Go With The Flow: BASIC FUEL SYSTEM ANALYSIS
Go With the Flow:BASICFUELSYSTEMANALYSISBY JOHNTHOMPSONWhile basic fuel systemdiagnosis may beginwith a fuel pressuremeasurement, itcertainly shouldn’tend there. Thesystem’s fuel volumeand flow also must bemeasured and evaluated.30July 2007
Montage: Harold A. Perry; photos and illustration: Bosch, GM & Jupiter ImagesThe 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 performed bearing this in mind. Diagnosingbasic fuel system problems requires anunderstanding of components, fuel system design, pressure and flow theory, aswell as diagnostic techniques. Let’s startwith fuel system components, beginningwith the last component first, to explainhow an injector’s flow rate is calculated.Fuel 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 of43.5 psi. One exception is Ford, whichrates its injectors at 39.5 psi as the standard pressure.Injector design flow ratings are measured in a static condition, which meansthey’re held open continuously. This isreferred to as a 100% duty cycle. However, once injectors are installed in an engine they’ll be pulsed with a varying dutycycle (depending on engine load requirements) measured in millisecond time increments. Operating injectors at 100%duty cycle would build up excessive heatwithin the injector windings, leading topremature failure. So in typical OE applications, injectors are never duty cycledabove an 80% to 85% on-time.Injector flow ratings are factoredwhen an OE manufacturer designs afuel system for a specific engine size.Specific 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 cycle.However, the fuel map assumes thatthe system’s design specifications willdeliver an expected pressure and volume of fuel to supply the injectors.Once installed in an engine, injectorflow output depends on three factors—the quantity of fuel flowing to the injector (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).If the designed pressure or flow volume is altered by a defect in a mechanical 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 load.Fuel 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. If calibrated rail systempressure is exceeded, excess fuel will bepermitted to return to the tank.Regulators typically fail by a ruptureddiaphragm resulting in engine vacuumdrawing raw fuel directly into the intakemanifold, poor seating of the fuel-pressure regulator resulting in fuel leakageto the return side or no return flow tothe tank whatsoever when a regulatorsticks closed.To give a practical example of how injector flow rate can be altered by multiple factors, let’s assume an increase infuel rail pressure at idle due to a stuckpressure regulator. An increase in pressure will result in increased injector output 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 overfueling of the engine’s cylinders? Duty cycle. Faced with this scenario, a PCM (inclosed loop) could reduce injector flowby reducing the injector pulse width.Conventional EFI systems utilize asubmersible fuel pump with a permanent magnet electric motor, a vibrationdamper and a relief valve to preventsystem damage from overpressure. Fuelenters the pump inlet tube by passingJuly 200731
Illustrations and photos: John ThompsonBASIC FUEL SYSTEM ANALYSISFig. 1Fig. 2through a sock-style filter and is pushedthrough the pump to the outlet by themotor.Conventional EFI systems also rely onthe fuel pressure regulator, not the pumpitself, to control pressure in the fuel rail.Any fuel not required by engine demandis diverted back to the fuel tank via thepressure regulator. Therefore, it’s important to remember that fuel pumps themselves only supply fuel volume; they donot create pressure in the fuel lines.Fuel pump current analysis is a technique that’s used to identify a deteriorating or defective fuel pump. It utilizes alow amperage probe to first calculate thecurrent drawn from a fuel pump’s electric motor, then transfer that informationto a lab scope waveform (Fig. 1 above)for visual analysis. This technique may allow 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 off.Examining the current “humps” inthe waveform created by the pump motor commutator bars will give you an accurate 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 remove and disassemble the pump. Evenone slightly worn commutator bar that’snot necessarily a problem will show upin the waveform.32July 2007You can calculate the rpm of thepump simply by picking out the repeating “signature” ID of that one commutator 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 calculated in Fig. 2.Despite this technique’s benefit ofquick and easy access to “rule in” a deteriorating 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. Typical auto fuel pumps draw 3 to 6 amps at5000 to 6000 