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Most of us are familiarwith the traditionalfuel loop. Typically, thisstarts and ends in thefuel tank, and utilizesone or more pumps to

supply a high volume of fuel at highpressure to the injector rail. The pressureis usually maintained by a regulator at-tached to the exit end of the fuelrail. Any unused fuel is bled off by theregulator and returns to the fuel tank viaa return line. A check valve in the pumpprevents backflow and maintains residualpressure in the supply line to avoid hotrestart problems due to fuel boiling (va-por lock). These systems (see Fig. 1 onpage 36) have been the norm for manyyears, but are now on the wane.

Returnless fuel systems, in contrast,

supply only the fuel thats currentlyneeded. This obviously eliminates theneed for a return line. From a manufac-turers standpoint, there are two advan-tagesreduced assembly costs and re-duced load on the evaporative emis-sions system. (The unused fuel return-ing to the tank in a looped system isusually quite warm as a consequence ofthe time spent in a hot engine compart-ment. Add in the vapor load resultingfrom high-volume flow and turbulence,and the evap system has a hard timekeeping up, especially in hot climates orurban stop & go driving.)

So, how does a returnless systemwork? How does it know how muchfuel is going to be needed? The photoon page 36 and Fig. 2 show two slightlydifferent implementations. The setup inthe photo, found, for example, in someSaturn and Chrysler vehicles, simplymoves the historical fuel pressure regu-lator to the inside of the fuelfilter. From there, any excess fuel runsvia a short line back to the tank, avoid-ing the heat of the engine bay.

Fig. 2 illustrates the setup now usedby many manufacturers. Here, all theaction takes place within the tank. Fuelis pumped into the supply line until itsfilled and pressurized. The pressureregulator is an integral part of the pumpmodule, and all excess fuel bleeds offinternally within the tank.

Since no fuel bleeds off via the regula-tor until the demands of the supply linehave been met, the system appears to

know how much fuel is required. Butits easy, really; all it has to do is keep theline filled and pressurized.

TestingTesting the fuel pressure in a traditionalloop system is fairly straightforward. Youmay connect a pressure gauge to a con-venient port if one is available. Failingthat, your gauge can be connected in-line beforethe regulator. Fuel pressureon most looped systems varies slightlydepending on engine vacuum, so besure to check the specs.

Of course, fuel pressure is only part ofthe driveability analysis story; you alsoneed to check fuel deliveryvolume. If youhave a gauge like the one shown in thephoto on page 38 (top), you can connectit in series before the beginning of thefuel rail and read not only the fuel pres-sure, but the volume as well. (I also liketo be able to see the fuel as it flows. Both

aeration and major contamination showup readily even at high flow rates.)

Dont have a flow gauge? Whiletheres no doubt that using equipmentwith a dedicated flow volume measure-ment scale is far easier and safer thanany other means, dont despair. You canmeasure return volume by running ahose from the pressure regulator outputinto a suitable container. (A glasskitchen measuring cup will do.) Ofcourse, flow ratemeasurement requiresmeasuring time as well as volume, solook at a watch with a secondhand. Most cell phones have a stop-watch feature (see Rate Calculator:How Much Is Enough? on page 40 fora chart of common minimum rates).

Pressure testing for returnless sys-tems is quite similar. If no port is provid-ed, youll have to connect in-line. Buthow do you check volume on these sys-tems? Even an in-line flow gauge will

34 March 2013

DIAGNOSINGRETURNLESSFUEL SYSTEMS

DIAGNOSINGRETURNLESSFUEL SYSTEMS

BY SAM BELL

Well beyond the simple crank/no-start,

fuel supply problems can cause a

range of driveability symptoms. The

advent of returnless fuel systems

has added another layer of

complexity to fuel supply diagnostics.

BY SAM BELL

Well beyond the simple crank/no-start,

fuel supply problems can cause a

range of driveability symptoms. The

advent of returnless fuel systems

has added another layer of

complexity to fuel supply diagnostics.Photoillustration:HaroldA.Perry;images:Thin

kstock&WieckMedia

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show only the amount of fuel actuallybeing consumed at that moment.

