return line or defective pressure regulator the engine will normally run rich and the fuel pump amperage draw will be much higher than normal The higher amper age draw will shorten fuel pump life and cause a repeat failure Fuel Pump Activation System It s also important to evaluate the fuel pump s activation system At the most basic level the crankshaft position sensor supplies a signal to the PCM that indicates the engine is crank ing or running On many older vehicles equipped with distributors the air flowing through the air flow meter closes a set of con tacts in the sensor that in turn activates the fuel pump relay In either case the fuel pump relay won t activate without some type of engine speed input The Fix For Failing Fuel Pumps Tank Tie In Modern fuel tanks are difficult to clean because of their internal baffling If a tank can be cleaned it s important to use high pressure soap and water to remove the dirt The tank should then be thoroughly dried by circulating air through the tank or air dried outside In many cases a fuel tank replacement is the most cost effective and safe alternative for preventing repeat fuel pump failures Scan It On late model vehicles it s usually much quicker to test the fuel pump with a scan tool that has the applicable bi directional controls and data stream displays If those features are available a technician can quickly evaluate the fuel pump s electrical performance by using the scan tool to activate the fuel pump A scan tool should also be used to down load trouble codes and data indicating if an anti theft or other vehicle sub system has disabled the fuel pump as part of its theft avoidance strategy In some cases the fuel pump circuit is disabled when the anti theft system identifies an improper or defective key in the ignition switch Scan tools must also be used to diagnose pulse modulated fuel pump systems Because pulse modulated sys tems are a story unto themselves we can t illustrate an example in this brief space In short the Powertrain Control Module PCM tailors the fuel pump pressure to meet various operating conditions by con trolling fuel pump speed by pulsing electri cal current to the pump When the pulse width requirements exceed normal parame ters the PCM stores a fuel pump related diagnostic trouble code Fuel Pump Diagnostics and Inspections Major causes of fuel pump failure include Misdiagnosis of the vehicle s problem Rust corrosion or other contamination in the vehicle s fuel tank Clogged or plugged fuel filters and strainers and Faulty on vehicle fuel pump wiring or connectors Electrical Issues If voltage is available at the fuel pump it s important to inspect the tank s electrical connector for looseness and burned connector pins Repeat failures are often caused by connectors that were burned due to excessive amperage draw of a defective pump If the fuel pump circuit is grounded to the frame perform preventive maintenance by cleaning the frame and connector and applying anti corrosion compound to the connection The professional multimeter can be used to measure current flow through the fuel pump On most vehicles a new fuel pump draws about three to five amperes of current This amperage or current draw can be measured at the fuel pump fuse located in the vehicle s fuse box Many technicians also like to display the fuel pump current flow on an automotive lab scope to determine if the fuel pump armature and brushes are in good condition If current isn t flowing through the fuel pump the fuel pump fuel pump ground circuit fuel pump relay or if equipped the fuel pump safety inertia switch might be defective If current is flowing through the fuel pump then it s highly likely that the external fuel filter might be clogged or that the filter sock attached to the fuel pump inlet might be clogged with dirt rust or algae contamination While it might be strange to consider alge in the system E85 and lower ethanol blends tend to be hygroscopic simply by their nature as alco hol based This can lead to a film of water on top of the fuel that collects algae and bacteria If current is flowing through the pump and the normal fuel pump operating noises aren t pres ent the inlet sock is probably missing and the mechanical part of the fuel pump is likely jammed with sand or other types of fuel tank contamination If the fuel pump s inlet filter is missing torn or broken make sure that the fuel tank baffle is securely attached to the fuel tank Fuel pump fail ures have been caused by a loose tank baffle bat tering the fuel pump until the inlet filter fell way allowing dirt to jam the pump assembly When replacing a fuel pump always Replace the filter or strainer with new Professionally flush clean or replace the fuel tank Inspect repair or replace suspect vehicle connectors and wiring and Refer to the Diagnostic Troubleshooting and Installation Guidelines document found in the fuel pump box Fuel Pump Voltage Test 1 Connect digital voltmeter to fuel pump wiring at fuel pump connector 2 Turn key on pump will only voltage only for a few seconds 3 Measure voltage at pump Reading should be within 0 5v DC of system voltage
