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Troubleshooting Your Fuel Pump

Posted by Mike on 1/22/2020 to Troubleshooting
Troubleshooting Your Fuel Pump

That is exactly the goal of this article, assisting you troubleshooting your fuel pump and thus prevent unnecessary repairs due to wrong diagnostics.

Fuel Pump

 

Tools Checklist

 

Before starting to diagnose your fuel pump ensure that you have the necessary tools and instruments to perform a complete and reliable test.

  • A suitable fuel pressure tester kit, preferably with a gauge fitted with a bleed-off valve
  • An appropriate OBD2 Scanner with enhanced mode 6 data stream capabilities
  • A digital multimeter with an adequate DC volt scale
  • A manual vacuum pump (optional)
  • Proper hand tools (screwdriver sets, open-ended wrench set, flare nut wrench set, etc)
  • Lint-free towels/rags
  • Safety goggles
  • Adequate nitrile gloves to prevent skin contact with gasoline
  • Appropriate container to collect excess fuel during the tests
  • Measuring container for fuel pump volume test (optional)
  • Manufacturers’ fuel pump specifications (optional)
  • Fire extinguisher

 

Fuel Pump Testing


Step-by-step Fuel Pump Diagnostic

The first thing to understand is that a fuel pump that cannot supply enough flow at the proper pressure must be considered defective and therefore requires immediate replacement.

Since finding pumps flowrate specs is rather difficult the procedure outlined in this section will use static and dynamic fuel pressure tests to diagnose the fuel pump condition.

For your reference, the table below shows the most relevant fuel system types currently available in the market along with their corresponding fuel pressure.

Fuel System Type

Expected Fuel Pressure

Carburator

3-7 psi

Throttle Body Injection (TBI)

12-15 psi

Multiport Injection (MPI)

30-45 psi

Sequential Fuel Injection (SFI)

40-60 psi

Gasoline Direct Injection (GDI) / low-pressure pump

50-60 psi

Gasoline Direct Injection (GDI) / high-pressure pump

2,900-4,500 psi

1. Check fuel tank level

Ensure that the fuel tank level is at least 50%. Low fuel levels may affect fuel pump performance and/or pressure and flow output.

2. Verify possible fuel leaks

Any leak in the fuel delivery system will affect both fuel pressure and flowrate, thus it is crucial to repair any leakage before continuing. Look for possible leaks in the fuel filter, fuel lines connections, clamps, fuel pressure regulator, and test port. Regarding the fuel pressure regulator, you can use a manual vacuum pump to test its diaphragm, any fuel drip is a clear indicator that its faulty and needs replacement. Please notice that mechanical fuel pumps installed on carbureted engines have a small orifice designed to allow a quick visual diagnostic of its internal diaphragm. Once again, if you notice fuel dripping from the pump you should replace it before continuing.

3. Check for clogged fuel filters

Most carbureted engines have translucent fuel filters that are easy to check. On the other hand, fuel-injected engines use metal filters that usually last 20,000 miles. Replace the filter if you have doubts about its condition. A clogged fuel filter may decrease fuel pressure significantly and thus affect gauge readings.

4. Verify engine oil level

Many fuel injection systems include a safety feature that shut-off the engine when the oil pressure drops below a certain threshold. This is usually accomplished by including an oil pressure switch in the electric fuel pump circuit. That’s why as part of the troubleshooting process is necessary to check the oil level. On Gasoline Direct Injection (GDI) vehicles not only oil level but also oil condition its crucial. Degraded engine oil may cause problems with DI high-pressure fuel pumps, so keep in mind this important point when diagnosing this type of system.

5. Check the inertia switch

Some fuel injected vehicles, notably Fords and a few Mazdas, can use an inertia switch designed to shut-off the fuel pump in the event of a collision. This switch can accidentally activate during normal driving if you fall in a pothole. Before continuing with the rest of the procedure refer to the owner’s manual to locate and check if this switch has been activated.

6. Verify fuel pump voltage supply

Depending on your vehicle you can use either the digital multimeter or the OBD2 scanner to verify fuel pump voltage. Any voltage below 12 VDC should be considered suspicious and will require a complete circuit check. Don't forget to check the fuel pump's fuse and relay during your inspection.

PRO TIP: it's a good idea to check again the fuel pump supply voltage during the dynamic pressure test (see below) to discard other types of electrical problems.