To see how much that might vary, Iused my wifes Scion xB. At a hot idle, its1.5L engine sips fuel at a barely percepti-ble rate: .02 gallon per minute (gpm), or.08 liter per minute (lpm). At 2500 rpm,consumption is still only .05 gpm (.19lpm). Finally, thoughnot that youdever want to rev an engine this hard, ofcourse!the little powerplant sucksdown as much as .12 gpm (about .45lpm) at 5000 rpm on a frigid morning. (Icheated and simulated this by substitut-ing some very high-ohm resistors for thetemp sensors on a well-warmed engine. Imay be crazy, but I m not stupid!)

So how do we measure the availablefuel volume without going to such ex-tremes? And how do we know howmuch is enough?

Let me take these two questions inorder. To determine maximum fuel vol-

ume delivery, I tried several tactics. Myfirst experiment was performed withthe engine off. I began by disconnectingthe fuel line under the hood, connect-ing my Zapp Fuel System Analyzer(FSA) in-line with a longpiece of fuelline back to the tank, then commandingthe fuel pump on via my scan tool andmeasuring the resulting volume. Withno restriction other than hose diameter(about 8mm), I recorded .58 gpm, plen-ty for any situation. Since there was norestriction to flow, the pressure herewas essentially zero.

Now I began slowly restricting theflow back to the tank, thereby increasingthe pressure. With a slight restriction, Iwas able to achieve a pressure of 20 psiwith a flow rate of .43 gpm. A moderaterestriction brought pressure up to 39 psi,while flow dropped to .25 gpm. As Ipinched off the line almost completely,pressure rose to 42 psi (the published

spec) but flow dropped almost tozero. This verified that the system setpoint was, in fact, the advertised 42 psi,and that the fuel pressure regulator inthe tank was now controlling fuel flowby simply dumping all excess fuel inter-nally.

I conducted my next experiment withthe engine running. I reconnected theoutlet of the FSA to the inletof the fuelrail, placing the FSA in series betweenthe pump and the injectors. I startedthe car and observed that the fuel pres-sure was the expected 42 psi. Now I be-gan to slowly restrict the inlethose ofthe FSA. Remember that before, as Iincreased the outletrestriction, thepressure rose from zero. Now I ob-served the opposite. As I began tocrimp off the supply line, pressure inthe FSA began to decrease. Interesting-ly, the engine continued to idle smooth-ly all the way down to a pressure of 20

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psi, or about half of that normally speci-fied. Below that pressure, the enginebegan to stutter, finally stalling as fuelpressure dropped below 10 psi.

But what about off-idle operation? I

used a throttle prop to hold enginespeed around 2000 rpm and repeatedmy experiment. Fuel trims once againgot a little crazy, but even at 15 psi, thelittle motor ran smoothly. Finally, I setthe throttle for 3500 rpm, achieving es-sentially the same result.

I learned several lessons from theseexperiments. First, fuel delivery volumevaries inversely with pressure, all other

things being equal. Second. a smoothrunning engine is no guarantee of ade-quate fuel pressure. And third, exces-sive fuel trim correctionsmayindicate afuel supply problem.

Service ConsiderationsAlthough admittedly less effective thanwhen coupled with off-car ultrasoniccleaning, a thorough fuel rail cleaningand engine decarbonization treatmenthave for many years been among the

most effective tools in my arsenal forrestoring proper driveability perfor-mance. The system I use allows me todisconnect both the fuel feed and re-turn lines and to substitute my cleaningmachine for the tank and pump.

Once the equipment is connected, thefirst step is a long cycle of cleaning theinjector inlet screens and the fuelrail. This is accomplished by running themachines built-in pump at a pressureabove the KOEO set point of the carsregulator, allowing a filtered mixture offuel and cleaner to circulate continuouslythroughout the rail. This circulation pro-vides a long period of contact betweenthe cleaning solution and the injector in-

let screens, during which previously ac-cumulated deposits can be dissolved andthen trapped in the machines filter.