rpm.Unfortunately, this is an average, andunless you’re familiar with the typicalamperage draw and rpm of the specificpump you’re actually testing, this average spec could mislead you. Just because a fuel pump appears to have “average” 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 current.This is a definite negative, and youshould be cautious in accepting currentanalysis as your only test.Ford electronic returnless fuel systems (ERFS) operate without a returnline to the fuel tank. Because there’s noreturn line, a pressure regulator attachedto the fuel rail is not needed. Despite thelack of a conventional regulator, theERFS does employ pressure regulationto control injector volume output.In theory, the PCM selects and sets afuel system operating pressure. ThePCM outputs a duty cycle command between 5% and 51% to the fuel pumpdriver module (FPDM) to control system pressure, using a fuel rail pressuresensor (FRP) for feedback. The FPDMdoubles the fuel pump command fromthe PCM and outputs a duty cycle command of its own to operate the pump. Incontrolling the pump’s on-time by toggling supply voltage, the system canmaintain any fuel system operating pressure 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. Any ERFS-related DTCthat may be set by the PCM is a directresult of the duty cycle of the diagnosticsignal returned to it by the FPDM.In operation, fuel is pumped fromthe fuel delivery module inside the fueltank, through a check valve and fuel filter, pressure transducer, fuel rail and finally through the fuel injectors. The fuelpump pumps only the amount of fuelneeded to keep the fuel rail at the desired or set operating pressure.Understanding how the FRP PID iscalculated is crucial to understanding the
BASIC FUEL SYSTEM ANALYSISsystem strategy. If the PCM desires 40 psi of pressure, 40 psi isthe target pressure it sets forthe injector nozzles, not for thefuel rail! It’s important to notethat the FRP PID on a scantool does not reflect the actualline pressure you’d see with afuel pressure gauge.The FRP sensor is not onlyresponsible for calculatingpressure in the fuel rail; usinga short vacuum hose attachedto the intake plenum, it alsoacts as a vacuum transducer.Using manifold vacuum to ex- Fig. 3trapolate pressure drop acrossthe injectors, the FRP sends a feedback were snapped from fully closed to widecalculation to the PCM. Negative psi open? A manifold vacuum drop to 0 in.can be calculated by halving vacuum Hg would be seen by the FRP as 0 psi atpressure measured in inches of mer- the injector nozzles while only 30 psi ofcury (1 in. Hg. .5 psi). By sensing pressure is present in the fuel rail. Inmanifold vacuum, the current negative this situation, the PCM would calculatepressure at the injector outlets (noz- that increased fuel rail pressure wouldzles) in the plenum is calculated by the be required to maintain the system tarFRP and added to the positive fuel rail get of 40 psi pressure at the injector nozpressure at the injector inlets.zles. The PCM would immediately sendFor example, 30 psi of fuel pressure in a command to the FPDM to increasethe rail added to 20 in. Hg sensed mani- the fuel pump duty cycle in order tofold vacuum ( 10 psi) would result in an raise the actual rail pressure.FRP PID reading of 40 psi. That is, 30Flow or current testing of electronicpsi at the top of the injectors added to returnless fuel systems requires the 10 psi of pressure present at the injec- pump to run continuously. You maytor nozzles equals 40 psi of pressure exit- command the pump continuously oning the injector nozzles.using a scan tool and sending a 50%What would happen if the throttle duty cycle command.Remember, just because you have“good” pressure and are satisfied withcurrent analysis of a fuel pump does not38 psimean that the pump is delivering thevolume of fuel the injectors will requireunder all operating conditions. Let’s discuss the differences among fuel pressure, volume and flow.the return side of the system.To trace a loss of residualpressure, perform a KOEO cycle to pressurize the fuel system, then clamp off supply andreturn lines one at a time to determine where the pressure isbeing lost. If pressure still dropsafter isolating both the supplyand return sections individually, the pressure loss could bedue to a leaking fuel injector(s).Fuel pressure is merely theamount of force (pressure)measured in pounds persquare inch (psi) exerted onthe available fuel volume. Mosttechs are very familiar with measuringfuel pressure with a gauge connected toa Schrader valve located on a fuel injector rail. What you may not realize is thatpressure is a one-way street. It can always be reduced but it cannot be increased, unless there’s sufficient volumeto sustain it. Using fuel pressure as youronly method of fuel system testingmeans you’re looking at only part of theoverall picture. Volume/TimeFuel Flow Volume and flow are not pressure. Aclosed faucet has system pressure but noflow. Too often techs rely on fuel pressure readings without realizing thatthere’s really very little fuel volume flowing through the system. Understanding38 psi Energy/VolumeFuel Pressure .53 GPMFuelPumpVoltage15 VoltsFig. 