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For more information about fuel pumps theory, improving fuel pumps accuracy, and calibration techniques, see the
Swisor Auto Parts Technical Reference Manual.

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Reference Manual



Reference Section--Electrical and Pressure Checks.

Preliminary --If the engine runs but displays driveability symptoms that may or may not be fuel-related (hard starting, hesitation, misfiring, power loss), first atempt to eliminate other possible causes of the problem.
1.Check the vehicle's on-board diagnostic system; correct any fault codes.
2.Check the ignition system: spark plugs, wires, coil, timing. Observe ignition patterns on a scope to identify ignition and fuel problems.
3.Check for vacuum leaks at hoses, throttle body, intake manifold, power brake booster, vacuum-powered accessories.
4.Check the EGR and PCV systems. A leaky EGR valve can imitate a vacuum leak, leaning out the air/fuel mixture. A clogged PVC valve or hose can cause a rich mixture, while an air leak may create a lean condition.
5.Run a power balance test to reveal weak cylinders, a power loss caused by leaky valves or clogged injectors.
Electrical --Check the fuse first. If it's blown, the most likely reason is a hot short in the relay, or the wiring between the relay and the pump. Next, check other electrical components using a voltmeter and wiring diagram.
Especially make sure the pump relay is receiving the correct voltage from the control module and is sending the correct voltage to the fuel pump. Attach voltmeter leads to the hot and ground terminals, turn the key on and check for correct voltage. Turn the key off, move the hot lead to the relay's output terminal, and turn the key on. The relay should click, and the voltmeter should read the specified voltage. You have only seconds to check this before the ECM shuts down the relay. If nothing happens (no click, no needle), test voltage output at the relay driver terminal on the engine control module. A voltage signal here shows the ECM is supplying power, but the relay isn't responding. (No voltage at the ECM would indicate a defective driver circuit.) The final check is continuity in the wiring between the ECM and the relay. If it's okay, replace the relay.
To isolate an electrical problem other than the relay, bypass other components one at a time using a jumper wire. If the jumper restores power to the fuel pump, replace the bypassed electrical component.
Pressure --If the engine runs but exhibits driveability problems that suggest fuel delivery problems (too much or too little), the only way to know for sure is by checking fuel-system pressures. Hook up a pressure gauge, start the engine, and compare readings to the manufacturer's specs.
Usually the operating pressure specification is at idle with the vacuum hose attached to the regulator.Maximum system pressure (wide-open throttle operation) is read with the hose disconnected or pinched off.
If the engine hesitates when accelerating, monitor fuel pressure during a test drive by fitting an EFI-approved hose (200 PSI) long enough to reach the pressure gauge wherever it can be observed from the driver's seat.
Read "deadhead" or maximum pump pressure (higher than maximum fuel-system operating pressure) by momentarily pinching off the return line while the engine is running, or by connecting the gauge directly to the fuel line (no tee).
Check the system's ability to hold pressure after shutdown by turning off the key and watching the gauge. If the pressure drops suddenly, the pump check valve may be stuck open. If the pressure bleeds off slowly, it may indicate a leaky check valve/fuel line/ fuel injector, or a faulty regulator.
Too High . The mostly likely cause is a faulty regulator or a clogged return line. Disconnect the return line at the regulator, install another hose and reroute the return fuel to an appropriate container. Start the engine and read the fuel pressure. If it's now within specifications, check for a blocked or kinked return line or a clogged or stuck roller valve in the line.
If fuel pressure is still too high with the return line disconnected, check the fuel volume flowing through the bypass hose. If it's little or none, pull a vacuum on the regulator with a hand pump. No increase indicates a faulty regulator. But if the flow increases, and the system pressure comes back down to spec, look for a leaky, blocked or misrouted vacuum line.
T oo Low. The many possibilities include: a clogged in-tank pump strainer; a plugged fuel filter; a blocked or kinked fuel line; a leaky pulsator, fuel line or hose; a leaky pump check valve; a defective fuel-pressure regulator; or simply the wrong fuel pump for the application.
Check for filter and fuel line blockages by teeing a gauge into the tank outlet line and reading fuel pressure there. A number at least several PSI higher than at the engine indicates a restricted filter or fuel line. If there's no difference in pressure between outlet and engine, the pump is weak or its strainer is clogged. Either way, you must pull to pump to find out.
Causes of low pump output include low voltage, a bad ground wire and a leaky or defective pump check valve. Usually the check valve is integral to the pump. If it's faulty you must replace the pump. High levels of alcohol in fuel can cause rubber check balls to swell and stick. A leaking coupling hose or the pump's pulsator also may cause low pressure in the system.
Flow .Some manufacturer's specs for checking pump output require measuring the volume of fuel delivered at idle over time. Do this quickly and safely by disconnecting the return line at the regulator and installing a bypass to a safe container. Check the system's pressure at the same time.


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