After 30 to 60 minutes of that initialcleaning, I fire up the engine to run onthe same cleaner and fuel concoction.(Unplug the cars own pump or loop itsfeed and return lines together first, ofcourse!) Once the motor is running, Iadjust the pressure to a slightly lower-than-normal level. In closed-loop opera-tion, this causes the injectors to openlonger to achieve the same fuel deliveryvolume, allowing more time for a thor-ough cleaning of the injectors innards. Iusually walk away at this point, lettingthe machine do the work until it runs

DIAGNOSING RETURNLESS FUEL SYSTEMS

Fig. 1 Traditional fuel injection sys-tems incorporate a high-volume pumpto ensure an adequate supply of fuelunder all operating conditions. Manyearly systems coupled a low-pressure,high-volume, in-tank presupply pumpto a high-pressure main pump. Moremodern systems locate the main pumpinside the tank. System pressures forsome TBI systems are as low as 13 psi,while other configurations feature sys-tem pressures as high as 75 psi.

This truncated return system, widely used on some Saturn and Chrysler vehicles, is across between the conventional fuel loop and the in-tank regulation of a pure re-turnless system. Here the pressure regulator and its return leg outlet are incorpo-rated into the fuel filter assembly. There have been some reports of faulty (or possibly

counterfeit) parts causing either extremely high pressures or a total lack of pressure.

36 March 2013

Fig. 2 In-tank pressure regulation is the hallmark of the now-common purereturnless fuel systems used by most manufacturers. In addition to reducedassembly costs, OEMs see an advantage in reduced evap load.

PCM

Fuel Rail/ Injectors

FuelReturn Line

Filter

Pump

Photos&illustrations:SamBell

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out of fuel and stalls. (Since the pro-longed low-pressure operation mayhave raised long-term fuel trim [LTFT],be sure to reset trims back toward nor-mal, even if only by test-driving after

the cleaning. And clear any codes orpending codes you may have set alongthe way, of course.)

Obviously, this cleaning strategy can-not work on a returnless system becausetheres no loop to allow for the inletcleaning cycle. In some cases, Ive beenable to find an adapter to allow me toconstruct a loop (see photo left be-low). If you choose to go that route,make sure you check thoroughly forleaks when you both install and removeyour adapters. New O-rings are some-times required, so be sure you have

some on handbeforeyou begin.But what about inlet screen cleaning

on the majority of returnless systems,where theres no return port on the rail?

You can use a pressurized cleaner or aconventional device like the one I use todissolve varnishes and similar de-posits. But any attempt to force largeparticulates through the injectors inletscreen is likely to backfiresometimesliterally! In some cases, there may be notruly good solution to this problemshort of pulling the injectors for off-carcleaning. Depending on the configura-tion and layout of the fuel rail, it maysometimes be possible to use a pressure

test port to allow for inlet screen clean-ing circulation to at least some of therail. Other alternatives may includedraining the rail and using a spraycleaner repeatedly, but this is not usual-ly effective except in cases of large-flakedeposits, such as those described next.

Some models have experienced inter-nal fuel rail deterioration. I ve seenmetallic flakes from these rails collect onthe inlet screens of some Taurus V6s andNeons. I n most instances, these werecars that were frequently and habitually

driven with the fuel tank very low, and Ihypothesize that excessive moisture con-densation in the fuel tank may have beena factor. In the case of one of the Neons,I was faced with a floating misfire as the

flakes shifted within the rail. Most ofthem eventually collected on the No. 3and No. 4 inlet screens.

Variable Fuel PumpsComplementing the basic returnlesssystems previously described is an addi-tional technological wrinklethe vari-able fuel pump. Both major versions ofthis idea are designed to prolong pumplife and reduce pump noise. One typeof system uses a dropping resistor, re-sulting in a two-speed pump. Here, thePCM monitors the voltage drop across

the resistor to verify low-speed pumpoperation and to calculate electrical cur-rent consumption. The other, a moretruly variable type of system, utilizes aPCM command to a pump-driver mod-ule, which, in response, provides a vari-able pulse width- or duty cycle-con-trolled power supply to the pump.

In my experience, such systems incor-porate a fuel rail pressure sensor to allowthe PCM to monitor pump performanceand more accurately calculate injectoron-time. In many cases, however, the re-ported scan data value for the fuel railpressure is substantially different fromthe actual pressure readings taken with a


In the Thompson Automotive Labs FuelZapp combination pressure and flowtester, the large central gauge readspressure while the two smaller gaugesshow manifold vacuum and exhaust

gas backpressure. A glass column andfloating ball provide volume data.