7. Read Data Trouble Codes (DTC)

It's always a good idea to take note of current and pending DTCs stored in memory. The newer the vehicle, the more advanced the self-diagnostic routine. Some brands provide valuable codes that help to pinpoint electrical-related problems. If you suspect that there is a faulty sensor or actuator, then fix or replace the necessary components before continuing the procedure.

8. Disconnect battery negative terminal

Once you have checked all points above disconnect the battery. This is considered good practice to avoid spark risks during fuel gauge installation/removal, even when it must be reconnected to proceed with some of the tests. It’s also strongly recommended to keep away any other spark source, avoid smoking during the entire procedure, and keep a fire extinguisher at hand at all times.

9. Remove the air cleaner

Engines fitted with carbureted or TBI systems often require removing the air cleaner to access the fuel inlet line. On both systems removing the air filter also allows you to watch fuel delivery in the throttle body and thus ensure there is fuel circulating in the lines.

10. Connect the fuel pressure gauge

PRO TIP: remember to release the pressure before connecting the fuel gauge as well as wearing safety goggles and gloves at all times during this procedure.

Depending on your fuel delivery system connecting the fuel gauge could be very easy or really challenging. Below are some pro tips to save you time and hassle:

Carbureted engines. Usually, the best location to attach the gauge is either on the fuel filter or carburetor's fuel inlet.

TBI systems. If you are using a professional fuel pressure kit you can choose between connecting through a tee at the throttle body or by using a special adaptor that replaces the fuel filter.

MPI and SPI systems. Most multiport systems come with a test port fitted with a Schraeder valve. This port facilitates the gauge connection when compared with other systems.

GDI systems. In the specific case of direct injection, many vehicles use two independent fuel pumps, a low-pressure pump, and a high-pressure pump. It’s possible that your vehicle doesn't have a test port since it's assumed that fuel pressure will be monitored through the scan tool.

11. Purge air

In fuel injection systems it's common that air enters the fuel lines during the gauge connection. That's why it's good practice to purge air before continuing to avoid misleading readings during the procedure. To purge air, you only need to turn the ignition key to ON position (engine OFF) and slowly open the bleeder valve in the fuel gauge. Remember to collect excess fuel in an appropriate container. Clean any remaining fuel with a towel or rag.

12. Perform a static fuel pressure test

This test only applies to vehicles equipped with an electric pump and consists of measuring the fuel pressure with the ignition switch ON (engine OFF). During this test, you will check two aspects. First, the fuel pump should comply with the expected pressure values (see table above) and secondly the pressure should not drop abruptly after the pump turns off. Assuming that you completed the previous steps and no leaks are present, a quick drop in pressure may indicate a faulty fuel pump or a leak in the pump's assembly inside the fuel tank. In any of the two scenarios, lower pressure than expected or an abrupt pressure drop, you will need to remove the pump from the tank for further inspection at a pump bench. Replace or repair as necessary.

13. Check fuel pressure at idle

Turn ignition switch OFF and then start the engine. Wait until the idle stabilizes and then take note of the gauge reading. Compare fuel pressure to specs. If the pressure fluctuates too much or is below specs then you have a bad fuel pump. Replace as necessary and repeat the whole procedure.

PRO TIP:

New fuel pumps are always suspect. Be sure to re-test after replacing a fuel pump.

Fuel Pump Test

14. Perform a dynamic fuel pressure test

The dynamic test consists of accelerating the engine to wide-open throttle (WOT) as quickly as possible and then let it idle for a few seconds. Doing this test 2-3 times while watching the fuel gauge will give you a clear idea of the pump's performance under load. The pressure must stay still (or increase due to the fuel pressure regulator). Any drop in fuel pressure during WOT is a clear indicator of a fuel pump unable to deliver enough flow to the system thus it should be replaced.

If you are diagnosing a GDI vehicle it is advised to go a little further and perform a road test. Place the values of the low-pressure pump, high-pressure pump, engine speed, and injectors pulse width on the scanner screen. Graph these values if possible. During the road test accelerate the engine from zero several times, force the transmission to downshift, drive uphill, in other words, test the engine under load as much as possible. Once at the shop, analyze the different graphs meticulously. On DI vehicles the main computer can manipulate fuel pressure at will to meet engine demand during different driving conditions. Compare fuel pressure values to specs when the engine was under load. Keep in mind that pressure may be lower during deceleration.

15. Clear any DTC stored in memory

Once you finish troubleshooting your fuel pump clear any DTC stored in memory and perform a road test. Check the memory again to ensure all electronic components are working as expected.

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