Testing Fuel Pressure and Flow

21-Sep-2012

Mityvac FST PRo Operation

Mityvac MV5545 Training Video FST Pro Fuel System Test Kit

Because fuel pressure is so critical to modern fuel ejection systems, current diagnostic techniques tend to focus only on the capability of the fuel delivery system to provide pressure. So while fuel volume is just as critical to engine performance, it is largely overlooked or assumed to be adequate. In many cases, checking fuel volume is considered only as a last resort. The fact is peak engine performance and efficiency depends on adequate fuel pressure and volume from idol to peak horse power. Diagnosing a fuel delivery system by considering pressure or volume alone, can be misleading and unreliable at best, especially if testing is not performed at peak engine output when its volume requirement is greatest. The Mityvac FST pro fuel system tester is the only on card diagnostic tool that allows a technician to simultaneously observe values from both pressure and volume while simulating peak engine requirements. This video demonstrates the proper application of the Mityvac FST pro, to achieve the most accurate and reliable diagnoses of a vehicles fuel delivery system

IDENTIFYING THE TYPE OF FUEL DELIVERY SYSTEM

NOTE: for the purpose of fuel delivery diagnostics it’s critical to determine if the fuel delivery system is return and return-less. As this will greatly impact the test procedure and chore responding results. On return fuel systems all of the fuel coming out of the fuel pump goes into the engine, the engine draws what it needs from the continues supply and the rest is returned to the fuel tank, returned fuel systems will have two fuel lines going to the fuel rail, one for fuel going in, and one for fuel going out. One a return-less. fuel system, the only fuel pump going to the engine is what it actually uses; access fuel coming from the pump is bypassed either in or just outside the tank. Return-less. fuel systems will have only one fuel line going into the fuel rail. Some manufactures including ford use a pressure sensor in the fuel rail to vary the speed of the fuel pump. These systems should be tested using the same method as a standard return-less. system. Multiple speed fuel pumps are an additional variable that must be taken into consideration prior to testing. Some vehicles vary the fuel pumps speed depending on engine requirements, in the case of a multiple fuel speed pump follow the vehicle manufacturer’s procedure to switch the fuel pump to its highest speed prior to testing. Once you’ve determined the type of fuel delivery system follow the FST users Manuel too properly install the fuel system tester. Be sure to note all safety precautions, and always wear safety glasses.

PREPARATION FOR TESTING

The key to determining whether a fuel delivery system is failing is to test it a peak engine output because that is when it’s under its greatest demand. To do this, you will need to know the vehicle manufacturer’s specification for fuel pressure and the approximate volume of fuel the engine requires at peak output. The ideal fuel pressure specification can be found in most service or repair manuals, the maximum fuel volume requirement can be determined using the maximum engine fuel volume requirements chart in the back of the FST users manual or can be calculated using the maximum fuel flow formula found on the AASA fuel pump manufacturers council website at fuelpumpinfo.org with a Mityvac fuel system tester connected and connections properly tested start the engine and allow it to idle. First, observe the fuel contamination and air bubbles as it flows through the clear flow tube in the flow meter. If the fuel is discolored or appears contaminated, have it chemically checked. And/ or drain and thoroughly clean or replace the fuel tank before continuing. Expect to see air bubbles when you first start the vehicle as air is purged from the tester and fuel line, but it is not normal to see air bubbles once the system is clear. The continued presence of air bubbles in the fuel stream is an indication of severe restrictions, typically at the fuel pump inlet strainer. Keeping this in mind watch for further evidence and blockage as you continue testing.

DIAGNOSING A RETURN FUEL SYSTEM: performance test

On return fuel systems at idol, the pressure gage will indicate the fuel pressure in the system, make note of the value, next note the volume of fuel by reading across the top of the flow on the FST flow meter and comparing it to the adjacent scale because the fuel pump operates at a constant output, and all of the fuel must pass through all the fuel rail. Diagnosing the performance of a return type fuel system is very straight forward if you know the pressure and volume at the rail. In fact, on the return system the test values for pressure and volume at idol represent the peak output of fuel delivery system. Start by comparing the test value for idol pressure to the manufacturer’s pressure specification is it normal, low or high. Next, compare the test value for idol volume to the engines maximum volume requirement you looked up earlier. If idle pressure is normal, and idle volume meets or exceeds the engines maximum requirement, the fuel delivery system is capable of performing to meet the engines requirements from idol to peak output. However if the pressure is out of specification and or the volume is below the maximum requirement of the engine, this would indicate a malfunction in the fuel delivery system. While a performance test will determine if there is a malfunction, additional testing should be performed to pinpoint the cause of the malfunction and support the final diagnoses.