434July 2007Extended engine cranking beforestart-up can indicate a loss of residualfuel pressure, which should remainconstant even when an engine hasbeen shut off for several hours. Pressure could be lost on the supply sideof the system by a bad fuel pumpcheck valve or a leaking supply line.On the other hand, with a conventional fuel system, it could also be lost by apoorly seated fuel pressure regulator onFuelPumpVoltage11 VoltsFig. 5.28 GPM
BASIC FUEL SYSTEM ANALYSISthat fuel pressure is a measurement offorce while fuel volume is a measurement of quantity is key. Fuel flow is thevolume of fuel a system can deliver overa given period of time. It’s also important to note that any fuel system’s maximum flow capacity is fixed by system design and can never be increased.As an analogy, your home waterplumbing supply is a system with a fixedcapacity. Only so much water can flowthrough the supply lines because all ofthe plumbing from the water main to thefixtures in your home is of a fixed diameter. This is why, if you’re taking a shower,you’ll immediately notice decreased water flow to the shower head when someone flushes the toilet in the same room.Fig. 6Some of the available volume of watersupplying the shower head has been diverted in order to flow to the toilet.Since a fuel system’s maximum flowcapacity is also fixed, a defective pressure regulator on a return-type systemcan divert fuel flow needed by the injectors (the shower head) back to thefuel tank (the toilet). An underperforming fuel pump or a restricted fuel filteralso can reduce fuel system flow to theinjectors. No matter what the cause, areduced system flow will result in reduced injector flow volume.Reduced flow can’t be remedied byincreased pressure. Here’s another example of the relationship among pressure, volume and flow. Assume you havegood supply in the fuel tank and a pumpthat’s in good operating condition. Split-36July 2007ting the supply line exiting the fuel pumpwould result in exactly the same amountof fuel volume moved by the pump, butonly half the flow in each of the twolines. Now let’s add a booster pump inthe supply line after the original pump,but before the split. The second pumpcan’t pump any more fuel than is beingsupplied to it by the first pump.Older systems commonly used acombination of tank-mounted pumpsand external inline pumps. The externalpump was there to boost pressure. Itdid nothing to increase the flow of fuel.Increased system pressure will not recover lost flow!System fuel flow depends on four factors—fuel volume available at the pumpinlet, delivery line capacity (size), thepump’s ability to pump sufficient supplyline volume and system pressure.But low flow can occur with seemingly normal fuel rail pressure. Here’san example. Pump supply voltage has adirect effect on fuel flow. If low charging system voltage is present or high resistance in the power or ground circuitto an electric fuel pump drops supplyvoltage, the pump’s capacity to deliverfuel volume will also drop.Voltage to a fuel pump motor can becompared to a fuel injector’s operation.Less voltage to the fuel pump motorwill mean less volume output, just likeless fuel pressure to an injector willmean less volume out its nozzle. Lowervoltage at the pump terminals decreasesmotor torque, resulting in decreasedvolume capacity for a given pressure.I used a 1998 Mazda Protegé with a1.5L engine to test both fuel pressureand flow. In Fig. 4 on page 34, a flowmeter capable of measuring fuel pressure as well as total system flow (including the flow of fuel being returned tothe tank) was connected inline to thefuel rail. The measured voltage supplyto the pump was 15 volts with a systempressure of just over 38 psi and total system flow of .53 gallon per minute (gpm).In Fig. 5, the voltage supply to thepump was dropped to 11 volts. A 47%drop in fuel flow resulted, with no significant reduction in fuel pressure. Don’tlet a scenario like this burn you. Manytimes voltage will be up to snuff right upto the connector entering the fuel tank.A breakdown in the wiring could be inside the tank between the tank connector and the pump. A poor pump groundor an old defective pump that’s drawingtoo much amperage can damage thewiring. If the circuit is not carefully inspected, you may have a voltage drop toa new pump after installation, reducingits capability to provide sufficient fuelvolume to the engine. A dented steel fuelline or a restricted fuel filter also can reduce available fuel volume without significantly affecting pressure.Be cautious about using fuel pressureas your only test. Crimping a return fuelhose may show pump pressure capability, but the volume of fuel being delivered at normal operating pressure isFig. 