Go With the Flow: BASIC FUEL SYSTEM ANALYSIS

1 BYJOHNTHOMPSONW hile BASIC fuel SYSTEM diagnosis may begin with a fuel pressure measurement, it certainly shouldn t end there. The SYSTEM s fuel volume and flow also must be measured and BASIC fuel SYSTEM diagnosis may begin with a fuel pressure measurement, it certainly shouldn t end there. The SYSTEM s fuel volume and flow also must be measured and 2007Go With the Flow: BASICFUELSYSTEMANALYSISGo With the Flow: BASICFUELSYSTEMANALYSISThe fuel injectors are atthe end of the line in anyEFI SYSTEM . The entirefuel SYSTEM and each ofits components exist toprovide proper injectorflow rate through the injector nozzlesand into the engine s cylinders. BASIC fuelsystem diagnosis should always be per-formed bearing this in mind. Diagnosingbasic fuel SYSTEM problems requires anunderstanding of components, fuel sys-tem design, pressure and flow theory, aswell as diagnostic techniques.

2 Let s startwith fuel SYSTEM components, beginningwith the last component first, to explainhow an injector s flow rate is injectors are designed and ratedfor the quantity of fuel that can flowthrough them at a given fuel pressureand duty cycle at mean sea level. Theamount of fuel that an injector can flowis measured in pounds per hour. Forrating purposes, most manufacturersspecify a standard operating pressure psi. One exception is Ford, whichrates its injectors at psi as the stan-dard design flow ratings are mea-sured in a static condition, which meansthey re held open continuously. This isreferred to as a 100% duty cycle. How-ever, once injectors are installed in an en-gine they ll be pulsed with a varying dutycycle (depending on engine load require-ments) measured in millisecond time in-crements.

3 Operating injectors at 100%duty cycle would build up excessive heatwithin the injector windings, leading topremature failure. So in typical OE ap-plications, injectors are never duty cycledabove an 80% to 85% flow ratings are factoredwhen an OE manufacturer designs afuel SYSTEM for a specific engine pressure and flow expectationsas well as a dynamic fuel map based onrpm and load of a particular engine arecalculated. This fuel map is the primarycontrol factor of injector duty , the fuel map assumes thatthe SYSTEM s design specifications willdeliver an expected pressure and vol-ume of fuel to supply the installed in an engine, injectorflow output depends on three factors the quantity of fuel flowing to the injec-tor (volume), the force behind the vol-ume of fuel flowing to the injector( pressure ) and the injector duty cycle oron-time command from the PCM(pulse width).

4 If the designed pressure or flow vol-ume is altered by a defect in a mechani-cal component of the fuel SYSTEM , or ifthe injector duty cycle is altered by thePCM due to an incorrect sensor input,injector flow rates will also be altered,ultimately affecting the goal of the fuelsystem, which is to deliver the requiredinjector output flow based on enginerpm and filters trap harmful contaminantsand are passive components that, whenrestricted, can cause immediate systemproblems by reducing fuel flow. Delayedsystem problems also will occur if a filteris no longer able to trap contaminatingparticles, which will then travel furtherdown the line and affect other systemcomponents (usually the fuel injectors). fuel pressure regulators restrict thereturn of fuel to the tank by a calibratedamount in order to maintain desired fuelrail pressure .

5 If calibrated rail systempressure is exceeded, excess fuel will bepermitted to return to the typically fail by a ruptureddiaphragm resulting in engine vacuumdrawing raw fuel directly into the intakemanifold, poor seating of the fuel -pres-sure regulator resulting in fuel leakageto the return side or no return flow tothe tank whatsoever when a regulatorsticks give a practical example of how in-jector flow rate can be altered by multi-ple factors, let s assume an increase infuel rail pressure at idle due to a stuckpressure regulator. An increase in pres-sure will result in increased injector out-put volume. The PCM does not havecontrol over the fuel volume beingpumped in the SYSTEM , nor can it controlthe pressure in the fuel rail . So howcould the PCM attempt to prevent over-fueling of the engine s cylinders?