DIAGNOSING A RETURN FUEL SYSTEM: dead-head test

The first test is to determine the maximum pressure the pump can reduce when all flow is restricted this is called a dead-head test. Performing a dead-head test will completely restrict the fuel flow, so be prepared to execute it quickly to prevent damage to the fuel pump. Locate the knob for the flow control valve on the side of the FST and rotate it 90 degrees clockwise from the open position to the close position, quickly note the value for peak pressure indicated on the pressure gage and the return the valve to the open position, this completes the dead-head test.

DIAGNOSING A RETURN FUEL SYSTEM: Bypass test

The second test is to determine the peak volume of the pump, when there is no restriction to flow, this is called the bypass test because the volume is measured and the fuel flow is bypassing the pressure regulator. To perform a bypass test turn the flow control knob from the open position, 180 degrees clock wise to the bypass position. You will notice the fuel flow from the bypass port located on the side of the tester above the flow control knob, while in the bypass mood note the peak flow by reading across the top of the float on the FST flow meter. After you’ve recorded this volume return the knob to the open position, if the vehicle stalls during this test simply return the knob to the open position and restart.

DIAGNOSING A RETURN FUEL SYSTEM: diagnosing the results

Performing a dead-head test, and a bypass test and addition to the original performance test will yield values for idle pressure, idle volume, peak pressure and peak flow for the vehicle. List these values out as they will all be taken into consideration when making a finale diagnosis. When performing the dead-head test it is normal for the pressure to peak at least 50 percent higher than the manufacturer’s pressures specification. If it can’t achieve this output, then consider the value for peak pressure to be low. When performing the bypass test for peak flow it is normal for the volume to reach over 0.7 gallons per minute. If it doesn’t consider the values for peak volume to be low. Using these criteria indicate on your list of test values whether each is low, normal or high.

DIAGNOSING A RETURN FUEL SYSTEM: fuel supply restriction

A restriction to the fuel supply reduces the flow of fuel to the engine, as long as the restriction is small enough the fuel pressure regulator can compensate for it and maintain the required pressure. However any compensation by the regular will cause a decrease in volume. As a restriction increases the volume will decrease until the engine has starved for fuel. On return fuel systems, restrictions are much more evident by observing fuel volume rather than pressure. When values for idle and peak pressure are normal but idle and peak volume are low this is a good indication there’s a blockage in the fuel supply line, typically caused by a clogged fuel filter or inlet strainer or pinched supply line. If the test values indicate a restriction replace the fuel filter, inspect the fuel supply lines and retest. If the test results are the same, you will need to inspect the fuel pump to determine if it has a clogged inlet strainer or some other source of blockage, once you’ve located the blockage repair it and retest.

DIAGNOSING A RETURN FUEL SYSTEM: faulty pressure regulator

A fuel pressure regulator creates pressure by restricting flow. Depending on how the regulator fails, it may create too much pressure by over-restricting the flow. Or too little pressure by under-restricting the flow. If the test value for idle pressure is high and idle volume is low or vice versa this is a god indication of a malfunctioning pressure regulator. In addition if the values for peak pressure and volume are both good it would support this diagnoses because these values are generated while fuel was bypassing the regulator, in this case, replace the fuel pressure regulator and retest.

DIAGNOSING A RETURN FUEL SYSTEM: malfunctioning fuel pump

If all four test values are low this would indicate a performance issue with the fuel pump, check the fuel pump electrical circuit and power supply only if they are good should you replace the fuel pump and retest

DIAGNOSING A RETURNLESS FUEL SYSTEM: performance test

On a return-less. fuel system, at idle the pressure gage will indicate the fuel pressure in the system, make note of the value, and observe the flow inside the clear flow tube. The float inside the clear flow tube should be at the bottom or very near it. This is normal because the only fuel flowing through the system is what the engine needs to idle, locate the flow control knob on the side of the FST tester and rotate clockwise past the close position and toward the bypass position, fuel will begin to flow from the bottom bypass port on the side of the flow meter and the flow will rise to indicate the volume. Continue to rotate the knob until the volume indicated by reading across the top of the float is equal to the maximum engine volume requirement you looked up earlier. Once you’ve dialed in this volume make note of the pressure reading on the gage the return the knob up to the open position if the vehicle stalls during the test, simply return the knob to the open position and restart. The test you just performed simulates a load on the fuel delivery system as if the engine where performing a peak horse power its critical to an accurate performance diagnoses because many malfunctions won’t become evident unless the engine us under load, when performing this test you are bypassing the volume of fuel equivalent to the engines peak demand. At that point the pressure noted on the gage is the peak demand pressure it represents the pressure the fuel delivery system is capable of producing when the engine is using its greatest volume. Having now collected the values for idle pressure and peak demand pressure compare them to the manufacturer’s specification. If the fuel delivery system is functioning properly, idle pressure should meet the specification, and peak demand pressure should be within 10 percent, if this is true, then the fuel delivery system is capable of performing to meet the engine’s requirements at peak output. However if idle pressure is out of specification and or the peak demand pressure is greater than 10 percent lower than spec, then the fuel delivery system is malfunctioning. While a performance test will determine if there is a malfunction additional testing should be performed to pinpoint the cause of the malfunction and support the final diagnoses.