Some returnless systems can be easily reconfigured to allow

for conventional injector cleaning/decarbonization treat-ments to work. For example, in some Ford and Volvo prod-ucts, the former fuel rail pressure regulator has been re-placed by a Fuel Rail Pressure Transducer in order to pro-duce a returnless system. Reversing the swap by putting aregulator back in allows easy and convenient access forsuch a cleaning. The hose shown here can replace the block-off for the old regulator port on some products, allowingthem to be looped for cleaning. Note that there is no provi-sion for pressure regulation directly with the hose; youllhave to rely on your cleaning equipment for that function.

These two fuel modules from a Chrysler Pacificas saddle

tank demonstrate the complexity of some returnless systemshardware. The drivers-side module contains the pressurepump. The internally routed tubing carries the fuel across thesaddle to the passenger-side module, which contains the reg-ulator and the supply line outlet to the engine. The regulatordumps any surplus fuel in the right-side tank, from which itsthen siphoned back to the left side. Both modules contain fuellevel sending units. Their signals are combined before beingdisplayed on the dash gauge. Here the tank is being emptiedfor cleaning prior to the modules being replaced, so only theleft (drivers-side) module is currently immersed.

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gauge. It appears that at least some man-ufacturers have chosen not to measurethe fuel pressure as we mortals normallydo. Our gauge readings tell us the pres-sureaboveatmospheric. In most applica-

tions Ive encountered, however, the re-ported values correspond to a relativepressurethat is, the pressure dropthroughthe injector, from the rail to theintake manifold. This is not the gaugepressure, nor the absolut e pr essur e,which is what the pressure gauge mightread if the vehicle could somehow betested in outer space.

Absolute pressure should not be un-familiar to us; after all, its precisely thesame frame of reference used in mani-

fold absolute pressure (MAP) read-ings. To calculate the expected gaugereading to verify a reported Fuel RailAbsolute Pressure (FRAP) PID fromyour scanners datastream, simply add

BARO if known, or normal local atmos-pheric pressure (about 14.7 psi at sealevel, roughly .5 psi less per thousandfeetabove sea level) back into yourgauge reading. The result should bevery close if the sensor is accurate.

But for those systems which reportneither absolute pressure nor gaugepressure, but instead report the pressuredifferencebetween the fuel rail and theintake manifold (Fuel Rail Pressure

Transducer [FRPT]), the calculation is a

bit more cumbersome. To correlate yourgauge reading, add atmospheric pres-sure or BARO as before, then subtractMAP. If MAP or BARO are expressedas inches of mercury (in.-Hg), multiply

this number by .491 to convert it to psi.Here are a couple of examples. In

the first example: FRAP reads 50 psi,elevation 2000 ft., direct BARO readingunavailable:

FRAP local atmospheric pressure 50 (14.7 (.5 x 2))50 (14.7 1) 50 13.7 36.3 psi

This is the expected gauge reading.In the second example, the gauge

reads 41 psi, F RPT reads 50 psi,MAP 11 in.-Hg, elevation 1000 ft.:

Gauge read ing local at mospheric

pressure MAP 41 (14.7 (.5 x 1)) (11 x .491)41 (14.7 .5) 5.401 41 14.2 5.4 49.8 psi

This is very close to thereported FRPT PID. (The differencemay be attributable to current weatherconditions. High atmospheric [baro-metric] pressure is associated withgood weather, low barometric pres-sure with rain and storms.)

As you can see, the gauge readingsfor the two methods of reporting thefuel pressure PID differ by about 12%,so it pays to know which sort of report-ing system is used. I f your databasedoes not provide a detailed answer, besure to check the PID KOEO. Sincethe engine is then off, MAP becomesequal to BARO. This means that sys-tems reporting FRAP will show a PIDequal to your gauge reading plus localbarometric pressure (as before.)

But what about systems showingFRPT, the pressure drop across the in-

jector? Since MAP is now equal to localatmospheric pressure, the reportedPID should match your gauge readingexactly, allowing you to determine easilywhich type of system youre dealingwith (see Whats the Matter? on page42 for help in diagnosing abnormalpressure, volume, or pump electricalcurrent consumption issues).