6 Duty cy-cle. Faced with this scenario, a PCM (inclosed loop) could reduce injector flowby reducing the injector pulse EFI systems utilize asubmersible fuel pump with a perma-nent magnet electric motor, a vibrationdamper and a relief valve to preventsystem damage from overpressure. Fuelenters the pump inlet tube by passing31 July 2007 Montage: Harold A. Perry; photos and illustration: Bosch, GM & Jupiter Imagesthrough a sock-style filter and is pushedthrough the pump to the outlet by EFI systems also rely onthe fuel pressure regulator, not the pumpitself, to control pressure in the fuel fuel not required by engine demandis diverted back to the fuel tank via thepressure regulator. Therefore, it s impor-tant to remember that fuel pumps them-selves only supply fuel volume; they donot create pressure in the fuel pump current analysisis a tech-nique that s used to identify a deteriorat-ing or defective fuel pump.

7 It utilizes alow amperage probe to first calculate thecurrent drawn from a fuel pump s elec-tric motor, then transfer that informationto a lab scope waveform (Fig. 1 above)for visual ANALYSIS . This technique may al-low you to decide if the amperage drawnby the circuit is typical. It s normal for apump s initial current draw to be higherwhen the pump is first energized from adead stop. As the pump starts to turnand push fuel through the SYSTEM , theamperage should drop and level the current humps inthe waveform created by the pump mo-tor commutator bars will give you an ac-curate idea of how the pump motorlooks internally. Any inconsistencies inthe visual representations you see in awaveform that took only milliseconds toacquire mirror how the armature wouldlook if you were to take the time to re-move and disassemble the pump.

8 Evenone slightly worn commutator bar that snot necessarily a problem will show upin the can calculate the rpm of thepump simply by picking out the repeat-ing signature ID of that one commu-tator bar. If the pattern repeats everyninth bar, then you know that the pumphas eight commutator bars, which inturn allows you to measure the time (inmilliseconds) it takes for one revolutionof the pump. Next divide 60,000 (1minute of time in milliseconds) by onemotor revolution time and you will havecalculated the rpm of the pump. Therpm of a very worn pump motor is cal-culated in Fig. this technique s benefit ofquick and easy access to rule in a de-teriorating or defective fuel pump, youshould always keep in mind that the onlyreal certainties in the waveform are theamperage draw, the rpm and the visualsignature of the pump armature.

9 Typi-cal auto fuel pumps draw 3 to 6 amps at5000 to 6000 , this is an average, andunless you re familiar with the typicalamperage draw and rpm of the specificpump you re actually testing, this aver-age spec could mislead you. Just be-cause a fuel pump appears to have av-erage rpm, average amperage drawand commutator bars that are uniformin appearance does not guarantee thatthe pump can supply the volume of fuelthe SYSTEM was designed for. The greatunknown of fuel pump current analysisis that you can t actually measure apump s volume output with is a definite negative, and youshould be cautious in accepting currentanalysis as your only electronic returnless fuel sys-tems (ERFS) operate without a returnline to the fuel tank. Because there s noreturn line, a pressure regulator attachedto the fuel rail is not needed.

10 Despite thelack of a conventional regulator, theERFS does employ pressure regulationto control injector volume theory, the PCM selects and sets afuel SYSTEM operating pressure . ThePCM outputs a duty cycle command be-tween 5% and 51% to the fuel pumpdriver module (FPDM) to control sys-tem pressure , using a fuel rail pressuresensor (FRP) for feedback. The FPDM doubles the fuel pump command fromthe PCM and outputs a duty cycle com-mand of its own to operate the pump. Incontrolling the pump s on-time by tog-gling supply voltage, the SYSTEM canmaintain any fuel SYSTEM operating pres-sure desired by the PCM (Fig. 3 onpage 34). The FPDM also generates adiagnostic signal that s transmitted backto the PCM on the fuel pump monitor(FPM) circuit to indicate if there are anyfaults present.