DIAGNOSING A RETURNLESS FUEL SYSTEM: bypass test

While idle and peak demand pressure is a good indication of the capability the fuel delivery system a bypass test to determine the peak volume output of the fuel pump is critical to pinpoint a malfunction in a return-less. fuel system. During the bypass test fuel is allowed to flow unrestricted from the tester in order to determine the peak volume capability of the fuel delivery system. To perform a bypass test turn the flow control knob from the open position 180 degrees clockwise, to the bypass position. While in the bypass mode note the peak flow by reading across the top of the flow on the FST flow meter. After you’ve recorded this volume return the knob to the open position, if the vehicle stalls during this test, simply return the knob to the open position and restart.

DIAGNOSING A RETURNLESS FUEL SYSTEM: diagnosing the results

Performing a bypass test in addition to the original performance test will yield values for idle pressure, peak demand pressure and peak flow for the vehicle, all of these values must be taken into consideration when making a final diagnosis. Start by listing the test values you obtained for idle pressure, peak demand pressure and peak flow. Compare the idle pressure to the manufacturer’s specification. It will be low, normal or high. Peak demand pressure will typically be slightly lower than the idle pressure. Consider peak demand pressure to be normal if its within 10 percent below the manufacturer’s spec. If it’s greater than 10 percent below the spec, consider it low. If its higher than the specification, consider it high. When performing a bypass test for peak flow it is normal for the volume to reach over 0.7 gallons per minute. If the value for peak volume is less than 0.7 gallons per minute, consider it low. Using these criteria, indicate on your list of test values whether each is low, normal or high. Now review your list of test values, if all the values are normal, the fuel delivery system is operating normally.

DIAGNOSING A RETURNLESS FUEL SYSTEM: fuel supply restriction

A restriction to the fuel supply reduces the flow of fuel to the engine, the method employed for regulating the pressure, whether it be a pressure regulator or electronic control of the fuel pump speed can typically compensate for small restriction. However as the restriction increases fuel volume will decrease until the engine is starved for fuel. If idle pressure is normal, yet peak demand pressure and peak flow are low, this is a typical indication of blockage in the fuel supply, possibly caused by a clogged fuel filter or inlet strainer, or pinched supply line. If the test values indicate a restriction, replace the fuel filter, inspect the fuel supply lines and retest. If the test results are the same, you’ll need to inspect the fuel pump to determine if it has a clog inlet strainer or some other source of blockage, once you’ve located the blockage, repair it and retest.

DIAGNOSING A RETURNLESS FUEL SYSTEM: faulty pressure regulator

A mechanical fuel pressure regulator creates pressure by restricting flow, while a system that regulates fuel pressure electronically, does so by varying fuel pump speed. In either case depending on how the regulating system fails, it will create too much pressure or too little pressure. This will be evident by the test values for idle and peak demand pressures both being low, or both being high, while the peak flow remands normal. If the fuel system is electronically regulated by the PCM, follow the vehicle manufacturer’s procedure to diagnose this system. If the system utilizes a mechanical diaphragm pressure regulator, replace it if possible retest. In many cases the fuel pressure regulator is not replaceable, requiring the replacement of the entire fuel pump module, before going through this expense, always check the electrical circuit and power supply to the fuel pump only if these are found to be good, should you replace the module. If an electrical problem exists, repair it and retest.

DIAGNOSING A RETURNLESS FUEL SYSTEM: malfunctioning fuel pump

If all three test values are low this would indicate a performance issue with the fuel pump. Test the electrical circuit and if there is a performance issue with the fuel pump, test the electrical circuit and power supply to the pump. Only if they are determined to be good should the fuel pump be replaced.

CONCLUSION

In the majority of cases a quick test combining the capability of the Mityvac FST fuel system with the proper fuel system diagnostic techniques, can determine the capability of the fuel delivery system. However, exceptions such as a intermittent malfunction caused by a sticky fuel pressure regulator or lose ground can cause the system to perform flawlessly one moment but fail the next. In addition, multiple malfunctions will significantly increase the complexity of pin pointing a failure, employing the proper tools, such as a pressure volume tester with bypass capabilities like the Mityvac FST, will always provide the best starting point. In cases where the diagnostic test results are borderline or inconclusive, consider connecting the FST in alternate locations. Possibly up stream of the filter or on the return side of the pressure regulator. By applying common sense and basic understanding of fuel delivery systems to the test results you should come up with a most likely conclusion, and there you have it, with the assistance of the Mityvac FST and this video, you should now be able to accurately diagnose a fuel delivery system, thank you for your time, see you round the garage.


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