7what you really want to know. If youhaven’t already, you’ll eventually encounter vehicles with one of the abovementioned flow restrictions that will effectively reduce the available fuel volume while having little or no effect onmeasured pressure at the fuel rail.Components that are underperformingbut not yet completely failed, such as apartially restricted fuel filter, a defectivepressure regulator or a worn fuel pump,are more easily identified by measuringflow and pressure together.Fuel pressure measured at the rail aswell as the required injector flow rate atidle may be within spec. But we need toknow the total system flow capacity atmax operating conditions of an engine,when nearly all of the available flow willbe used to sustain injector flow require-
BASIC FUEL SYSTEM ANALYSISments. Vehicle fuel system specifications generally include fuel pressure butdo not list fuel flow specifications. Depending on engine displacement, typical fuel system flow capacity will rangefrom .4 to .8 gpm. However, fuel flowrequirements can be easily establishedfor all engines.Calculating the flow requirements ofany fuel system is dependent on onlytwo factors—the size of the engine thesystem supplies and the rpm range atwhich the engine is operating. To illustrate this point, refer to Fig. 6 on page36. An engine that has a displacementof 383 cu. in. (or 6.3 liters of total combustion chamber volume) that’s runningat an idle speed of 750 rpm will displacea calculable amount of air per minute.At this rpm, the engine’s fuel systemwould be required to supply only .07gpm of fuel to keep that engine idlinghappily.Now let’s assume you’re driving thiscar and passing a truck on a two-lanecountry road. The same 383-cid engineoperating at 5500 rpm would requireover a half-gallon of fuel per minute(Fig. 7). Any less and the engine will beunderfueled.Under normal operating conditions,an engine will almost never demand thetotal output flow capacity of its fuel delivery system. The exception would bewide-open throttle (WOT) at redline.So, if the measured flow capacity of thefuel system meets or exceeds calculatedflow volume required at max rpm, fuelflow will always be sufficient, at any engine rpm and load.On the other hand, if the engine’stested maximum fuel flow falls evenslightly short of these requirements, theengine will not be able to supplyenough fuel to the injectors under alloperating conditions. This engine mayrun perfectly fine at lower rpm, butwhen the demand vs. actual supply lineis crossed, the injectors (and the engine)will literally run out of gas. This is whyit’s so important to measure fuel flow.So how will you measure fuel flow?Your methods of assessing fuel systemflow should be just as precise as whenyou use a torque wrench on head boltsor a micrometer on brake rotors. Flow-Circle #2238July 2007ing gasoline from an unpressurized fuelline into a graduated container, timedby a stopwatch, is not an accurate wayto measure a system’s gpm capability.It’s also an obvious safety risk. A wornpump may be able to provide sufficientflow until it must generate enoughtorque to overcome normal system operating pressure. The fuel system analyzer shown in Figs. 4 and 5 combines afuel flow meter, fuel pressure gauge,manifold vacuum gauge and exhaustpressure gauge in one unit. All criticalmeasurements are combined, plus thefuel supply can be visually inspected forcontamination or cavitation.Use your basic understanding of thestrategy of the system you’re trying todiagnose. Use basic fuel pump currentramping techniques to rule in but notrule out system faults. Most importantly, an understanding of the basic theories of fuel pressure, volume and flowmay be the most valuable tool of all.Visit www.motor.com to downloada free copy of this article.
the fuel pressure regulator, not the pump itself, to control pressure in the fuel rail. Any fuel not required by engine demand is diverted back to the fuel tank via the pressure regulator. Therefore, it’s impor-tant to remember that fuel pumps them-selves only supply fuel volume; they do not create pressure in the fuel lines.
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Le genou de Lucy. Odile Jacob. 1999. Coppens Y. Pré-textes. L’homme préhistorique en morceaux. Eds Odile Jacob. 2011. Costentin J., Delaveau P. Café, thé, chocolat, les bons effets sur le cerveau et pour le corps. Editions Odile Jacob. 2010. 3 Crawford M., Marsh D. The driving force : food in human evolution and the future.