Additional ConsiderationsNo discussion of fuel system diagnosis orservice would be complete without con-


40 March 2013

DisplacementCID/L)............... .......... 91.5/1.5 ....... 122.0/2.0........ 152.6/2.5........ 262.4/4.3Max rpm. . . . . . . . .. . . . . . . . . .. . . 6000 ....... . .. . . 7500........ . . . . . . . 6500....... .. . . . . . . 5000Air cu. in./min.) . . . . . . . . . .. 274,500....... 457,500....... . .. 495,950....... . .. 656,000Air lbs./min.). . . . . . . . .. . . . . . 12.12 ....... . .. . 20.21....... .. . . . . . 21.90........ . . . . . . 28.97Fuel lbs./min.) . . . . . . . . . .. . . . .83 ....... . .. . . . . 1.38....... . . .. . . . . . 1.5...... . . . . . .. . . . . . 1.98Fuel U.S. gals./min.) ....... .13 ............... .22.................. .24.................. .32Fuel L/min.)............... ... .49 ............... .83............... ... .91................. 1.21

While manufacturers specifi-cations vary, a good rule ofthumb is to expect fuel de-

livery volumes in the range fromabout a pint every 15 seconds forsmall-displacement motors to abouttw o to three times that a mount forhighe r displacemen t o r very high-per-forman ce eng ines. Most productionvehicles need no more than a quartof fuel every 15 second s. Using eq uip-ment with a dedicated flow volumemeasurement device is much saferthan any other method.

H e re a r e a f e w m o r e r u le s o fthum b: A typical g asoline eng ine us-es less than one gallon of fuel toma ke 1 horsepow er for one hour. Asfuel pressure increases, fuel pumpdelivery volume decreases and pumpelectrical current consumption in-creases. Brake Specific Fuel Con-sumption (BSFC) varies considerablydepending on basic engine designand performance criteria. High-per-formance, naturally aspira ted en-gines may operate in a range of .4

to .5 lb./hp/hr., while t he ir fo rced-in-duction counterparts are g enerallyin th e ra ng e o f .6 t o .75 lb./hp/hr.

Gasolines density is roughly 6.2lbs./gal. When choo sing a fue l systemfor high-performance applications,standard practice is to oversupplyfue l by 20% to 25%, w ith t he injec-tors operat ing a t a n 80% duty cycle.

I created the t able show n here us-ing t he Thom pson Auto mot ive Lab sFuel Calculato r provided w ith thecompanys Fuel Systems Analyzer(ht t p : //ww w.thompsonautomo t ive

labs.com/Fuel_Zapp). The ta ble a s-sumes 100% volumetric efficiency,so n at urally aspirated eng ines w illrequire less fuel than shown, whileforced-induction powerplants mayreq uire some w ha t mo re. The vol-umes show n a re ab solute minimumrequirements to maintain adequatemixture formation. A substantial re-serve safety factor of at least an ad-ditiona l 25% is the norm, w ith ma nysystems supplying as much as doublethe m inimum req uired a mounts.

Rate Calculator: How Much Is Enough?

Overall: Approximately 12 g a l./min . o r .5 L/min . (roug hly 1 o z./sec.) is usua lly mo re

tha n sufficient fo r norma lly aspirat ed g asoline eng ines up to a bout 5.0L displacement.

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sidering fuel quality issues. Excessive al-cohol, water or other contaminants maycause serious driveability problems. Mostof us have experienced at least one no-start resulting from misfueling a gasolineengine with diesel. Excessive alcohol, tochoose another example, also can causeproblems, including aeration and leanrunning due to a split in the hose be-tween the pump and the outlet from thetank, especially in older vehicles orwhere the wrong connecting hose hasbeen installed between the pump andthe tank outlet. Alcohol can also cause in-

jector winding faults, spray pattern andatomization issues and performance ormileage complaints.

Contamination may come in manyother forms underlying a variety ofcomplaints, including misfires, hesita-tion and lack of power. This is onereason why seeingthe fuel supply inaction is so important. Where flowrates are high, contamination may bemore difficult to spot, but returnless

systems offer a very clear view of fuelat idle with the appropriate equipment.

