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Changes and Improvements to Fuel SystemsPosted 6/7/1999By Jim Linder
Much has been said and done over the years with fuel injection systems and, most importantly, the fuel supply systems. Often these changes have been gradual and went totally unnoticed. For example, when the pump manufactures had large amounts of damaged pump units due to a technician deadheading the system, they installed a pressure relief valve in the pump unit to prevent damage. Ever notice that the late model tbi units only go to 20 psi when deadheaded, when the older units went to 50 psi? This article will cover some of these gradual changes over the last few years and take a peek into the future systems. Starting at the fuel tank:
At one time the specifications for filtration was 70 micron (the strainer sock), 20-30 micron (the fuel filter) and 10 micron (the fuel injector upper screen). The utilization of gerotor high pressure pumps, new deposit resistant disc spray tip injectors and returnless (demand) fuel systems have required a tighter spec for the new models to be 33 micron (the tank strainer sock), 15-20 micron (the fuel filter) and 10 micron (the fuel injector upper screen). Special care must be taken with these new systems and pump replacement requires a tank flush to prevent a repeat failure.
FUEL SUPPLY AND RETURN LINES:
RETURNLESS DEMAND SYSTEM Many vehicles are starting to use a "returnless" or "demand" fuel system. Returnless means that only one fuel line comes from the fuel filter up to the injectors. These vehicles have no return line from the engine back to the fuel tank. This is done to reduce heating of the fuel by the engine, and therefore, reduce evaporative hydrocarbon emissions. These systems use an in-tank mounted regulator that is not vacuum based and requires a somewhat different diagnostic routine for pressure and volume testing discussed later in this article. Tighter hydrocarbon emission laws will likely cause this system to be installed on more models as time goes on.
FUEL PUMPS Many vehicles use a high output roller vane pump operating at about 3500 rpm, drawing from 5 to 10 amperes of current. Much information may be gained with a current waveform and by knowing the ampere draw and actual pump speed. We suggest that the technician perform the three basic tests for fuel pump condition and performance:
Most systems are operating at higher pressures of about 35 to 60 psi. These units will use a pressure relief valve in the pump to regulate fuel pump pressure to a maximum of 60 to 110 psi, depending on application. In the case of a restriction to flow, this valve opens and dumps the excess back to the fuel tank. A pulsator may also be used to dampen the roller vane pulses, and must be installed properly to be effective.
GURU TIP! Most pumps on today's vehicles should NOT be tested deadheaded as damage to the pump module may occur!
Many of today's vehicles incorporate a modular type unit. These units use the pump, tank fuel level sender and the strainer into one unit mounted in the fuel tank. These units may have a reservoir filled with captured fuel to keep the pump submerged during cornering. Fuel pump noise may be reduced using these units. GURU TIP! These units are much more prone to in-tank debris as they capture a volume of fuel and keep it in the reserve section. Therefore dirt stays where it can and will damage the pump. Where does dirt come from you may ask ? The answer is: You purchase this dirt sometimes for a $ 1.09 a gallon ! FUEL SUPPLY TESTING The age-old methods of fuel pressure and supply (volume) testing must also be looked at and modified with these newer systems. For years we have blown some fuel into a jar, called it a pint in 15 seconds and went on with our day! Newer demands on the fuel systems need a somewhat tighter specification than ever before. FUEL PRESSURE TESTING
NORMAL FUEL FLOW AT IDLE
The example given shows the actual fuel flow needed on a common V-6 engine application at idle. This is a measure with a flow meter at 25 cc's per minute. Note the small amount of fuel required to run this 99 Buick engine at idle. (Some imports idle at 10 cc's per minute !)
(A normal gasoline engine uses from 20 to 30 cc's per minute at idle; the example shown uses only 24 cc's per minute at idle on the '99 3800 Buick V6 engine. Note that there are just drops of fuel at this flow rate.) AN EXAMPLE OF A COMMON FUEL SYSTEM PROBLEM Let's say a certain fuel system is designed to flow 0.4 U.S. gallons per minute through the return line to the fuel tank. If the fuel pump had a problem and only delivered 0.3 U.S. gallons to the fuel tank, the pressure would still be the same and the vehicle has a loss of 25 percent of its available volume! This could be caused by a defective fuel pump, excessive clearance, improper application for vehicle or internal leaks, etc. Now to test the above vehicle and determine the weak fuel pump, the technician would need to perform a volume test, somehow measuring the flow volume with the fuel pump running and measuring the liquid in a container as per the earlier picture. This method is very unreliable due to restriction at the schrader valve hookup and the actual measurement of the fuel in a container. (Usually with a coffee can, is the first ring a pint?) ACCURATE FUEL VOLUME TESTING The preferred method of measuring fuel volume is to use a volume meter. Two different tests are performed here for accuracy. Step one uses a hookup in series with the supply system, and shows the regulator controlled flow through the system. This test shows how much fuel is used by the injectors (again a very small amount of 20- 30 cc's per minute and not usually noticed on the flow meter) and how much is returned to the fuel tank through the regulator.
Step two is to disconnect both the fuel supply and return lines and loop the flow meter with the system, with the pump powered up and no engine rail attached. This test will disclose the actual flow of the system unloaded for a maximum volume test. The spec for this test is 0.4 to 0.5 U.S. gallons per minute on most vehicles.
Since the introduction of this type of volume testing we have caught many pump units (in some cases new) running a proper fuel pressure specification with less than proper volume! Information on this volume tester is available at http://www.gbreman.com.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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FILTRATION
MODULAR FUEL PUMP UNITS
Fuel pressure testing from factory supplied schrader valve is the most common method of hookup. Many things may be checked with a simple pressure check. A system that will not hold rest pressure may identify a faulty:
While much may be learned from a common pressure check, it should not be used alone to confirm a properly operating system! You may have good fuel pressure and have a lower than required volume from the pump!
Note that this fuel rail system ('99 Buick 3800) flows slightly over 0.3 U.S. gallons per minute of actual fuel delivery through the system (at 50 psi) and out the pressure regulator returning to the tank. At idle, approximately 90 percent of the system fuel volume is returned to the fuel tank!
In step two, the fuel system is looped from the fuel pump outlet line and through the fuel return with the actual rail and regulator unhooked to test the pump's unloaded volume. Note that the total pump volume is now 6 U.S. gallons per minute at only 10 psi!