If youre taking a fuel sample alongthe way, make sure you use a clean con-tainer made of glass or some other suit-able material. Allow the fuel to stand forsome time so that any water or contami-nants can settle out. Detecting excessivealcohol concentrations requires a bitmore work, but a number of simple kitsare readily available. Most producequantitative results in as little as two tothree minutes. Remember that the go

juice in most metropolitan areas is oxy-genated with as much as 10% alcohol.

Good fuel is usually readily transpar-ent, although normally dyed to allow af-ter-sale (or after-spill!) identification.Cloudiness or turbidity often accompa-ny excessive alcohol and water content,but may also arise from very fine partic-ulates in suspension. Friends who workas rental fleet technicians report en-countering an increasing incidence ofapparently intentional fuel watering


42 March 2013

This article can be found online atwww.motormagazine.com.

Pu mp Cur re nt* Sy st em Pr essu re Su pp ly Volume (Flow )

Low . . . . . . . . . . . . . .Low . . . . . . . . . . . . . .Low . . . . . . . . . . . . . . . . . . .

Normalto Low . . . . . . . . . . .Low . . . . . . . . . . . . . .Low . . . . . . . . . . . . . . . . . . .

Normalto High . . . . . . . . . . .Low . . . . . . . . . . . . . .Low . . . . . . . . . . . . . . . . . . .

High . . . . . . . . . . . . . .Low . . . . . . . . . . . . . .Low . . . . . . . . . . . . . . . . . . .

High . . . . . . . . . . . . . .High** . . . . . . . . . . . .Low . . . . . . . . . . . . . . . . . . .

High . . . . . . . . . . . . . .High** . . . . . . . . . . . .Norma l to High . . . . . . . . . .

The chart below can help you di-agnose fuel supply problems inconvent iona l return-loop sys-

tem s. To a da pt it fo r use w ith return-

less systems, install your pressure andvolume gauge in the feed line, thenrun a return line ba ck to the t ank us-in g a s u i t a b le f l ex ib le n eo p r en ehose. Slow ly and ca refully crimp th ereturn hose until flow stops. Caution:Fuel pressure should reach the pub-lished maximum system pressure.

Make sure all couplings are tightly

secured. Now slow ly open yo ur crimpuntil the pressure dro ps barely belowthe maximum just observed, thenuse the chart show n here.

In th e case of variable-speed (dut y-cycle-cont rolled or current -limiting )pumps, it may be necessary to bypassthe fuel pump driver module or relay

either through bidirectional scan too lcommand or physically at the modulew iring. If you ha ve no ot her means ofmeasuring volume, have an assistant

use a w ell-ma rked m ea suring cup orsimilar device along w ith a stopw atchat the end of t he return hose.

Pump actuation using a typical re-mote starter (dea d ma n) push butto nallow s for a q uick response in caseMurphys law tries to horn in on t heaction.

Whats the Matter?

*Pump current relat ive to norma l know n-go od. iATNsponsors can find ma ny such va lues in t he WaveformArchives at http: / /members. iatn.net / .

**High system pressure in a re turnless system should g olow if a return path is provided.

Look For

Low pump circuit volta g e; incorrect pump. Drivercomma nd o r faulty mo dule; inlet restriction (clogg edsock); w eak pum p

Fuel leak inside ta nk; coupler ho se, inte rna l pressure

regulator fault , pulsation damper leak, clogged filterRestricte d fue l filter; fau lty com bo f ilte r/reg ulat orassembly on truncate d return system

Restricte d supply (clog g ed sock); pump int erna llyboun d up (high mechan ical resistance)

Pressure reg ulat or stuck closed; ret urn line restricted

Faulty pump driver mod ule; incorrect pum p comma nd

whenever pump prices climb.

ConclusionReturnless fuel systems call for somenew diagnostic techniques, but atten-tion to basic principles of construc-tion and operation should go a longway toward helping to ease the transi-tion. Basic fuel supply testing proce-dures can still be carried out withminimal adaptation, although somework may be required to square fuelrail pressure scan data with conven-tional gauge readings. The deadend fuel rails lack of a return linemay result in more injector inletscreen blockage, since debris is notreadily flushed away by ongoing flowthroughthe regulator and back to thetank. In some cases, traditional on-carinjector cleaning may be ineffective,requiring new techniques, equipment

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